From owner-qrp-l@Lehigh.EDU Fri Dec 13 08:23:02 1996 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id IAA12924 for ; Fri, 13 Dec 1996 08:23:01 -0500 (EST) X-Received-x: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id IAA12924 for ; Fri, 13 Dec 1996 08:23:01 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35360-16246>; Fri, 13 Dec 1996 08:15:28 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <35334-37240>; Fri, 13 Dec 1996 08:14:33 -0500 Received: from mail11.digital.com (mail11.digital.com [192.208.46.10]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with ESMTP id IAA152359 for ; Fri, 13 Dec 1996 08:14:31 -0500 Received: from us4rmc.pko.dec.com by mail11.digital.com (8.7.5/UNX 1.5/1.0/WV) id IAA31896; Fri, 13 Dec 1996 08:02:58 -0500 (EST) Received: from asic.enet by us4rmc.pko.dec.com (5.65/rmc-22feb94) id AA20530; Fri, 13 Dec 96 08:04:40 -0500 Received: from asic.enet; by us4rmc.enet; Fri, 13 Dec 96 08:04:40 EST Message-Id: <9612131304.AA20530@us4rmc.pko.dec.com> Date: Fri, 13 Dec 96 08:04:40 EST Reply-To: randolph@asic.ENET.dec.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: 13-Dec-1996 0800 To: "Low Power Amateur Radio Discussion" Subject: N1OOQ RX #1: Basic receiver scheme, 1st IF choice X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Status: RO No. 1 Let's start with the basic conversion scheme of this RX, and why I did it that way. The design inspirations were the big receiver projects in the ARRL book, "Solid State Design for the Radio Amateur". Get that book if you don't have it. It's a gold mine for the measly $10 or so. The 160m receiver with band converters in the Advanced Receiver Concepts chapter is the W1FB "His Eminence" receiver. The W7ZOI contest grade CW station is at the back of the book. Both first appeared in QST in the 70s. I should note that this design isn't really up to the latest and greatest methods of RX design. Quieter and more linear designs exist or are in development by amateurs. Quieter means you hear more signals that are really there, because the radio doesn't lose them in it's own noise. More linear means that the radio itself generates less signals that aren't really there, because the circuits are designed to enhance only the real signals and to supress the spurious ones originating in the circuits themselves. Also, this is not going to be a portable radio! It'll probably draw something like 300-500 mA from the 12V supply. It will be a good performer, though (I hope, anyway! The design is entirely on paper at this date.) I used the W1FB idea of a single-band RX with converters for my RX. This is one method of designing a dual-conversion RX. It's similar to what a transverter does on receive. The first conversion is from whatever band you're listening to, say 20m, to the band of the RX, 32 MHz in my RX. This is an easy way to get any signal in the MF/HF spectrum into your RX. Rather than having to tune all ham bands 1.8-29.7 MHz, the radio only needs to tune a single band, 32 MHz. The band converter for 20m doesn't need to do any tuning. It just converts the whole 20m band, 14.0-14.35 right up to 32.0-32.35, where the radio can tune it. This 32 MHz band is the 1st IF, the 1st intermediate frequency. Any band you want to listen to can be converted to this band, where the radio can easily deal with it. The selection of what 1st IF freq to use is based on the nature of the circuit used to do the converting - a mixer. Mixers will take any two input frequencies and "mix" them to get two different output frequencies: input1+input2, and input1-input2. One of these output frequencies is our IF, and the other gets filtered out by the IF circuits, for instance: 18+14 = 32, 18-14 = 4. The 18 MHz freq is known as the LO, the local oscillator. Our 20m band module will have an 18 MHz oscillator in it. As I said, the mixer will take ANY two frequencies and mix them to get the output frequencies, so we must ask: Can any other input frequency on our antenna mix with the local oscillator to give us an output at the IF? The answer can be had by adding and subtracting the LO from the IF freq: 32-18=14 MHz, our RF freq 32+18=50 Mhz This 50 MHz freq is called the image. A signal at the image freq will be heard in our receiver if we don't filter it out. The filter before the mixer which performs this function is often called the pre-selector. The image rejection of the receiver is the measure of how attenuated this signal will be compared to the signal we want to receive. If we pick a low IF freq, the image will be nearby. Suppose we use 160m, 2 MHz, as the IF, as W1FB did in the "His Eminence" receiver: Our LO for 20m is: 14-2=12 MHz 2+12=14 MHz, our RF freq 2-12=-10 MHz, our image freq The -10 is valid. The signs don't matter much, becuase of the +/- nature of the output of the mixer: 12 +/- 10 indeed gives us outputs at 2 and 22 MHz. So the image is at 10 MHz - not all that far away from the desired signal at 14 MHz. It's difficult to construct filters that attenuate this image without killing the desired signal. In my RX, the image is at 50 MHz, while the desired signal is at 14 MHz. This is easily filtered without hurting the 14 MHz signal. I expect excelent image rejection on 20m. Our mixer has two input signals: the LO, and the RF, which is the signal we want to listen to. BOTH of these signals have images, but so far we've only dealt with the LO image. If we take the IF +/- the RF: 32-14=18 MHz, our LO 32+14=46 MHz, our RF image This is another frequency we must filter out if we don't want to hear it in our RX. It's right up there with the 50 MHz LO image, so it shouldn't be a problem. Note that this image will only appear if there are incoming signals from the antenna at 14 and 46 MHz at the same time, making it less of a problem than the 50 Mhz image, which would appear every time a signal at 50 MHz arrived at the antenna. There is yet one more thing to think about. What if a signal appears on the antenna at our IF freq, 32 MHz? This could leak through the mixer and right into the radio. If our 1st IF is at an HF freq, it's too near the signals we want to hear to easily filter. Moving the IF up into the VHF range eliminates the problem. IF rejection is another spec you see on radio maufacturer's data sheets, and in magazine test reports. We now need to filter 32, 46, and 50 MHz when receiving 14 MHz. This is easy: construct a lowpass filter that cuts off at something like 16 MHz. It will have very high attenuation at those freqs. This is why a VHF 1st IF is a good idea. It's easy to get rid of a lot of crud that otherwise would require very tight, hard to build filters on every band module. I picked this particular VHF frequency because the LO freqs for many of the ham bands will fall on common microprocessor crystal or oscillator freqs, or can be gotten by doubling a microprocessor crystal freq. Only a couple of custom crystals will be needed, for 160m and 80m. Also, 32 MHz isn't too terribly high up in the VHF range, so the actual layout of the 32 MHz circuits won't be too critical. I didn't want to have to deal with chip caps and the like in my 1st IF circuits. These N1OOQ RX postings are intended to be a learning exercise for the QRP-L folks, so if you have questions or comments, post them! How's the level of detail? Too much or too little? ============================================================================== Tom Randolph N1OOQ NE-QRP 419 QRP-L 87 ARRL randolph@asic.enet.dec.com ============================================================================== From owner-qrp-l@Lehigh.EDU Wed Dec 18 15:19:11 1996 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id PAA26999 for ; Wed, 18 Dec 1996 15:19:10 -0500 (EST) X-Received-x: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id PAA26999 for ; Wed, 18 Dec 1996 15:19:10 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35390-51050>; Wed, 18 Dec 1996 15:09:07 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <35122-43880>; Wed, 18 Dec 1996 15:08:18 -0500 Received: from mail11.digital.com (mail11.digital.com [192.208.46.10]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with ESMTP id PAA217683 for ; Wed, 18 Dec 1996 15:08:12 -0500 Received: from us4rmc.pko.dec.com by mail11.digital.com (8.7.5/UNX 1.5/1.0/WV) id OAA20399; Wed, 18 Dec 1996 14:57:23 -0500 (EST) Received: from asic.enet by us4rmc.pko.dec.com (5.65/rmc-22feb94) id AA11983; Wed, 18 Dec 96 14:59:09 -0500 Received: from asic.enet; by us4rmc.enet; Wed, 18 Dec 96 14:59:09 EST Message-Id: <9612181959.AA11983@us4rmc.pko.dec.com> Date: Wed, 18 Dec 96 14:59:09 EST Reply-To: randolph@asic.ENET.dec.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: 18-Dec-1996 1455 To: "Low Power Amateur Radio Discussion" Subject: N1OOQ RX #2: 2nd IF choice, spurious responses X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Status: RO No. 2 The basic receiver will be a superhet that tunes a small band at 32 MHz. We need to pick an IF freq, which will actually be the 2nd IF. The same criteria apply: we want the IF to be a good way away from the received band for decent image rejection and IF rejection. In this case, after the first conversion, it's all much less critical. The incoming signals have been filtered to a certain bandwidth by the band module pre-selector, converted to 32 MHz, and again filtered to a definite bandwidth, so we have only a relatively small chunk of spectrum around 32 MHz to deal with. Any signal that might cause an image or IF feedthrough response has been substantially attenuated. 32 MHz being up in the VHF, almost any lower HF freq will do for a 2nd IF for good image and IF rejection. Another problem, again originating in the nature of the mixer, is spurious signals. Mixers generate low-level harmonics of both input signals, and these cause low-level outputs on their sum and difference freqs. Any of these that happen to fall near our IF will be heard in the receiver as a spurious signal: one that doesn't really exist. Obviously, it would be in our best interest to pick a low HF range 2nd IF that causes a minimum of spurious outputs. Unfortunately, there's no easy way to do this. Software exists to help with the process, but a lot of cut-and-try is required. The classic way is to form a table: | RF x1 x2 x3 x4 x5 x6 x7 | 32 64 96 128 160 192 224 ----------+-------------------------------------------------- LO x1 20 | 52,12 84,44 116,76 . . . x2 40 | 72,8 104,24 x3 60 | 92,28 x4 80 | . x5 100 | . x6 120 | . x7 140 | 268,12 x8 160 | x9 180 | 372,12 The table can be extended out to any arbitrary size, but after about the 9th harmonic the spurs start to get too weak to worry about. For the 12 MHz 2nd IF we'll need a 20 MHz LO. The mixer products above show us that a couple of spurs will be present: 7xLO - 4xRF, and 6xRF - 9xLO. These are both fairly high harmonics, so will be weak if audible. How weak? Looking at the tables in a catalog of commercial double-balanced mixers gives us numbers in the -60 to -75 dB range for these particular spurs - around 10 s-units below the real RF signal, at 6 dB per s-unit. This is a decent IF freq. Most others I tried had substantially more spurs. A double-balanced mixer is a mixer specifically designed to cancel the harmonics that cause these problems, which is why the spurious products are so small. We can figure out what incoming RF freq will cause the spur. The spur will only be audible when that RF freq is within the passband of the receiver. 7xLO - 4xRF = IF 6xRF - 9xLO = IF 140 - 4xRF = 12 6xRF - 180 = 12 4xRF = 140 - 12 6xRF = 12 + 180 RF = (140 - 12)/4 RF = (12 + 180)/6 = 32.0 = 32.0 These will only be audible when the receiver is tuned within a KHz or two of 32.0 MHz, the bottom edge of the band, and there's a signal there. This probably isn't a big problem. The bottom edge of every band will get converted in its particular band module to 32.0 MHz, so the spur will always be at the bottom edge of the particular band: 3.50 MHz, 7.00 MHz, 14.00 MHz, etc. There's one more thing to think about for this 2nd IF: we will be building crystal filters at this freq, and we'll need a BFO at this freq. The crystal filters will determine the bandwidth of the receiver: 500 Hz for CW, 2500 Hz for SSB. If we want to use commonly available microprocessor crystals, we want the IF to be on one of those freqs. We have a wide selection between 1 and 24 MHz. I tried a few different freqs before settling on 12 MHz. We could get rid of one of the spurs by picking a slightly different IF. 12.288 MHz crystals are available. The other spur would move up into the band we want to listen to, though. Also, we lose our nice 20 MHz LO. As is, we can count the LO freq and ignore the first two digits to get our exact RF freq. With a 12.288 MHz IF and a 19.712 MHz LO, we cant. There are now programmable digital freq readouts for sale that can handle this, though. These N1OOQ RX postings are intended to be a learning exercise for the QRP-L folks, so if you have questions or comments, post them! How's the level of detail? Too much or too little? ============================================================================== Tom Randolph N1OOQ NE-QRP 419 QRP-L 87 ARRL randolph@asic.enet.dec.com ============================================================================== From owner-qrp-l@Lehigh.EDU Fri Dec 20 15:01:19 1996 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id PAA16662 for ; Fri, 20 Dec 1996 15:01:18 -0500 (EST) X-Received-x: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id PAA16662 for ; Fri, 20 Dec 1996 15:01:18 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35345-48668>; Fri, 20 Dec 1996 15:00:44 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <35290-48668>; Fri, 20 Dec 1996 14:59:48 -0500 Received: from mail12.digital.com (mail12.digital.com [192.208.46.20]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with ESMTP id OAA158140 for ; Fri, 20 Dec 1996 14:59:44 -0500 Received: from us4rmc.pko.dec.com by mail12.digital.com (8.7.5/UNX 1.5/1.0/WV) id OAA31240; Fri, 20 Dec 1996 14:51:36 -0500 (EST) Received: from asic.enet by us4rmc.pko.dec.com (5.65/rmc-22feb94) id AA27351; Fri, 20 Dec 96 14:53:23 -0500 Received: from asic.enet; by us4rmc.enet; Fri, 20 Dec 96 14:53:23 EST Message-Id: <9612201953.AA27351@us4rmc.pko.dec.com> Date: Fri, 20 Dec 96 14:53:23 EST Reply-To: randolph@asic.ENET.dec.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: 20-Dec-1996 1449 To: "Low Power Amateur Radio Discussion" Subject: N1OOQ RX #3: images and spurs for all bands X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Status: RO No. 3 In no. 1, we determined that the image and IF rejection on 20m would be good. ALL of the bands will have their own image and IF rejection numbers. In no. 2, we looked at spurious output from the 2nd mixer. ALL of our mixers will have similar spurs. To be thorough, we should check all of this. IMAGES that will mix to 32.000 MHz band bottom edge LO LO image RF image ---------------------------------------------- 160 1800 30200 62200 33800 80 3500 28500 60500 35500 40 7000 25000 57000 39000 30 10000 22000 54000 42000 20 14000 18000 50000 46000 17 18000 14000 46000 50000 A filter that cuts off everything above about 19 MHz would be more than adequate to kill all of these images and the 1st IF of 32.0 MHz. We should be able to get excellent image rejection and IF rejection on all six bands. 1ST MIXER PRODUCTS at or near 32.000 MHz band LO x RF x @ RF of ~ dB down ------------------------------------------ 160 - - 80 0 9 3555 >85 2 7 3555 >85 40 1 8 7125 >85 3 6 7125 >85 30 - - 20 8 8 14000 >85 17 8 8 18000 >85 ------------------------------------------ 15 1 2 21500 45 3 0 - 10 (birdie at 33.0 MHz) 12 9 4 24200 60 10 6 2 28000 55 8 0 - 35 (birdie at 32.0 MHz) I cheated a bit and used a PC program to find these spurs, but a table, such as in article no. 2, for each band would work. Our six bands have only a few very weak spurs. This is good, as the 1st IF is wideband and untuned. That means these spurs will be audible across the entire band if RF hits the antenna at the freq listed. There's nothing stopping energy anywhere in the particular band from getting into the 1st mixer, and the LO will always be at the same freq, ready to mix it to 32.0 MHz. If there's a megawatt shortwave broadcaster on 7125 at night, I could be in trouble on 40m! I included the three high bands to show one reason why I decided to drop them. Some big harmonics of the LO for 15m and 10m would be very close to the 1st IF. These birdies would suck up a lot of the available power in the IF amp. This would leave little available power to amplify the signal we actually want to listen to, making our receiver deaf! In the 10m case, the birdie is right on the IF freq. We'd be able to hear it, good and loud! These N1OOQ RX postings are intended to be a learning exercise for the QRP-L folks, so if you have questions or comments, post them! How's the level of detail? Too much or too little? ============================================================================== Tom Randolph N1OOQ NE-QRP 419 QRP-L 87 ARRL randolph@asic.enet.dec.com ============================================================================== From owner-qrp-l@Lehigh.EDU Mon Dec 23 11:20:13 1996 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id LAA21536 for ; Mon, 23 Dec 1996 11:20:12 -0500 (EST) X-Received-x: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id LAA21536 for ; Mon, 23 Dec 1996 11:20:12 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35075-41628>; Mon, 23 Dec 1996 11:19:56 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <34859-41628>; Mon, 23 Dec 1996 11:18:44 -0500 Received: from mail13.digital.com (mail13.digital.com [192.208.46.30]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with ESMTP id LAA190902 for ; Mon, 23 Dec 1996 11:18:31 -0500 Received: from us4rmc.pko.dec.com by mail13.digital.com (8.7.5/UNX 1.5/1.0/WV) id LAA27774; Mon, 23 Dec 1996 11:05:36 -0500 (EST) Received: from asic.enet by us4rmc.pko.dec.com (5.65/rmc-22feb94) id AA06946; Mon, 23 Dec 96 11:07:24 -0500 Received: from asic.enet; by us4rmc.enet; Mon, 23 Dec 96 11:07:25 EST Message-Id: <9612231607.AA06946@us4rmc.pko.dec.com> Date: Mon, 23 Dec 96 11:07:25 EST Reply-To: randolph@asic.ENET.dec.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: 23-Dec-1996 1102 To: "Low Power Amateur Radio Discussion" Subject: N1OOQ RX #4: block diagram, filter tradeoffs X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Status: RO No. 4 Ok, at this point we've gone far enough along with the verbal description that it's time to attempt a partial block diagram: preselector mixer diplexer amp preselector mixer diplexer amp |\ |\ +-----+ +---+32 MHz| \ +-----+ +---+12 MHz| \ a | _ | - | |----->| \ | _ | - | |----->| \ n--->| / \ |--->|X|--->| | | /--->| / \ |--->|X|--->| | | / t | | - | |--+ | / | | - | |--+ | / +-----+ ^ +---+ | |/ +-----+ ^ +---+ | |/ 13.8-14.7 MHz | \ 30-34 MHz | \ | / | / | \ 50 | \ 50 18.0 MHz | 20.0-20.5 MHz | LO gnd VFO gnd | | ----------+---------- | 20m band module We've examined the selection and problems of IF frequencies, now let's start going over the tradeoffs of these various circuits in detail. The preselectors are simply bandpass filters. Before this, I've been saying that a simple lowpass filter would be plenty to keep crap out of the receiver. That's true as far as image and IF rejection. There's more, though. We have introduced the IF amps above in the block diagram. Amplifiers in an ideal world would be totally linear. In other words, the output would be an exact amplified replica of the input. In the real world, all amplifiers add some distortion to the signal. This distortion is similar to that in the mixer products table of article no. 2. Every signal into the amplifier mixes slightly with every other signal, producing output that we don't want. This is IMD, intermodulation distortion, or intermod. Also, all amplifiers have a definite amount of power that they can source to the next stage. If one strong signal, well out of the band we're interested in listening to, were to use up all of that power, there would be none available for the signal we want to hear. Our receiver would seem "deaf" for no apparent reason. This is known as overloading or desense. Yet another problem is caused by the mixer: Any random noise, at any freq, reaching it will get mixed with the signals we want to hear, making them noisier. This is called noise modulation. Obviously, it would behoove us to only allow a minimum range of signals into the mixer and amp. A filter that passes the whole band we're interested in, and not much more, is ideal. In practice, very small passbands are difficult to get without introducing a lot of insertion loss. The filter is made just wide enough to reduce the loss to an acceptable level. Also, very narrow passbands tend to result in extremes of component values. A slightly wider than ideal filter is sometimes necessary in order to be able to build it with real components. There are several easily-constructed bandpass filter types to choose from: Butterworth, Chebyshev, Elliptical, etc. Any of theses are readily built with practical inductors and capacitors at HF. The differences are in the mathematical description, the slope of the cutoff, and the change in phase vs freq. We only need to concern ourselves with the cutoff of our preselectors. For the most part, it's not too critical how sharply the filters cut off the unwanted freqs. The signals that will cause the biggest problems are many MHz away - the images and the IF. They will be heavily attenuated no matter which filter type we select. When the slope of the cutoff isn't critical, Butterworth is probably the best choice. Its passband is flat, and it's easily tuned by peaking a signal. All of the preselectors are Butterworth except the 160m filter. At 1.8 MHz, the cutoff is indeed critical: the AM broadcast band is only 200 KHz away. A Chebyshev filter provides the sharper cutoff we need, while still being fairly easy to tune. The passband is not flat like Butterworth, but has some slight ripple. Elliptical filters allow even sharper cutoff than Chebyshev, but there are several tuned traps that aren't easily adjusted without a spectrum analyzer. The 32 MHz bandpass filter before the 2nd mixer is considerably less critical than the band module filters. It's there mainly to stop any oddball mixer products of the 1st mixer from reaching the 2nd mixer. Also, we want to cut down the bandwidth of the noise generated in the 1st IF circuits before it reaches the 2nd mixer. In modern synthesized rigs, this would be a fairly broadband crystal filter. With our wideband 1st IF, 32.0-32.5 MHz or so, we can't use crystals, so we just chop it down as narrow as possible without too much insertion loss, using a Butterworth filter. This is the highest-freq LC filter in the receiver, so some of the components will have impractically small values if the bandwidth is too small. A bandwidth of around 4 MHz is about the best compromise between images, noise, and component values. These N1OOQ RX postings are intended to be a learning exercise for the QRP-L folks, so if you have questions or comments, post them! How's the level of detail? Too much or too little? ============================================================================== Tom Randolph N1OOQ NE-QRP 419 QRP-L 87 ARRL randolph@asic.enet.dec.com ============================================================================== From owner-qrp-l@Lehigh.EDU Thu Jan 2 15:21:25 1997 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id PAA06637 for ; Thu, 2 Jan 1997 15:21:24 -0500 (EST) X-Received-x: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id PAA06637 for ; Thu, 2 Jan 1997 15:21:24 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35218-34127>; Thu, 2 Jan 1997 15:20:47 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <35208-19536>; Thu, 2 Jan 1997 15:19:50 -0500 Received: from mail12.digital.com (mail12.digital.com [192.208.46.20]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with ESMTP id PAA34072 for ; Thu, 2 Jan 1997 15:17:54 -0500 Received: from us4rmc.pko.dec.com by mail12.digital.com (8.7.5/UNX 1.5/1.0/WV) id PAA12358; Thu, 2 Jan 1997 15:10:37 -0500 (EST) Received: from asic.enet by us4rmc.pko.dec.com (5.65/rmc-22feb94) id AA00347; Thu, 2 Jan 97 15:01:26 -0500 Received: from asic.enet; by us4rmc.enet; Thu, 2 Jan 97 15:01:28 EST Message-Id: <9701022001.AA00347@us4rmc.pko.dec.com> Date: Thu, 2 Jan 97 15:01:28 EST Reply-To: randolph@asic.ENET.dec.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: 02-Jan-1997 1456 To: "Low Power Amateur Radio Discussion" Subject: N1OOQ RX #5: mixer tradeoffs, IMD, intercepts X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Status: RO No. 5 We'll now look at the tradeoffs of mixers. There is a lot to cover, so this will come in two postings. The concepts learned here will also be useful when we get to the IF amps. There are many kinds of circuits that we can use as mixers. A simple diode goes a long way as far as just mixing two signals and getting an output. The problem with simple mixers is that a lot of stuff that we don't want also comes out the other end. In particular, the two input signals and all their harmonics make a real mess of the output. Circuits can be designed using balance to cancel out certain of the inputs. A single-balanced mixer cancels one of the inputs but lets the other pass through to the output. This is useful in some situations where one incoming signal is very small compared to the other. The best types of mixers are double-balanced. These use a circuit that cancels both input signals and their harmonics at the output, leaving you with a fairly clean output. The residual junk can be easily filtered to insignificant levels. Double-balanced mixers also offer isolation between input ports. This is important to avoid the signal from one input getting into the other input and modulating the source, resulting in even more junk. All mixer circuits are limited in the amount of power they can handle without distorting. All of them add some noise to the signal. The major tradeoff between the various types of double-balanced mixers is between distortion and noise. Generally, those mixers with conversion gain contribute less noise and more distortion, and those with conversion loss give more noise and less distortion. Gain means that the signal gets amplified as well as mixed; the output is bigger than the input. This of course requires active devices. The amplification of the active devices means less drive power is needed from the oscillator. Loss means that the signal gets attenuated as well as mixed; the output is smaller than the input. Diode mixers generally have conversion loss. More power is need from the oscillator to drive this type of mixer, 5 to 20 mW being typical. The receiver's mixers are all double-balanced diode rings. This type of mixer is similar to a bridge rectifier, has about 7 dB conversion loss, and is pretty clean under large signal conditions. These mixers are particular about the impedance on the three ports. In order to realize their peak performance, they must see close to 50 ohms across a wide bandwidth on all three ports. The diplexer handles this at the output port. It passes the IF signal on to the 50 ohm input of the amplifier, while terminating all other frequencies into a 50 ohm resistor. It's essentially just a bandpass filter to the IF output, and a bandstop filter to the 50 ohm resistor, the band in question being the IF +/- a MHz or two. At the oscillator input port, a 3 dB resistive attenuator will provide 50 ohm termination. We can easily afford to throw away 3 dB's of oscillator power to get a nice, clean 50 ohms. At the signal input port, we can't attenuate without killing the sensitivity of our receiver. We'll have to rely on the 50 ohm antenna or the 50 ohm output of the previous IF amp to provide termination through the bandpass filters. We will be using about 20 mW to drive the mixers in this receiver. With the 3 dB attenuators, this means we'll need 40 mW out of the oscillators. IMD is the biggest distortion worry in mixers. Close together signals generate small IMD products in the mixer that appear nearby, close enough to appear within the band we're listening to and get detected just like a real signal. This is due again to harmonics of the two signals, generated by the mixer itself, mixing and producing output. If the two signals are separated by, say, 10 KHz, the IMD products will appear at 10 KHz above the higher signal, and 10 KHz below the lower signal: signal signal | | | | | | | | IMD | | IMD | | | | | | | | <- 10 KHz -> <- 10 KHz -> <- 10 KHz -> Two close, strong stations will generate IMD products like these. If we happen to be operating at one of the IMD freqs, we'll hear the IMD as QRM every time both strong stations have the key down at the same time. The IMD output of a mixer can be judged by its 3rd-order intercept point. You may have heard this term before. The concept is easy to understand and is applicable to any active stage in a radio. At normal low power levels, the IMD products in mixer circuits are well below the level of the signal itself. As the signal power is increased, the level of the IMD products rises much faster than the level of the signal itself. At some theoretical point, the circuit is putting out as much junk as signal. This is the intercept point. In practice, most circuits' gain levels off well before it reaches that level, but the concept is useful for comparison and prediction. | o . | o. IPo |- - - - - - - -o <----- intercept point | .o| | . o | . o | . = signal out | . o o = IMD out Pout | . o | | . o IPo is the output intercept | . o | IPi is the input intercept +------------------- for any circuit, IPi = IPo - gain Pin IPi Intercept is usually rated in dBm, decibels relative to a millwatt. Our diode-ring mixers have output intercepts of about +14 dBm. The input intercepts are IPi = +14 - (-7) = +21 dBm. What does this mean? With an input signal of +21 dBm, which is 126 mW, the theoretical IMD output will be as high as the theoretical signal output. For every dB we reduce the input, this 3rd-order IMD will drop by 3 dB. We can now predict how much junk will come out with a certain level of signal in: input level signal dB down IMD dB down IMD dB down from IPi from IPi from signal ---------------------------------------------------------------------------- Pin IPi-Pin (IPi-Pin)x3 (IPi-Pin)x3 - (IPi-Pin) 0 dBm 21 dB 21x3 = 63 dB 63-21 = 42 dB -10 dBm 31 dB 31x3 = 93 dB 93-31 = 62 dB -20 dBm 41 dB 41x3 = 123 dB 123-41 = 82 dB -30 dBm 51 dB 51x3 = 153 dB 153-51 = 102 dB Obviously, the IMD products drop to an imperceptable level much more quickly than the signal power drops. Plugging higher IPi values into the equations would give us lower IMD levels. This tells us that mixers with a higher intercept rating cause less distortion than those with a lower rating. We should use a mixer with as high an intercept as we can get for lowest IMD. If we're willing to pay the price in oscillator power, diode-ring mixers are among the better in this respect. This receiver was designed to be a home station rig, with performance up to commercial rig standards, so I went all out. Many QRP rigs are designed for minimum current draw from batteries, so low power is a must. Active mixers such as the NE602 are a blessing in such cases, even though IMD is a bit higher. There are experimental active mixers in development with intercepts as high as +50 dBm, with relatively low oscillator power. Future receivers may have 20-30 dB more IMD resistance than anything currently available! We can use the intercept concept to evaluate the IMD handling of the entire receiver as well as the mixer alone. 3rd-order intercepts are often listed in receiver specs and magazine test reports. A simple analysis is easy to do. If we reasonably assume everything after the mixer is pretty linear, we need only consider the circuitry from the antenna to the mixer to get a fair estimate of the receiver IPi. The only thing in the path is the preselector filter. The LC circuit of the filter contributes no IMD, only insertion loss. Insertion loss is added directly to the IPi; 1 dB of loss means a signal 1 dB stronger is need to generate the same IMD products. The filter does indeed have about 1 dB of insertion loss, so we can expect the IPi of the receiver to be around +22 dBm. The values in the table above are good estimates of what will happen in the first mixer. These N1OOQ RX postings are intended to be a learning exercise for the QRP-L folks, so if you have questions or comments, post them! How's the level of detail? Too much or too little? ============================================================================== Tom Randolph N1OOQ NE-QRP 419 QRP-L 87 ARRL randolph@asic.enet.dec.com ============================================================================== From owner-qrp-l@lehigh.edu Mon Jan 6 20:33:57 1997 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id UAA17681 for ; Mon, 6 Jan 1997 20:33:56 -0500 (EST) X-Received-x: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id UAA17681 for ; Mon, 6 Jan 1997 20:33:56 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35070-19619>; Mon, 6 Jan 1997 20:33:34 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <34906-21408>; Mon, 6 Jan 1997 20:31:30 -0500 Received: from VNET.IBM.COM (vnet.ibm.com [199.171.26.4]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with SMTP id UAA43213 for ; Mon, 6 Jan 1997 20:31:19 -0500 Received: from RALVM8 by VNET.IBM.COM (IBM VM SMTP V2R3) with BSMTP id 5374; Mon, 06 Jan 97 20:31:03 EST Message-Id: <199701070131.UAA43213@nss2.CC.Lehigh.EDU> Date: Mon, 6 Jan 97 18:55:25 EST Reply-To: rflight@VNET.IBM.COM Sender: owner-qrp-l@lehigh.edu Precedence: bulk From: rflight@VNET.IBM.COM To: "Low Power Amateur Radio Discussion" Subject: Re: N1OOQ RX #5: mixer tradeoffs, IMD, intercepts (Looooooong!!) X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Status: RO Nobody seems to be addressing the question "What is 3rd order Intercept", and it is an important question to answer if one is to understand its meaning in the context of mixer performance. First, it is a mathematical expression, and shouldn't be identified with mixer performance alone. In radio communications, this reference generally applies to intemodulation distortion as a measure of performance, and as such is used to specify amplifier performance as well as total system performance. It is generally considered the upper boundary of another measure of performance called dynamic range where the lower boundary is a function of bandwidth and noise. A linear system is defined as one whose output is a faithful reproduction of the input modified only by the gain of the system (note that loss is a negative gain). If we plot the input versus the output of such a system on a linear graph (i.e. evenly spaced units of power, volts, current, etc.), the result will be a straight line with a slope of 1. This means that for every unit increase on the input, the output increases the exact amount at all points on the line. In mathematical terms this is: Y = G * X Where: Y = the output value which varies as a function of the input X = the input value which can be varied. G = the gain of the system which remains constant. We have not yet determined how to make a system perfectly linear, and in the case of a mixer, we purposefully attempt to make it very non-linear. A nonlinear system is one which exhibits an apparent gain change as a function of increasing input signal level. At small signal levels, the output signal may follow the input signal 1 for 1 and exhibit a constant gain. It is indeed a linear system in this region. As the input signal increase, the output starts to compress, and the apparent gain appears to be lower. This continues until the output reaches what is termed saturation, where increasing the input signal doesn't appear to cause any significant change in output signal. In this case, the system is highly non-linear. A mixer is operated near this region. Mixing products are produced when devices are operated in this region. They are called spurious products when by convention, they are not wanted. A more accurate term for these signals are intermodulation products. In a strict sense, spurious products are signal artifacts that are derived from other than the process of mixing two tones to produce a third, often result from self oscillation or injected unwanted noise. In a perfect world, the mixing process produces sum and difference frequencies. This results from the mathematical multiplication of two sinusodial waveforms. In reality, the output of a mixer contains an attenuated version of each of the input sinusoids, a tone which represents their sum, one that represents their difference, 2 times each, etc. and having the mathematical relationship of M x N where M and N are integer multipliers of each of the two fundamental frequencies input into the mixer. The "order of the product" is the terminology used to specify the result represented by multiplying a specific harmonic of each tone. If the second harmonic of the first tone (M) is multiplied by the fourth harmonic of the second tone (N), an 8th order product will result. Note that there are a number of combinations that can produce an 8th order result (i.e. 1 x 8, 2 x 4, 4 x 2, and 8 x 1). Order only identifys the multiplier relationship that exists to produce any given tone resulting from the process of mixing two fundamental waveforms. To get back on track, the 3rd order intercept relates to the behavior of the third order mixing products produced by the non-linear performance of a system being evaluated. There always exists two such signal components, the 1 x 2 and the 2 x 1 (called one by two and 2 by one products). These 3rd order products are of special interest, because they are the strongest (highest signal level) mixing products that can appear as intermodulation products that cannot be eliminated by filtering in any receiving system of arbitrary bandwidth. This can be easily demonstrated mathematically but is expedient to demonstrate by example. Consider a very narrow CW receiving system with a 10 Hz "Brick Wall" filter. This is a filter that receives everything within the 10 Hz bandwidth, but nothing outside these limits regardless of signal strength. If two signals appear within this 10 Hz bandwidth such that one is 5 Hz from the bottom of the filter, and the second is only 2 Hz higher (i.e. 7 Hz from the bottom), the third order products created by the nonlinearities in this system will appear at: (2*5)-7 = 3 Hzfrom the bottom of the filter and (2*7)-5 = 9 Hz from the bottom of the filter Notice that the two inband signals appear inband as well as both of the mixing products. (i.e. signals appear at 3, 5, 7, and 9 Hz from the bottom). Thus even a filter as narrow as 10 Hz doesn't protect you from exposure to 3rd order mixing products. Three important factors to realize from this: 1) Two signals within the same bandwidth may or may not be a problem. This is a parameter to which all linear systems are exposed (mechanical as well as electrical), and as such is a universal measurement of "goodness" of a system that can be applied at a component level on the bottom, all the way to the top at the system level. 2) The fundamentals are always higher in amplitude than the mixing products they produce. In many balanced mixer designs, the fundamentals are significantly suppressed at the IF port so that they can be more easily filtered. 3) The "IF" of a mixer is a third order product, but it is not the mixing artifact that is specified in terms of Intercept point with those devices. More on this later. Back to the definition... as a mathematical expression. If a system follows a true mathematical third order response, the output products will change at a proportional rate greater than the input level is increased. Most simple systems (single active device) follow this behavior very well up to the point where the device becomes saturated, and the third order products themselves start to become compressed and ultimately reach a point of saturation where no increase in input signal level will cause the 3rd order products to increase in strength. If one plots the response of the output 3rd order products as a function of input signal level, you will observe that the output increases at a rate equal to 3 times the change at the input until the output begins to compress. Before the 3rd order products begin compressing, the plot produced by graphing the output 3rd order response is a straight line with a slope of 3 to 1. If both the fundamental response above (with a slope of 1 to 1) is plotted on the same graph as the 3rd order response (with a slope of 3 to 1), and the straight line segments are extended to the point where they meet, the point where they meet is called the 3rd order intercept point. That is the point where the 3rd order response curve intercepts the linear response curve. It is important to note that this point is not achievable in practice, as the device being measured reaches saturation long before attaining it. In addition, you will recall that the 3rd order signal levels are always smaller than the fundamental tones. It is deterministic in that it can be computed from measured results, but is fictituous in the sense that a system cannot be evaluated at that point. It is a measurement of "goodness" in the sense that the higher the intercept point, the more linear the system performance (i.e. the desired output is a faithful reproduction of the input modified by the gain (loss) of the device or system. Higher is better can be observed by drawing a graph of input versus output, with two lines representing the fundamental (1 to 1) slope and the 3rd order (3 to 1) slope intersecting at an arbitrary point (e.g. +10 dBm). Avantek, Watkins Johnson, and Mini-Circuits are 3 application note sources for an excellent tutorial on this, as well as a source for graphical aids for performing device/system evaluations. If you draw a second line with a 3 to 1 slope that intersects the graph at a higher point (e.g. +20 dBm), and compare for a given input signal level the respective 3rd order products for each of the two intercept points, you will observe that the 3rd order products are much lower for a given input level on the device with the higher intercept point (+20 dBm). In simplistic terms, the device with the highest 3rd order intercept point can handle a proportionally stronger input signal level without producing unwanted intermodulation products in the system. In simple mathematical jargon, the 3rd order intercept point is the point where a line with a slope of 1 to 1 intercepts a line with a slope of 3 to 1. Back to the specifics of a mixer... Since a mixer is by design a non-linear device, it's third order response is a desirable artifact, and as such, mixers are optimized for this. In this special case, the 3rd order intercept as a measure of "goodness" has little meaning. Instead, the term "conversion loss" is used. Conversion loss simply means that a signal at the RF input produces an IF output signal (i.e. 3rd order mixing product that is at some fixed level offset below the RF input. The IF is a reproduction of the RF input signal modified by the gain of the device, and either the sum or difference in frequency with respect to the Local oscillator which is generally at a much higher level than the RF input signal. Observing that the IF is at a fixed level difference with the RF input level, implies that the RF to IF conversion process behaves linearly (i.e. a change in input level reflects an equivalent change in output level). This is indeed the case until... you guessed it... the mixer begins to go into saturation. It is the RF to IF conversion linearity that is being addrressed when using the term 3rd order intercept when specifying the performance of mixers. It is measured in exactly the same way, except the spectrum in the IF passband is observed as the output rather than the spectrum in the region near the RF signal. As with other linear devices, the higher the 3rd order intercept point the better the device. Now the question remains whether the 3rd order intercept point should be specified with respect to the input or output. Technically, one cannot measure the input intercept point, since the intermodulation products result from two signals (and only two signals) at the input, they only appear at the output. It seems reasonable that since the measurement is at the output, the specification should be made with respect to the output. Indeed this is true.... but.... Consider the case where the gain is negative.... The output intercept point at some value on the output represents a higher value if you subtract the gain of the device (with a mixer this is generally negative). If you sell mixers, you might be inclined to prefer a measurement 6 dB or so higher than your competition is advertising. To keep everybody even on this, mixers by convention are specified with respect to their input 3rd order Intercept point. Consider the case where you wish to know how well a receiver performs in a given environment where many large inband signals exist. One receiver may have a very high output intercept point, but also a high minimum gain, where another may have a lower intercept point with lower gain and able to handle a much higher input signal level without producing intermodulation products. To level the playing field in this case, receiver systems are specified with respect to input intercept point. Consider the case where the maximum output level is desired for a given system requirement (e.g. an audio power amplifier). In this case, maximum drive power with minimum distortion is the most important measure of "goodness" In this case, 3rd order intercept point is only meaningful if referenced to output power. However.... since amplifiers appear at the front end as well as the back end (and at all points in between) amplifiers don't conform to convention. Thus caveat emptor is the rule when evaluating these components and/or devices. It is important therefore to understand the difference between input and output 3rd order intercept point in order to determine how to convert between each. Recall that the 3rd order products are measured at the output of the device under test. The result of this measurement is the 3rd order output intercept point by definition. The intermodulation product levels are measureable in this case. Input 3rd order procucts in contrast are not measurable, and one has to resort to deterministic means to establish this value. It is a simple matter to convert from one to the other however, and all that is required is knowing one or the other and the gain of the device being specified. Again by example... If an amplifier with 10 dB of gain has an output 3rd. order intercept point of +30 dBM, the input intercept point is the output 3rd order intercept point reduced by the gain or +20 dBM in this example. Of course the reverse of this is true as well where the input 3rd order intercept point of +20 dBm is specified, one would conclude the output third order intercept point would be +30 dBm. It should be clear that it matters not which is specified. The device or system performance if completely determined by either, and only the gain of the device or system is required to convert between the two. Caveat emptor again applies here, since many (dare I say most) "systems" have some form of gain control. Evaluation of receiver performance should be based on all variables of gain including the effects of AGC to fully characterize the performance of the system in an operational environment. 72 de N3GO, Gary Raleigh, NC From owner-qrp-l@Lehigh.EDU Mon Jan 13 11:47:26 1997 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id LAA05155 for ; Mon, 13 Jan 1997 11:47:24 -0500 (EST) X-Received-x: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id LAA05155 for ; Mon, 13 Jan 1997 11:47:24 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35153-25389>; Mon, 13 Jan 1997 11:46:45 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <34910-45871>; Mon, 13 Jan 1997 11:45:49 -0500 Received: from mail1.digital.com (mail1.digital.com [204.123.2.50]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with SMTP id LAA47658 for ; Mon, 13 Jan 1997 11:45:32 -0500 Received: from us3rmc.pa.dec.com by mail1.digital.com (5.65 EXP 4/12/95 for V3.2/1.0/WV) id AA17348; Mon, 13 Jan 1997 08:36:28 -0800 Received: from asic.enet by us3rmc.pa.dec.com (5.65/rmc-22feb94) id AA16325; Mon, 13 Jan 97 08:37:42 -0800 Received: from asic.enet; by us3rmc.enet; Mon, 13 Jan 97 08:37:43 PST Message-Id: <9701131637.AA16325@us3rmc.pa.dec.com> Date: Mon, 13 Jan 97 08:37:43 PST Reply-To: randolph@asic.ENET.dec.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: 13-Jan-1997 1134 To: "Low Power Amateur Radio Discussion" Subject: N1OOQ RX #6: more mixer tradeoffs, noise, dynamic range X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Status: RO No. 6 Mixers not only distort the incoming signals, but add some noise to them as well. Distortion is a regular change in the shape of the sinusoidal waveform, an adding of harmonic components, but noise is random. It amounts to continuous random changes of the amplitude and phase of the waveform. All circuits add noise. The only way to avoid it is to cool the circuits to absolute zero, as most of the noise has its origin in the thermal energy of the components. Noise is rather different than distortion, as the amount of noise added by a particular circuit isn't necessarily linked to the gain of the circuit. It's helpful to think of the added noise as a fixed chunk of noise power. A weak but clean incoming signal will be similar in strength to the added noise power; the signal-to-noise ratio will be low at the output. A strong, clean signal will be much more powerful than the added noise power; the signal-to-noise will be high. A circuit will, however, amplify or attenuate any noise already on the input signal according to the gain. If the incoming signal is very noisy, the small chunk of extra noise added by the circuit will be covered up by that input noise. If the incoming signal is very quiet, the extra noise added by the circuit will be very noticable when the signal is small. Once a signal is amplified up out of the noise floor, we needn't be fussy about adding a small extra chunk of noise to it, as the signal will totally overwhelm that little bit of noise. Only the stages before the first gain stage need be painstaking about noise. Again, all circuits add noise, even passive ones. Many circuits, for instance, our antenna, can be modeled as a resistor. The noise power in a resistor is: -23 Pn = kTB, where k is Boltzmann's constant, 1.38 x 10 W/K. T is the temperature in Kelvin, about 290K at room temp. B is the bandwidth we're interested in. Note that the noise depends only on the temperature. Hotter circuits add more noise. As a baseline, any resistor at room temperature has a noise power of -174 dBm in a 1 Hz bandwidth. Converting to typical receiver bandwidths: CW bandwidth noise = -174 + 10 log 500 where 10 log x is the = -174 + 27 = -147 dBm conversion from ratio x to dB. This can also be SSB bandwidth noise = -174 + 10 log 2500 looked up in a table of = -174 + 34 = -140 dBm dB vs power ratio. So the noise power on a room temperature antenna, at a bandwidth of 500 and 2500 Hz, is -147 dBm and -140 dBm, respectively. This amount of noise is always present on the antenna, and sets a limit on receiver performance. Signals weaker than about -140 dBm (about 0.03 uV peak in a 50 ohm antenna) will be lost in the noise of the antenna. A real receiver actually adds some noise to this, so the noise floor is somewhat higher, maybe -130 dBm for an HF receiver. This is known as the minimum discernable signal, or MDS. The difference between the natural noise and the MDS is the noise figure, or NF, of the receiver. NF is a convenient way of specifying how much noise is added by a particular circuit. It is just the ratio signal-to-noise-in/signal-to-noise-out, called the noise factor, converted to decibels. A signal-to-noise ratio is signal power/noise power, so the overall definition of noise figure is / Psig_in / Pnoise_in \ NF = 10 log ( --------------------- ) \ Psig_out / Pnoise_out / Note that this equation has no meaning if we don't specify what the input noise is. Since all circuits add noise, there is always input noise, -174 dBm or more in a 1 Hz bandwidth, as above. The NF is generally specified in just that way, with the input noise being that from a 290K resistor. An ideal circuit would have a NF of zero, adding no noise, only gain or loss, to the noise input. As an example, let's say our circuit has only a room-temperature resistor at the input, zero gain, and a 5 dB NF, a ratio of about 3.2:1. The noise power at the output is Pnoise_in x 3.2, and the signal power is just Psig_in. If the circuit had gain, the noise power at the output would be Pnoise_in x 3.2 x gain, and the signal power would be Psig_in x gain. Either way, signal-to-noise-in/signal-to-noise-out is about 3.2, so NF is about 5 dB. Since all circuits add some noise, the NF is always greater than 0. The noise added by a particular mixer depends on the design of the mixer. Active mixers have noise figures in the 1 to 10 dB range, depending on the circuit type and the active devices used. Diodes add little noise, but they do have insertion loss. Such attenuating circuits are adequately modeled as a resistor for noise purposes, and as such add a constant amount of noise, as above. The noise in this resistor model is the same as that used when specifying NF, -174 dBm, but the signal is attenuated. The signal-to-noise ratio at the input of the combined circuit is therefore degraded by the amount of attenuation, and the circuit adds its attenuation directly to the NF of the following circuits. Hence, our diode-ring mixer adds 6-7 dB to the NF of the IF amplifier. We can get an estimate of the receiver's overall NF by similarly analyzing the circuitry between the mixer and the antenna. Again, there will be only a bandpass filter with insertion loss (and additional NF) of around 1 dB. The NF of our receiver will be the NF of the IF amp + 7 dB + 1 dB. Assuming a 5 dB NF for the amp, the receiver NF will be about 13 dB. The receiver's MDS can now be estimated: -147 dBm + 13 dB = -134 dBm 500 Hz bandwidth -140 dBm + 13 dB = -127 dBm 2500 Hz bandwidth We have, in these last two articles, established two limits on the receiver's performance. At the strong signal end, IMD in the front-end components sets the limit on how big a signal we can copy without distortion. At the weak signal end, noise in the front end limits how small a signal we can copy without drowning in the noise. This clean-signal spread is called the dynamic range of the receiver. We can put a number on it: DR = (signal level where IMD products rise above front-end noise) - MDS In article #5, we saw that IMD products at an input signal of -30 dBm, one microwatt, were about 153 dB below the IPi of +21 dBm, or -132 dBm. This is similar to the MDS we calculated above, so this is approximately the point at which the products will become audible. With some algebraic manipulation of the equations in that article, we can get a formula for the exact signal level: MDS + 2 IPi sig = ------------- 3 So the signal levels that will cause audible IMD are: (-134 + 2 x 21) / 3 = -31 dBm 500 Hz bandwidth (-127 + 2 x 21) / 3 = -28 dBm 2500 Hz bandwidth and the DR is: (-31) - (-134) = 103 dB 500 Hz bandwidth (-28) - (-127) = 99 dB 2500 Hz bandwidth This being an HF receiver, some sensitivity (i.e. low NF) was sacrificed to maximize DR. Atmospheric noise will generally drown the noise of the receiver's front end at these freqs. We could add a preamp to boost sensitivity if we don't mind losing some of the mixer's nice, high IPi. A +10 dB preamp would reduce the +21 dBm IPi to +11 dBm. Assuming a preamp NF of 5 dB, MDS would be -140 dBm + 5 dB = -135 dBm, an 8 dB increase in sensitivity. The signal level that will cause IMD is now (-135 + 2 x 11) / 3 = -38 dBm, and the DR is (-38) - (-135) = 97 dB, a loss of only 2 dB in DR. This is a reasonable tradeoff, and a switchable preamp would likely be a useful addition to the receiver. IMD in the preamp itself would then become a limiting factor in the RX performance, and should be carefully handled. These N1OOQ RX postings are intended to be a learning exercise for the QRP-L folks, so if you have questions or comments, post them! How's the level of detail? Too much or too little? ============================================================================== Tom Randolph N1OOQ NE-QRP 419 QRP-L 87 ARRL randolph@asic.enet.dec.com ============================================================================== From owner-qrp-l@Lehigh.EDU Tue Jan 14 15:02:31 1997 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id PAA05880 for ; Tue, 14 Jan 1997 15:02:22 -0500 (EST) X-Received-x: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id PAA05880 for ; Tue, 14 Jan 1997 15:02:22 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <34987-11531>; Tue, 14 Jan 1997 15:02:08 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <34941-42762>; Tue, 14 Jan 1997 15:00:17 -0500 Received: from mail1.digital.com (mail1.digital.com [204.123.2.50]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with SMTP id PAA43250 for ; Tue, 14 Jan 1997 15:00:10 -0500 Received: from us3rmc.pa.dec.com by mail1.digital.com (5.65 EXP 4/12/95 for V3.2/1.0/WV) id AA25328; Tue, 14 Jan 1997 11:54:48 -0800 Received: from asic.enet by us3rmc.pa.dec.com (5.65/rmc-22feb94) id AA26821; Tue, 14 Jan 97 11:56:02 -0800 Received: from asic.enet; by us3rmc.enet; Tue, 14 Jan 97 11:56:02 PST Message-Id: <9701141956.AA26821@us3rmc.pa.dec.com> Date: Tue, 14 Jan 97 11:56:02 PST Reply-To: randolph@asic.ENET.dec.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: 14-Jan-1997 1452 To: "Low Power Amateur Radio Discussion" Subject: N1OOQ RX #7: diplexer tradeoffs X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Status: O Between the mixers and the IF amps are the diplexers. These circuits are designed to pass a small range of freqs on to the amps, while terminating all other freqs into a 50 ohm resistor. This prevents reflection of any mixer products back into the mixer's output port, where it could degrade the mixer's intercept point. Also, this protects the IF amps from having to deal with too large a range of input freqs. The input impedance of the amps can be controlled to look like 50 ohms across a wide bandwidth, just what we want on the mixer output, but every signal other than the IF is just potential distortion-causing junk to the amp. For instance, the first couple of harmonics of the LO are strong enough to cause concern. We don't want to use up the IF amp in amplifying these junk freqs. In some cases, the vast majority of the junk we want to filter is above the IF, and a simple lowpass/highpass arrangement will work. This is usually the case if the IF is very low, for instance 455 KHz, or if we're mixing to audio in a product detector. In this receiver, with its VHF and HF IF's, bandpass/bandstop circuits are needed. They consist of simply a parallel tuned circuit to ground, through a 50 ohm resistor, and a series tuned circuit to the input of the amp. The parallel circuit blocks only the IF, terminating all other freqs in the resistor. The series circuit passes only the IF, blocking all other freqs. The 50 ohm input of the amplifier at the IF, combined with the 50 ohm resistor at other freqs, gives a reasonably smooth 50 ohm termination at all freqs. If the tuned circuits aren't well matched in freq, there will be some bumps in this smooth termination, so some tweeking of the diplexers is a must. The bandwidths (the Q of the tuned circuits) are kept somewhat wide, a couple of MHz, to make this tuning easier. This also keeps insertion loss to a minimum, preserving our NF. The diplexers are made up of LC tuned circuits. We have to make sure there are no L's or C's in the preceeding or following circuits that will interact with these and cause resonances that we don't want, as we need to do with any LC filter. Strange resonances will cause impedance bumps and reflections, messing up all our careful termination. The diplexers in this receiver are unidirectional, that is, the diplexing action will not occur in the reverse direction. The IF would be passed backwards to the input, but every other freq would be reflected rather than dissipated in a resistor. This might be cause for concern in some receiver systems, where impedance after the diplexer isn't consistent across a wide freq range, and signals get reflected back toward the mixer. You may have heard the similar term, "duplexer", before. A diplexer isolates two very different frequency bands, like our IF and all other freqs. This isn't terribly difficult to do with LC filters. A duplexer isolates two very close frequecies, for instance, the input and output freq of a VHF repeater. This is rather difficult, and is done with tuned cavities. These N1OOQ RX postings are intended to be a learning exercise for the QRP-L folks, so if you have questions or comments, post them! How's the level of detail? Too much or too little? ============================================================================== Tom Randolph N1OOQ NE-QRP 419 QRP-L 87 ARRL randolph@asic.enet.dec.com ==============================================================================