From owner-qrp-l@Lehigh.EDU Tue Sep 8 02:51:30 1998 Received: from fidoii.CC.lehigh.EDU (fidoii.CC.lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.8.5/8.8.5) with ESMTP id CAA02287 for ; Tue, 8 Sep 1998 02:51:29 -0400 (EDT) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.Lehigh.EDU with SMTP id <12661-25328>; Tue, 8 Sep 1998 02:50:50 -0400 Received: from nss4.cc.Lehigh.EDU ([128.180.1.13]) by fidoii.cc.Lehigh.EDU with ESMTP id <12566-71148>; Tue, 8 Sep 1998 02:49:23 -0400 Received: from m7.boston.juno.com (m7.boston.juno.com [205.231.100.196]) by nss4.cc.Lehigh.EDU (8.8.8/8.8.5) with ESMTP id CAA36038 for ; Tue, 8 Sep 1998 02:49:09 -0400 Received: (from gsurrency@juno.com) by m7.boston.juno.com (queuemail) id DNLUMNWD; Tue, 08 Sep 1998 02:48:29 EDT Message-Id: <19980907.234655.9694.0.gsurrency@juno.com> Date: Mon, 7 Sep 1998 23:46:54 -0700 Reply-To: gsurrency@juno.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: gsurrency@juno.com (Gary L Surrency) To: "Low Power Amateur Radio Discussion" Subject: The Low-down on Low-pass filters (long) X-Mailer: Juno 1.49 X-Juno-Line-Breaks: 0-1,9-10,15-16,18-22,32-33,35-43,49-55,57-59,64-65, 69-73,78-79,84-88,92-99,102-103,106-107,114-118,125-126,130-131, 139-143,145-151,153-155,161-162,169-170,179-180,183-184,190-191, 202-206,215-216,223-226,232-237 X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: RO Gang, After responding to several people regarding the necessary changes to low-pass filters (hereafter referred to as LPF) in several popular QRP rigs, I have decided to post this information to the list in the hopes that more people may benefit and be willing to tackle this issue on their own. Please read carefully to understand what is going on here, and don't be afraid to try some ideas on your own. My thanks to Wes Hayward - W7ZOI, and Doug DeMaw - now a SK, for their insight into the origin of this information, waaay back in 1977! ;-) Also, please don't ask me to provide the exact values for every XYZ QRP rig's LPF that may be out there, since I don't own them all (yet), and haven't the time to look at each and every one, either. If you lack the equipment or understanding to make these changes, then leave well enough alone. I may make this info available soon in an issue of one of the popular QRP magazines, if there is sufficient interest. ******************************************************************************************** I was recently reading in an old copy of the ARRL's Solid State Design for the Radio Amateur, by Wes Hayward, circa 1977 publish date. On page 61, Wes and Doug described a way to pre-align filter networks using a 50 ohm bridge and a low-level RF source. Hmmmm, I thought. That sounds an awful lot like my new MFJ-259B I recently acquired. Wes has confirmed to me in a separate email that he still employs this method to align his homebrew rigs. I have just adapted the procedure to use the commonly available antenna analyzers that many of us own, rather than using separate pieces of lab equipment, such as an impedance bridge and low-level RF source. Read SSD if you want more info. OK, from the text, the collector impedance of a PA amplifier can be reasonably approximated by: Zcollector= Vcc*2 / (2 x Pout) For 5 watts at 13.8 volts, then: Zcollector= 13.8*2 / (2 x 5) Zcollector=19.044 ohms So, I paralleled a couple of 39 ohm 1/4 watt resistors (about 18.5 ohms) and tack soldered them across the PA collector in my SW40+. Then, with the MFJ attached to the antenna terminals, I proceeded to find the collector Z of the rig as designed. I later tuned the low-pass filter with the MFJ set at 7.040 Mhz to get exactly 50 ohms, and no X component. Worked like a champ! :-) Old SW40+ low-pass values: New low-pass values: C37 470pf C37 470pf (same cap) L3 16 turns on a T37-2 L3 16 turns on a T37-2 C38 .001uf (1000pf) C38 820 pf L4 16 turns on a T37-2 L4 19 turns (18 tight) on a T37-2 C39 470pf C39 470pf (same cap) The original low-pass filter was resonant below 7.0 Mhz, somewhere in the 6.0 Mhz range if I remember correctly, and the Z was not a perfect match to the 50 ohm antenna output impedance, but it was closer with a 25 ohm collector test resistor. This confirmed what Dave B. mentioned to me about the original values that were derived from the GM series. As you adjust the inductors and capacitors in the LPF, you will gain the knowledge of whether the L or C values need to be changed from the original values by watching the results on the SWR / Z meter. It is quite an eye opener. SW30+ changes: I've also modified the LPF in my SW30+, but I'm gonna leave this one for you folks to try, now that you have the idea. If I get bombarded with requests for the component values, I'll have to go look at it and pass them along. But I hope owners of the SW30+ will take the initiative that I did, and give it a try. ;-) You might think you could use the LPF values listed below for the 38S, but the SW30+ comes with T37-6 (yellow) cores, rather than the T37-2 (red) cores supplied in the 38S kit. If you make the necessary changes to the inductors for the appropriate core, you can achieve the desired result. Or, use red cores if you have them. ;-) Norcal 38 Special changes: So, onward to my two 38S's. Both of them were darn close to optimum value, from the mods many of us came up with. Slight tweaking of the toroids' windings brought them into perfect 50 ohm impedance match.The new 38 Special LPF values are as follows: C501 560pf silver mica (s/m) L3 11 turns on a T37-2 (red) evenly spaced around the entire core C28 1000pf s/m L4 12 turns on a T37-2 (red) spaced near the top half of the core C29 560 pf s/m Note: C501 is marked on the PCB; C505 on the schematic. C28 on the PCB is marked C25 on the schematic, and C29 on the PCB is marked C26 on the schematic, pg. 11 of the manual. I found some of the 1000 pf s/m caps varied a bunch in actual measured capacitance, but the adjustment to the toroids' inductance should allow compensation for that. 910 pf to 1100 pf might be OK, too. Subsequent calculations after changing the low-pass filter, seem to indicate the actual collector Z may be closer to 13.6 ohms if you use the 7 watt output value in the original equation, but it's close enough and works well as-is. I again used the 2 parallel 39 ohm resistors on the PA drain for about 19.5 ohms Drain impedance. Assume Drain impedance for collector impedance in any of this discussion when we are speaking about the IRF510. :-) Zcollector=(13.8v)*2 / (2*7watts) Zcollector=13.60 ohms A 15 ohm test resistor may be more appropriate, for the greatest output if your's is a little anemic. I found only small differences in measured output level and antenna Z looking into the rig's antenna jack when I tried different collector test resistors. Getting the SWR reduced thru the LPF makes the most improvement, as it is usually off by quite a margin. It's kind of like having your antenna tuner mis-matched, but you don't realize this until you do these measurements on the PA LPF. The 38S MOSFET PA runs darn cool, and is no doubt capable of greater output, but why bother? Other components would likely need changing, and fellow ScQRPion Dan, N7VE has already shown the MOSFET is capable of as much as 25 watts or more, *if* the supply voltage is increased. Both of my 38S's have Mike Gipe's PIN diode mod, 'cuz without it - the 1N4007s get warm at 5 watts and above. The IRF510 is very cool under continous keydown, with a small Radio Shack extruded TO-220 heat sink, pn. 276-1368, on one of the rigs, and the IRF510 in the other rig is heat sinked to the aluminum chassis and isolated with a silicon thermal pad from an old PC power supply. There seems to be very good efficiency, as I measured a DC supply current of ~830 ma @ 7 watts output with a 13.8 VDC supply. Norcal NC40A changes: The NC40A lowpass filter needed a little work, since it was originally set up for only 2 watts of output. The changes I made were: Old values: New values: C45 330pf 470pf L7 18 turns on a T37-2 16 turns on a T37-2 C46 820 820pf (same cap) L8 18 turns on a T37-2 19 turns on a T37-2 (or, 18 tightly wound) C47 330 pf 470pf Very much the same as the SW40+. Both rigs are stable at various antenna loads, better than before the changes. Even with no antenna for a brief moment, the output seems clean and stable. I also believe I can lower the voltage rating on the protective zener from its present 43v value after these changes, as it seems the reduced collector Z allows lower collector voltages to appear. Gotta try that still. The PA transistors on each rig are well within their temperature ratings with normal CW duty cycle, and will even tolerate 10-20 seconds of keydown without excessive temperatire rise or damage. I have to make some current draw measurements on the SW40+ to calculate the total transmitter efficiencies, but the NC40A draws about 610 ma at 5 watts output. That's very efficient, and marked less current demand than many rigs I have checked at 5 watts output. With these values, I was able to achieve ~ 7 watts output on the NC40A and almost 9 watts out with the SW40+ and SW30+, using my OHR WM-1 wattmeter and 13.8 v DC. 5 watts was easy to set with the drive level control reduced. Of course, I am using a 2N3924 final on the NC40A - but a MRF237 works just as well. The stock 2SC2078 is just fine on the SW40+ and SW30+, and it seems happy at the higher power levels with a small TO-220 heat sink. The stock h/s in the NC40A is rather small, so keep its output to 5 watts or less, and play it safe or get a bigger h/s. An NTE401 is a good candidate h/s that is widely available. These results were the same on all the QRP rigs I have tried it on, and the output looks clean. Anyone with access to a spectrum analyzer is invited to check the spurious output and report their findings. A look at the low-pass filters in a couple of my S&S rigs tends to verify the component values I came up with through actual tests. They also can produce as much as 8-9 watts if everything is blueprinted to the max. I have reduced the drive to hold the output to a more reasonable 5 watts to preserve the PA's and be "legal" QRP. The spurious output is likely to be less too, if the PA isn't driven too hard. I should mention I saw no difference in measured collector Z (actually, LPF output Z) when 13.8 v DC was applied to the PA. Of course, I had to have the two 39 ohm resistors connected on the antenna side of the collector DC-blocking cap to prevent frying them! ;-) So, there is little or no change in static transistor capacitance with Vcc and no drive applied, or with the power off. The Z and X readings on the MFJ-259B are quite sensitive to changes in the inductor windings and/or the capacitors. It's also interesting to substitute different collector test resistances and see the resulting changes in the output Z readings of the low-pass filter, using the antenna analyzer. I encourage you to experiment. Summary: The PA has to "see" 15 to 19 ohms impedance to work hard enough to achieve higher output. And, with the low-pass filter optimized with the SWR analyzer, you can be sure the correct RF is passing thru it easily and any power shortcomings must be due to something else, like low drive or voltage drop to the PA collector. With the analyzer, you can see big changes in the LPF characteristics as you change the turns spacing on the toroids. This is the only way I've found to be certain the LPF acts the way it should, and you'll be satisfied it is correct and isn't just left to the chance behavior of the theorectical values. This is certainly a cool way to utilize your SWR / impedance analyzer to check out filter networks in your radio. MFJ mentions this application in the MFJ-259B manual, but they leave the motivation and details up to the reader. As an example, I have also used my MFJ-259 to align the bandpass filters following the TX mixer in my two TAC-1's. Once you get familiar with the instrument and its possibilities, you'll no doubt come up with some of your own ideas on how to more fully utlilize its potential. DISCLAIMER Please, please, don't transmit into your antenna analyzer connected to your rig's antenna connector. Be safe, and disconnect *both* the power and key so it's less likely you will do this while working on your rig. Only when you are satifisfied with your LPF alignment, and the test equipment is removed from the antenna jack, should you test the results. You have been forewarned! vy 72, Gary Surrency AB7MY QRP-L #571 Chandler, AZ (near Phoenix) _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. 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