From owner-qrp-l@LEHIGH.EDU Sun Sep 28 19:56:12 1997 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 TAA20957 for ; Sun, 28 Sep 1997 19:56:11 -0400 (EDT) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.Lehigh.EDU with SMTP id <34842-55184>; Sun, 28 Sep 1997 19:49:52 -0400 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.Lehigh.EDU with ESMTP id <35480-55184>; Sun, 28 Sep 1997 19:48:45 -0400 Received: from zia.aoc.nrao.edu (zia.aoc.nrao.edu [146.88.1.4]) by nss2.CC.Lehigh.EDU (8.8.5/8.8.5) with SMTP id TAA243036 for ; Sun, 28 Sep 1997 19:48:34 -0400 Received: (from pharden@localhost) by zia.aoc.nrao.edu (8.6.12/8.6.10) id RAA16008 for qrp-l@lehigh.edu; Sun, 28 Sep 1997 17:48:32 -0600 Message-Id: <199709282348.RAA16008@zia.aoc.nrao.edu> Date: Sun, 28 Sep 1997 17:48:32 -0600 Reply-To: pharden@aoc.nrao.edu Sender: owner-qrp-l@LEHIGH.EDU Precedence: bulk From: Paul Harden To: "Low Power Amateur Radio Discussion" Subject: O-SCOPES: PART 2 X-To: qrp-l@LEHIGH.EDU X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: RO OSCILLOSCOPES - BASIC USE AND MEASUREMENTS by Paul Harden, NA5N PART 2 - LET'S MAKE SOME MEASUREMENTS DC voltages, AC voltages, time period and frequency ---------------------------------------------------------------------- NOTE: This is a text version of an article appearing in the Summer 1997 issue of "QRPp." The article contains numerous illustrations and photos of oscilloscopes displays, which unfortunately can not be included in a text file. NOTE ON LIMITED BANDWIDTH SCOPES. Today's scopes have 500MHz bandwidths or higher. Likely your scope is much less than that. A limited bandwidth scope is still very useful to the QRPer. Say the bandwidth of your scope is 5MHz. This does not mean you can't see a 7MHz (40M) signal ... it just means that the calibration of the scope is no longer valid. The peak-to-peak value of the display is not correct and much smaller than it really is, and the sweep rate may be in error. But still, you may likely beable to resolve individual cycles higher than the cited bandwidth to a certain degree, and make the gain and phase measurements that follow (since they are based on RATIOS). Most of the examples in this article explore many regions of a QRP rig without the benefit of any great bandwidth. Experiment with your scope to learn its limitations. !!! IF POSSIBLE, SPEND THE MONEY TO GET A GOOD SCOPE PROBE AND MAKE MEASUREMENTS WITH A GOOD GROUND CONNECTION TO GET THE MOST OUT OF THE BANDWIDTH YOU HAVE. BASIC MEASUREMENTS. It is assumed you have your scope relatively calibrated as described in Part 1, and familiar with the front panel controls. For the sake of the following discussions (since illustrations can not be included), it is assumed the scope has 4 vertical and 4 horizontal divisions. DC VOLTAGES. Say you want to check the T-R switch (Transmit-Receive) in your QRP rig. Usually this will be a transistor (or inverter gate, such as in the 38-Special). The key line goes to the base, which is pulled HI to some positive voltage (on key UP), and goes LO to ground when the key is DOWN (or closed). Setup your scope for DC voltage at 2v/div. and a slow sweep speed (say 100mS/div). Set the trace so the bottom graticle (division line) is 0v. Place the scope lead on the T-R switch transistor base. Say the trace deflects two divisions. This would be 4vdc bias on the base. Now close the morse code key. The trace should drop to 0v. The purpose of the T-R switch is to generate a POSITIVE voltage on key down, which is taken from either the collector or the emitter (depending upon the circuit configuration). Say it comes off the emitter. Move the scope probe to the emitter. Now you should have about 0v with the key UP, and with the key DOWN, the voltage should jump to some positive voltage, often +12v. In this case, the trace will go off the top of the screen. Change the scope to 5v/div. Re-verify that 0v is the bottom graticle. On key DOWN the trace jumps up 2 divisions. The key DOWN voltage is thus +10v. If the emitter is "stuck" at +10v on both key up and down, the transistor is not switching. If the base signal above is correct, then likely the transistor is bad. While this test could be done with a DVM, the integration time is slow requiring long key downs to get the proper voltage. A scope will also show you how clean the switching is, or if there is an AC voltage (or RF noise) riding on the T-R voltage. Scopes are thus good DC voltmeters, with about a 5% reading accuracy. AC VOLTAGES. Here is where a scope pays for itself by making AC voltage (and frequency) measurements. You must remember that AC voltage displayed on a scope is PEAK-TO-PEAK VOLTAGE, while a voltmeter or DVM measures AC voltage in RMS (root mean square). RMS voltages read on a DVM will be ABOUT 1/3rd the peak-to-peak voltage (Vpp) shown on a scope. Or specifically, Vrms = .707 x Vpeak = 0.5(.707 x Vpp) = .35 x Vpp If the signal on your scope -----|--**--|------|--**--|------| looks like that in the quasi- | * * | | * * | | illustration, at 2V/division, |* *| |* *| | then the signal would be *----- * -----*------*------* 4V peak-to-peak (4Vpp), or | |* *| |* *| 1.4Vrms if read on a DVM or | | * * | | * * | voltmeter. -----|------|--**--|------|--**--| VERT: 2V/DIVISION For example, let's measure the output voltage and frequency of the sidetone oscillator in your QRP rig. Setup the scope for 1v/div, AC volts, and a sweep speed of 1mS/div. Connect the scope probe to the audio output of your rig and set the volume control on key DOWN so the audio sinewave is 2 division peak-to-peak. This would then be 2Vpp AC, and should look similar to the illustration above. AC FREQUENCY MEASUREMENT. With this same waveform, we might as well see what frequency our sidetone or transmit-offset is at. Most operators prefer the side- tone to be about 700Hz. With the same setup as above, trigger the scope for a stable waveform and the time base sweep to display 2 or 3 cycles. Center the waveform on the center horizontal graticle so the sinewave goes one division above, and one division below the center graticle. Now move the HOR POSition so the first "zero crossing" of the sine wave is on the first or second vertical graticle. With this setup, zero-crossing would be where the sine wave crosses the center horizontal graticle. Now measure the time it takes to make one complete sine wave, from one zero-crossing (sine wave going positive) to the next positive going zero crossing. Say one complete sine wave takes one and half horizontal divisions. At 1.0mS/div., this would be 1.5mS per cycle. Frequency is the reciprocal of time, such that the sidetone frequency is: f = 1/t = 1/1.5mS = 667 Hz (Sidetone frequency is the tone heard on key DOWN). This may be a little low to your liking. To raise it to 700Hz, calculate the period of 700Hz, which is t = 1/f = 1/700 = 1.4mS. At 1.0mS/div, you can adjust your XMIT OFFSET on key down until zero-crossings (or the positive peaks) are 1.4 divisions apart. This will be 700 Hz. (The XMIT OFFSET is not adjustable in all rigs ... such as the 38-Special. In this case, it usually requires changing the value of a capacitor on the XMIT MIXER and usually discussed in the instruction manual). QUALITY OF THE WAVEFORM is another feature of a scope that is unsurpassed, since your are "seeing" the waveform in real time. For example, say the audio output from your QRP rig is not a clean sine wave, that is, it has a slant to it, or the rise time takes longer than the fall time. This could be due to improper time constant on the audio amplifier coupling capacitors or improperly biased amplifiers. Or, say the audio output sine wave is flattened at the top, looking sorta like a square wave then a sine wave. This would be a raspy sounding sidetone, and due to the audio power amplifier being overdriven and in gain compression (clipping). You should beable to see this effect by turning the volume control to its maximum level, overloading the output audio amplifier (unless your QRP rig has anemic audio like some). The o-scope is an invaluable tool for detecting and diagnosing such distortions and impurities in the signal quality. The audio output of a QRP rig, whether the sidetone or an on-the-air signal, should be a fairly pure sine wave. If not, something is wrong, from a poor product detector action, poor filtering after the product detector, poor coupling capacitors, or severe noise being introduced into the audio at some point. By tuning in an on-the-air signal and plotting the Vpp at different audio frequencies, you can plot the filter response of your QRP rig. This will be discussed in detail in a later section. You can also adjust your BFO on the product detector for the maximum Vpp of the received signal for centering the signal in the filter passband. Note how these important tests and adjustments (sidetone, filter response and setting the BFO) are a few of the things that can be done on a scope with a very limited bandwidth ... since you're not looking at anything beyond the audio range. Once you get comfortable making the above voltage, time and frequency measurements, you might want to go through your QRP kit with the schematic and record the various DC and AC voltages and waveforms at pertinent locations in the circuit. This will be a great aid in the future should your rig develop a problem. NOTE however, that signal levels from the receive mixer through the IF crystal filters are VERY weak and can not not be seen on even an excellent scope. The main exception to this would be the local oscillator (LO) drive on pins 6 and 7 on a NE602. They are usually in the order of 100mVpp. !!! NOTE: You can't hurt anything by probing around the circuit of your QRP rig. The biggest mistake made by beginners is to let the ground lead come loose and drag along the tops of components, which can short out the power supply or damage a component ... OR when putting the scope probe on an IC pin, to slip and let the probe touch two pins at once. This will short out the two pins, which in some cases, could cause damage to the IC. For example, on a NE602, measuring the Vcc voltage on pin 8. A slip to Pin 7 (the OSC input) could destroy the internal oscillator if the pin 8 to 7 short persisted a second or two. END OF PART 2 72, Paul Harden, NA5N NA5N@Rt66.com