From owner-qrp-l@LEHIGH.EDU Sun Sep 28 19:55:39 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 TAA20951 for ; Sun, 28 Sep 1997 19:55:37 -0400 (EDT) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.Lehigh.EDU with SMTP id <35377-55184>; Sun, 28 Sep 1997 19:53:56 -0400 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.Lehigh.EDU with ESMTP id <35484-51862>; Sun, 28 Sep 1997 19:52:16 -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 TAA164468 for ; Sun, 28 Sep 1997 19:52:07 -0400 Received: (from pharden@localhost) by zia.aoc.nrao.edu (8.6.12/8.6.10) id RAA16155 for qrp-l@lehigh.edu; Sun, 28 Sep 1997 17:52:05 -0600 Message-Id: <199709282352.RAA16155@zia.aoc.nrao.edu> Date: Sun, 28 Sep 1997 17:52:05 -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 4 X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: O OSCILLOSCOPES - BASIC USE AND MEASUREMENTS by Paul Harden, NA5N PART 4 - MEASURING PHASE SHIFTS ---------------------------------------------------------------------- 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. MEASURING PHASE SHIFTS Phase relationships between two signals at the same frequency can be measured with 2-5 degree accuracy with a scope, although more suited for a dual-trace scope. The REFERENCE signal is applied to CH. 1 and the signal to be phase measured to CH. 2. For proper phase measure- ments, ensure your dual trace display is in the CHOPPED mode, not ALTERNATE mode. (Alternate mode can effect the triggering position for the second, or CH.2 sweep). There are many methods to do this. One is to stretch out the signal so it takes 4 full divisions, so each division is 90 degrees of phase. By measuring from a common point on one signal to the other (zero crossings or from the peaks), the phase can be measured. For example, say you are making a phased array antenna system, in which one feedline must cause a 90 deg. delay. You calculate the electrical length for a quarter wavelength [L=(246/f) x velocity factor] and cut the coax to that length. You are now working on blind faith that you have exactly 90 degrees. With a scope, you can measure it fairly accurately by injecting a signal into one end with a signal generator (at the frequency of interest) and a 50-ohm load on the other. Connect the scope CH.1 to the coax input (signal generator end) and CH.2 to the load end and measure the phase. Trigger the scope and move the horizontal position and/or the time base vernier so the positive peak of the CH.1 sinewave is on the first vertical graticle line and the second positive peak is on the : : +117 degree fourth vertical graticle, as --->: :<--- Phase Shift shown in the illustration to : : based on PEAKS the right. Now measure the **-----|------|------|-----** phase by noting where the | * | : | | * | first positive peak on CH.2 | * | : | | * | occurs. Say it occurs about CH.1-|------*------|------*------| 1.3 divisions to the RIGHT of | | *: | * | | the CH.1 positive peak. Since | | :* | * | | one division is 90 degrees, |------|-@@@-***-----|------| using this method, then | @| : @ | | | 1.3 div. x 90 deg. = 117 deg. | @ | @ | | YOUR DELAY LINE IS TOO LONG! CH.2-|-@----|------|-@----|------| Cut off an inch or two at a @ | | @ | | time until the CH.2 peak is | | | @| @| one division from the CH.1 |------|------|------|-@@@--| peak (or on the 2nd vertical DUAL-TRACE PHASE MEASUREMENT graticle as shown in the illustration) for precise tuning of the delay line. Another method is to make the CH.1 signal to be two divisions high, and center it between the two divisions, such that the zero-crossing points are on the middle graticle line. Where the CH.1 sinewave signal crosses zero going positive is the 0 deg. REFERENCE; the positive peak is 90 deg.; the negative going zero crossing is 180 degrees, etc. For CH.2 to be 0 90 degrees 90 degrees delayed from CH.1, the : : CH.2 sinewave should cross zero, : : going positive, right under the |---**|-----|-----**----| 90 degree peak of the CH.1 signal. | * * | *| * | If the CH.2 zero crossing is farther | * |* | * | * | to the right from the CH.1 positive |Z----|-Z---|--Z--|---Z-| peak, the phase shift is MORE than * | * | * | *| 90 degrees. Back to the example of | | * |* | * the coaxial delay line, you would |-----|----**-----|-----| cut an inch or two at a time until Z=Zero-crossing points the CH.2 zero crossing is directly underneat the CH.1 positive peak (the 90 degree point). And still yet another method of comparing the phase between two signals on a dual-trace scope is to accurately measure the period it takes for one complete sine wave on the CH.1 reference channel. Say it is 140nS (that would be 7.14 MHz, by the way). Now say the CH.2 signal is 50nS delayed from the CH.1 signal. The phase shift would be: Phase = 50ns/140ns x 360 degrees = 129 degrees POSITIVE OR NEGATIVE PHASE SHIFT? One thing you must remember is how to "read" phase shifts on an oscope. When comparing two signals as described above, remember that if the CH.2 signal peak is to the RIGHT of the CH.1 peak, then the CH.2 signal is OCCURING LATER IN TIME than the CH.1 signal, because time is traveling from left to right. If the CH.2 peak is say 90 degrees to the LEFT of the CH.1 peak, then the CH.2 signal occured in time BEFORE the CH.1 signal. This would then be a -90 degree phase shift, or 270 degrees. Think about this carefully before you start cutting the coax on that delay line! PHASE MEASUREMENTS ON A SINGLE TRACE SCOPE Phase measurements can be made on a single trace scope as well. First, connect the REFERENCE signal, using a BNC "T", to both the VERTICAL INPUT to the scope and the EXTERNAL TRIGGER and select EXTERNAL as the trigger SOURCE. Adjust the TRIGGER LEVEL so the zero- crossing occurs at the beginning of the trace on the first vertical graticle. Now remove the reference signal from the scope's vertical input (but NOT the external trigger input) and connect the signal to be phase measured to the vertical input ... WITHOUT altering the time base or trigger level. The sinewave of the signal to be tested should be on the CRT, with the trace being triggered from the external trigger input, or the reference signal. The sinewave now on the CRT likely will not have it's zero- |----**-----|-----|**---| crossing starting at the first | * |* | * * | vertical graticle as the reference | :* | * | :*| * | signal did, but some place else. |-*---|--*--|---*-|----*| On the illustration to the right, |*: | * | *: | * the zero crossing occurs about * : | *| * : | | 0.3 divisions to the RIGHT. This |-----|-----**----|-----| can now be converted to the phase : : : angle in degrees. In the -->: :<--.3 div. : illustration, one complete cycle :<----------->: 2.5 div. takes 2.5 divisions, and the phase delay from the reference is 0.3 div. SINGLE TRACE SCOPE The phase shift is therefore: PHASE MEASUREMENT Phase shift = 0.3 div/2.5 div. x 360 = 0.12 x 360 = 43 degrees The SINGLE-TRACE scope method is a little easier to do if you make the sine wave of the reference to be 4 divisions for one cycle, thus making 90 degrees per division. The phase angle can be guestimated a little quicker with the signal to be phase measured. It is noteworth to mention that the above examples, measuring the phase through a delay line at 7 MHz, would require an oscope with a 20MHz or higher bandwidth, if for no other reason, then just assure that the time base is fast enough to display 1 or 2 sinewave cycles on the CRT. If at your fastest sweep speed, the 7MHz signal is displayed as many cycles, then obviously the accuracy that you can determine the phase angle will be highly degraded. LOW FREQUENCY PHASE SHIFTS ON A LIMITED BANDWIDTH SCOPE. If your scope has a limited bandwidth of only a few MHz or less, there are still useful phase measurements that can be performed. One interesting experiment is to measure the phase shift of the audio signal at different frequencies as it travels through the stages in a CW audio filter. This is done by putting the input to the CW audio filter on CH.1 and the output on CH.2. What is the phase shift of the wanted vs. unwanted frequencies? Recall that an audio filter works by cancelling out (180 degree phase shift) the unwanted signals, while re-enforcing (0 degrees) the frequencies you wish to pass. There will only be one audio frequency for which there is a 0 degree phase shift. This will be the "pole frequency" of the active filter, or the frequency you wish to have the maximum gain. For CW QRP rigs, this should be around 700 Hz. And finally, on a limited bandwidth scope, the phase angles of higher frequencies can be determined by applying the reference signal to the vertical input and the signal to be phase measured on the external horizontal input. This will form a lissajous pattern, the angle or tilt will signify the phase angle. END OF PART 4 -------------------------------------------------------------------- This is the last part of this series of articles on OSCOPES posted to QRP-L. There will be another series on scope measurements posted in the future (many months) that will include some advanced techniques, such as measuring sideband rejection, tuned circuits, filter responses, group delay, VCO phase noise, etc. This will be the contents of part 2 of the oscope article for the Winter QRPp. I haven't written it yet or made hardcopies of the scope displays. But following publication in QRPp, I will convert it to a text file and post it to QRP-L as I did this one. PS - making those waveform illustrations really sucked swampwater! 72, Paul Harden, NA5N NA5N@Rt66.com