Unwanted modulation in crystal oscillator - Page 1
Forum section: electronics / rf-microwave

author: mauri
author_rank: Newbie
timestamp: August 24, 2017, 10:15:49 pm
content: hello,a few years ago I built a Qrp kit transceiver, one with NE602, there are manyinstances of those, I built the one by F6BQU, the 'Toucan', a cheap,nice, little kit.Last month, when trying to increase its output power a little bit,I spotted a low frequency ripple in the transmitter output.At first I thought there was a parasitic oscillation somewhere andI put bypass caps, shunt resistors, etc,, to no avail.Last week I traced back the cause at the very beginning of thetx chain, in the 4MHz LO (which is also a NE602 - SA612AN, actually).Measuring with a scope the LO I noticed what seemed to me a 800Hzmodulation over the 4MHz LO signal.I tried to replace the SA612 with another one, in case it was faulty,but that didn't change anything. Then I removed the 4MHz crystal (HC49U) andI put it in a different separated circuit, a simple test oscillator made byone transistor (Colpitts, very simple), and I noticed the 800Hz modulationwas still there. I replaced the crystal with a similar one (3.932 MHz) and I didn't see any spurious modulation, so I thought the 4MHz crystal was the culprit and I bought acouple new (HC49S).Last Monday I mounted the new crystals and, to my dismay, I saw theunwanted modulation again (this time is 300Hz with one of the new crystals and100Hz with the other).I attach scope pictures. I measured the tx output applying the scope probe rightupon the dummy load, and the LO on SA612 pin6. As I'm a little bit paranoid,I put capacitors between the scope probe and the circuit, on both the signaland ground leads, to avoid loops, but that didn't make any difference.I also attach the original diagram (the actual one is a little different as I havemodified it a bit to run on a different band, but the TX LO is basically the same).The transceiver is powered with batteries.I looked for some insight in sacred texts as well as over the internet but I didn'tspot anything useful.What could cause that ? Can a simple crystal oscillator generate a modulated signal ?Or is it just some scope artifact I'm not aware of ?ThanksMaurizio

author: LA7SJA
author_rank: Regular Contributor
timestamp: August 25, 2017, 04:12:09 am
content: Check PIN1 on IC1 (78L05) with your scope, the VXO is an excellent source of FM modulation.Johan-Fredrik

author: nugglix
author_rank: Regular Contributor
timestamp: August 25, 2017, 08:51:00 am
content: Given that the modulation shows in a different circuit the TO should look for things common to both circuits.Power supply, measurement (probe leads, ground leads), how the circuits are built and so on.

author: mauri
author_rank: Newbie
timestamp: August 26, 2017, 10:35:54 pm
content: hi,thanks for replying.@ Johan-Fredrik : I checked pin1 on IC1 but didn't find any signal there, just DCToday I have built a simple test oscillator running on batteries and I have tried 6 crystals with it.That unwanted modulation appears with some crystal and not with others, which I find puzzlling.The appearance of the unwanted modulation seems to be not dependent on the crystal frequency,while it seems that branded crystals are less affected or not affected at all (crystal Q ?).The frequency of the unwanted modulation is different for every crystal (even with the same frequency),and is somewhat affected by the surrounding components (it changed a little when I replaced thetransistor with a different one ).If it was just a scope artifact shouldn't it appears more or less on *all* crystals ?At the same time, I doubt that most crystals around are faulty and nobody noticed that,so something is going on that I don't understand.Please let me know your take.ThanksMaurizio

author: Tomorokoshi
author_rank: Super Contributor
timestamp: August 27, 2017, 02:42:03 am
content: With the transceiver turned off, can you pick up any signals on the scope or spectrum analyzer in the 4 MHz range? Could the 400 Hz be the beat frequency due to interference between your transceiver and something else like a switching power supply?Does the 400 Hz modulation change at all due to the orientation or position of the transceiver?

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2017, 04:54:23 am
content: Quote from: mauri on August 26, 2017, 10:35:54 pmIt looks like a digital storage oscilloscope artifact to me. It could show up do to mixing between the sampling frequency and the oscillator frequency so it would vary with different frequencies.To test this, I would put a trimmer into the oscillator and adjust its frequency slightly to see what effect that has on the oscilloscope's display.

author: Ian.M
author_rank: Super Contributor
timestamp: August 27, 2017, 05:54:37 am
content: Build a RF detector probe and see if that shows any signs of modulation on the oscillator.

author: voltz
author_rank: Frequent Contributor
timestamp: August 27, 2017, 08:17:26 am
content: I second the idea this is a scope artifact.Here's why.You have tried two completely different oscillator circuit designs, both show the problem - that pretty much rules out circuit design.You only see the problem at multiples of 1Mhz (4Mhz in your case) and not at 3.932. Again this is suspicious.You always see the modulated signal at multiples or 100Hz.. Like 300, 400 or 800 - again suspicious. Crystals simply dont do that.Put that all together and its looking your scope is sampling at some multiple of 100Hz, this is manifesting itself as a beat note in effect which is looking like AM modulation, but in fact its not there. To test this as already suggested by David Hess, try a timmer cap across the crystal and adjust away. You should see is big change in the frequency of the 'am modulated signal'Good luck with your QRP rig!

author: mauri
author_rank: Newbie
timestamp: August 27, 2017, 11:29:48 pm
content: RF fellows,thanks for your suggestions.@ Tomorokoshi:I have checked the spectrum and I can't see anything but the FM band.I think that the probe cable acts as an antenna because I clearlysee in the spectrum a 88-108MHz rectangle. I discovered that just afterbuying the scope and since then I always use some filtering.I don't have any switching power supply (I use batteries), so thereshouldn't be any RF source around.@ Ian.M:I have measured the oscillator output by means of a calibratedRF probe, after applying a cap trimmer in series with the crystal.I did that with the 4MHz HC49U crystal (the one that exhibits moreof that unwanted modulation) and the 6553kHz crystal (the one thatshows little or no modulation).With the 4MHz HC49U crystal I got- 1.168V DVM reading (which is about 1.3Vpp according to theprobe manual) when the cap trimmer is fully closed (max.capacitance, 100pF),- 1.163V (also about 1.3Vpp) with cap 3/4 closed- 1.148V (1.25Vpp) with cap half closed- 1.103V (1.2Vpp) with cap at 1/4- 0.478V (0.6Vpp) with cap open (min. capacitance, it should be 5pF)With the 6553kHz crystal I got- 1.147V (1.25Vpp) with cap fully closed- 1.121V (1.2Vpp) with cap half closed- 0.894V (1 Vpp) with cap almost open (oscillation stops when itis fully open)Such values seem pretty consistent with what I measured with thescope and pretty linear. If I understand the purpose of thistest, an actual modulation should cause some spike in the readings,correct ?@David Hess and voltz:as suggested, I have put a 5-100pF capacitive trimmer in series withthe crystal, and this is what I got.The frequency of the unwanted modulation changes wildly when turningthe cap with the 4MHz crystals (both the HC49S and the HC49S), whileit basically doesn't change with the remaining crystals.Unfortunately I haven't got any crystal which is exact multiple of1MHz, other that one of 18MHz, so I tried that too and it also behavesexactly like the 4MHz crystals in this respect. In detail:- the 4MHz HC49S crystal shows a 88Hz modulation when the cap is fullyclosed (it was 150Hz with no cap), 6.6Hz when it is half closed, and205Hz when the cap is almost open (oscillation stops if fully open)- the 4MHz HC49U crystal shows a 463Hz modulation when the cap isclosed (it was 600Hz with no cap), 290Hz when it is half closed, and2.2kHz when it is open- the 18MHz crystal shows a 1.6kHz modulation when the cap is fullyclosed and 595Hz when it is half closed (oscillation stops turningthe cap any further)With the three crystal above, the scope behaves in a particular way:when increasing the time base I see that sort of snake on the screen,well, increasing the time base a little further makes a sinusoidto appear on the screen which has the same amplitude as the fundamental,it looks the same as the fundamental, but the scope cannot trigger it.This behaviour is quite unclear to me and sounds very much likean artifact. That doesn't happen with any of the remaining crystals.- the 3932kHz crystal exhibits a small modulation with 40us periodwhich doesn't change turning the cap- the 3579kHz crystal exhibits a small modulation with 20us periodwhich doesn't change much either turning the cap- the 6144kHz crystal exhibits a small modulation with 200us periodwhen the cap is closed and 140us when the cap is half closed, thendisappears- the 6553kHz crystal doesn't exhibit any measurable modulation, justthe mains (50Hz) or harmonics of that appears, very low in amplitude(full data and pics are in the zipped attachment)As I understand your point, this should prove that the unwantedmodulation I see on the scope screen is actually not there.This raises a question though. When I noticed all that first in mytransceiver, it was caused by a 4MHz crystal in the LO that madea 800Hz modulation to appear (that was the crystal I threw awaybecause I thought it was faulty). What struck me was that Ifound the very same modulation in the tx output measuring acrossthe dummy load, so I thought there was an actual parasitic modulationthat travelled through the entire tx chain up to the output (picturesof that are in first post).Now, if that 800Hz in LO was actually not there because it was just"mixing between the sampling frequency and the oscillator frequency"how do you explain the 800Hz in the tx output ? Is it not thereeither and also caused by the same artifact ?If this is the case, that would also explain the discrepancy betweenmy wattmeter reading and the power computed based on the scopemeasurement, which is always in excess.And, most important, if there is indeed such artifact, how can Iworkaround it ? Is there a way to regain trust in my scope ?You know, if I spot a 50Hz or 100Hz in the scope screen I easilyunderstand what the source is and dismiss it, but how am I supposedto recognize the cause of a 833Hz ghost modulation which unfortunatelylooks real to peasants like me?I'm looking forward to hearing your comments.ThanksMaurizio

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2017, 12:02:54 am
content: Quote from: mauri on August 27, 2017, 11:29:48 pmerrrrrrr. . . . You want to see if the crystal oscillator is amplitude modulated with a several hundred Hz signal. Why would you use the RF probe on a DVM that cant respond fast enough to follow the modulation?USE A RF PROBE WITH YOUR SCOPE. Put the 'raw' crystal output on one channel and the detected signal from the probe on the other. If the detected signal follows the displayed envelope your oscillator is really modulated by *something*. If there is no modulation, the detected signal will be a DC level, and then you know the observed envelope variation is a scope artefact. As a final sanity check, connect the negative end of a 1.5V cell to your oscillator ground, and probe its positive end instead of the RF probe output. If it doesn't give you a flat line you have serious scope problems or maybe a ground loop with a significant current flowing round it.

author: voltz
author_rank: Frequent Contributor
timestamp: August 28, 2017, 09:13:06 am
content: Hi,Your results from adding the trimmer cap and seeing a large change in the modulation is consistent with a scope artifact. And also why other crystals away from dead on 4Mhz frequencies appear not to show the problem. Also your observation of 'snake like' patterns while changing the scope timebase around 4mhz points the finger to artifacts too.But what can be done?..Well you could try an analog scope if you have one. That would show a true signal without any digital sampling.Its the reason some of us keep both analog and digital scopes handy. Sometimes problems like this can happen.There may be a menu option in your scope to change the sampling frequency but i cannot comment on that, its a maybe.Another sanity check, try listening to the oscillator on a receiver. Tune it in for zero beat, observe if there is any modulation on the signal. If 400 or 800Hz is present, it would be clearly audible. The suggestion here is that there would be none, just a clear signal.And yes as mentioned, you could try to demodulate the signal with a probe or make a simple diode detector and view that on the scope for any audio signals.

author: Kalvin
author_rank: Super Contributor
timestamp: August 28, 2017, 02:58:25 pm
content: If you have a RTL SDR-dongle which can be tuned down to 4MHz, make a small loop antenna and place the loop close to the xtal oscillator. By zooming to a fundamental you should see whether there is any modulation or not. Does the modulation change when you probe the circuit with our oscilloscope? You can also use oscilloscope with a small loop antenna to pick up the oscillator's signal without touching the circuit itself. Is the modulation still present?

author: mauri
author_rank: Newbie
timestamp: August 28, 2017, 11:22:08 pm
content: RF folks,I have done my homework, and here is the result.@Ian.M :ok, it seems I have entirely missed your pointAs I understand, we need a probe to discard the RFand measure on the scope the AF-like signal which issupposedly modulating the crystal oscillator.My probe is a RF/AF and I'm not sure is suited forthis, so I just removed the clipping diode fromthe oscillator and put it on the oscillator outputalong with a 47nF cap, to act as a probe.The 47nF cap should bypass the RF and still letus measure something in the AF range, correct ?Well, with this setup I can measure the oscillatoroutput but I get nothing (in the scope) acrossthe 47nF cap, not even a DC.On the other hand, I can see the 1.5V DC of the battery,it looks flat with all timebases.@voltz :I have made the experiment with the receiver.Now I remember that when I first saw that 800Hz ripplein the tx output I did quite that, I grabbed my receiverto see if I could hear the 800Hz tone in AM, and I heardnothing, just the unmodulated carrier of my transceiver.Unfortunatley, I trusted my scope more than my earsI have tried the oscillator in three position of the cap(three frequencies) and each time I zero-beated and heardnothing different than what expected. The frequency of thereceiver and the one on the scope matched perfectly at zerobeat (zero beat is not quite perfect due to the rx SSB BW, butI think I came close). The scope showed a 1.3kHz modulationwith fully open cap, so it would have been quite audible.I also tried in AM and heard nothing either, while a 1.3kHzAM should be audible.@Kalvin :I haven't got a SDR dongle, so I attached a loop to thesecond channel as suggested, and picked up a weak signal.The loop signal behaves much like the osc. output, andit *does* show the unwanted modulation exactly like thefirst channel. Is this result different than expected ?

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2017, 12:00:22 am
content: Here is another test you can do; hook your DSO to the output as usual, set the timebase to expand the waveform so you can see the individual RF cycles, and then set your DSO to infinite persistence or envelope mode to capture the variation if it exists.

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2017, 02:08:10 am
content: Your RF probe circuit is no good. You've got a capacitor in series with the diode with no other DC path so it cant rectify.Seehttp://www.qsl.net/g3oou/simplerfdetectors.htmlfor probes that work.

author: Kalvin
author_rank: Super Contributor
timestamp: August 29, 2017, 07:52:53 am
content: Quote from: mauri on August 28, 2017, 11:22:08 pmMy thought was that the oscilloscope might induce or conduct some interference into the circuit which will the modulate the oscillator output. If you disconnect the oscilloscope from the circuit altogether and use only the loop antenna only to sniff the oscillator output do you still see the modulation? If you still see the modulation, I have no clue what is going on.

author: Kalvin
author_rank: Super Contributor
timestamp: August 29, 2017, 08:01:09 am
content: Looking at your oscilloscope time base, you are measuring MHz range signal with slow sample rate. Probably you are seeing aliasing due to slow sampling rate which shows as modulation due to timing difference between the oscilloscope and the oscillator?

author: dmills
author_rank: Super Contributor
timestamp: August 29, 2017, 09:31:25 am
content: Some digital scopes have a mode where they correctly interpolate sample values for screen display, as opposed to just joining the dots, try turning this on if you have it (It tends to make slow squareish waves look worse, but makes sines be actual sines on the display) ......The issue is largely that the sample values lie ON the signal, but not necessarily on the PEAK values of the signal, so if your scope just joins the dots (Computationally cheap) you will see modulation like this as the scopes sample points slide past the signal peaks (It is particularly obvious when the signal frequency is **ALMOST** a rational fraction of the scopes sample rate). The cure is to place a resampler between the ADC and the screen display (in the digital domain) so that rather then joining the dots you are doing reconstruction, but that takes real processing power in the scope, so guess what gets dropped to hit a price point?The other nasty digital scope trap is that they usually do not have anti alias filtering appropriate to the sample rate, especially when running at lower sample rates, you would think that always running the ADC flat out and doing the decimation in the digital domain would make this if not 'easy' (Nothing is at multi GHz rates) at least a reasonable thing to do.You got to understand your instruments! And sometimes that old 465B really is the right tool for the job.Regards, Dan.

author: voltz
author_rank: Frequent Contributor
timestamp: August 29, 2017, 11:32:45 am
content: Quote from: mauri on August 28, 2017, 11:22:08 pmYup, this confirms it. There is no modulation taking place (as thought).. This is purely caused by scope artifact.

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2017, 07:52:01 pm
content: Quote from: dmills on August 29, 2017, 09:31:25 amThis brings up a good point. The sin(x)/x reconstruction filter should produce the correct result and it is not computationally difficult however the sin(x)/x reconstruction filter used in the Rigol DS1000Z series is known to be faulty producing a reconstruction which does *not* encompass the sample points. Maybe that is what is causing the flawed display.QuoteThe anti-aliasing filter if present should be a linear. The waveform being displayed in this case shows AM modulation which no filter should be producing.

author: mauri
author_rank: Newbie
timestamp: August 29, 2017, 11:16:21 pm
content: RF people,here we are again with my homework@David Hess :I set the infinite persistence as suggested, pictures in attachments.I didn't find an envelope mode in my scope though, so I have also triedpeak mode (also attached).@Ian.M :ok, I built the first RF probe described in the document youkindly suggested. I haven't got a Ge diode so I used anordinary Si one, after removing the clipping from theoscillator its swing is large and we shouldn't have anyproblem in detecting its signal.I first hooked the RF probe to my receiver when listeningto a 1kHz beat whistle, to check if an AF-like signal getsthrough it. The 1kHz signal does show up in the scope.Then I hooked the RF probe to the oscillator, and the scopedid show a large DC, which seems to be quite flat with alltimebases (there is a small ripple in the DC, but aftermagnifying it looks like the 4MHz signal itself, it's notthe AF-like thing we're looking for).@Kalvin :I have tried the loop alone as suggested, hovering over theoscillator without touching it.It is confirmed that the ghost modulation does appear in thesame way as when it was connected directly.@dmills :according to the scope manual there is a sin(x)/x interpolation,which is On by default, but it is active only at 50nS timebase andfaster. I have tried to set it on and off while the timebasewas 50nS and 20nS but I didn't notice any difference at all.

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2017, 11:32:33 pm
content: If you get a SOT23 dual series Schottky diode, you can make a very nice version of the second one. Ideally use a RF Schottky diode, but an ordinary small signal one will work. No PCB layout - scratch build it on a scrap of double sided copperclad with isolation gaps cut where you need them, insulate it with Kapton tape then cover it with copper foil soldered to the backside ground plane to form a screening can.This won't be only time you need a sanity check on your scope!

author: albert22
author_rank: Regular Contributor
timestamp: August 30, 2017, 02:08:33 pm
content: This video may help with aliasinghttps://www.tek.com/support/faqs/what-aliasing-and-how-do-i-detect-it-and-fix-it-my-oscilloscope

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2017, 02:58:25 pm
content: Quote from: mauri on August 29, 2017, 11:16:21 pmYour photographs seem to show the normal peak-to-peak high frequency noise that infinite persistence, envelope, or peak detection will pick up.So I think you have just been bitten by the DSO or perhaps Rigol DSO reconstruction bug; for whatever reason, the sample rate is insufficient to reconstruct the signal and you are instead seeing aliasing which is what it looks like.

author: mauri
author_rank: Newbie
timestamp: August 31, 2017, 12:12:36 am
content: RF commoners,after reading your comments about aliasing I searched thescope manual and it seems that the sampling rate decreasesas the timebase increases, while in a different mode called 'long-mem'it decreases much less.After doing some more tests, I found out that the snake-likepatterns you identified as aliasing starts appearing at 20us timebase(which should correspond to 10MSa) while with such 'long-mem' onit starts appearing at 1mS timebase (which should correspond to 20MSa).So the artifact onset seems indeed due to the sampling rate.Such long-mem mode is less affected, and it's not quite clear to mewhile it is not the scope default mode, I suppose there is some drawbackaround. As I understand based on your wise comments, with such cheapinstruments the choice is just as having the damn storage oscilloscopeto bite you on the left or right buttock.I also found out that setting Peak Detect the artifact vanishesentirely, at all timebases (irrelevant of long-mem or not).Actually, the manual says "To Avoid signal aliasing, select Peak DetectAcquisition". That seems to be ideal for analyzing a RF with longtimebases, which is exactly what I was trying to do when I bumpedinto the artifact, I was looking for the rise and decay time of mytx output. But once again I still have to learn what the drawbacksare with this mode, as dmills rightly pointed out I got to knowmy instruments

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2017, 02:29:45 pm
content: Quote from: mauri on August 31, 2017, 12:12:36 amIt is a performance issue because processing long records takes longer so fewer records will be processes in a given amount of time.QuoteAny DSO may suffer from this problem under the wrong circumstances and except for the sin(x)/x reconstruction issue on the Rigols, they are not any worse than most DSOs.Back in the late 1980s, Tektronix came up for a way for a DSO to detect if aliasing was likely present; just compare the trigger and sample frequencies. And it worked! Unfortunately, it would have been used by marketing of the other oscilloscope companies to denigrate their products so the idea was dropped. "Look at how bad Tektronix DSOs are! They even tell you when aliasing is present!" Marketing triumphs over engineering every time.QuoteNot all DSOs support peak detection but it is my first choice for some measurements and if I want a sanity check. DSOs without it can still check for aliasing using persistence or envelope detection.

author: mauri
author_rank: Newbie
timestamp: September 01, 2017, 12:08:06 pm
content: Quote from: David Hess on August 31, 2017, 02:29:45 pmso there is some workaround in the end.Now that the picture is getting clearer to me, I can see the beauty of the trap I have fallen inall that looks now like the most classic example of sync. sampling and aliasing as depicted in the literature,everything matches perfectly.In a sense, it has been a fortune to bump into this problem. You know, the very few things I know I have learnedbecause something went wrongIt has also been a fortune to read all the comments posted in this thread, I have read and appreciate each of them.