Sine wave gen's output constantly going off phase?

Thread Starter

WineJ

Joined Aug 23, 2015
16
I hope I can get the answer to my question. Using Tektronix THS720A I was checking two identical 160 kHz outputs of two identical controllers. With all settings on O-scope being the same, I have on the screen a steady sign wave from controller A. When I check output from controller B, the same exactly sine wave is constantly "running" throughout the screen (sort of on the old analog O-scopes, where you have to use time/div controls to make it steady). I tried to use time/div adjustment of THS720A to "froze" it, but the only effect is that the wave shape is being stretched or compressed. The only way to capture it steady for controller B is to use RUN/HOLD control in HOLD mode.
Both wave shapes are exactly the same (an approximated sine wave). Both controller A and B were checked with their respective loads disconnected. Would it be 160 kHz generating circuit of controller B a potential problem? It appears as the sine wave is being produced, then there is an instant interruption, and when new wave starts going out again, it's thrown of phase; then same thing happens over and over again, keeping the wave shape on the screen constantly "running"?
 

Thread Starter

WineJ

Joined Aug 23, 2015
16
Unfortunately I don't have neither the # nor schematic for 160 kHz generating modules; they are more like sealed units. Two 160 kHz units are totally independent from each other and are apart from each other, so I took reading on A, then on B. All the same O-scope settings, the same channel 1, yet one wave sits on the screen steady and another is constantly moving. It got to be something within the wave generating circuit, which makes difference on the O-scope screen for unit B. A soon as I get a replacement, I will swap the unit B, but my question is rather theoretical: would it be some sort of brief interruptions in the 160 kHz output, which makes the sine wave to constantly "run" across the screen?
 

AlbertHall

Joined Jun 4, 2014
11,505
The signal generator should not be able to cause this effect on the 'scope screen. The scope trigger should keep the waveform start point steady on the screen even if the frequency changes. Can you keep the waveform steady by adjusting the trigger controls?
One possible cause is high frequency noise on the generator output which might confuse the 'scope trigger circuitry.
 

DickCappels

Joined Aug 21, 2008
7,722
For the phase between two sine wave generators not to slip in phase with respect to each other their outputs much be identical at all times. The only way to achieve that is to lock one generator to the other or supply a single clock to both generators. There are other schemes but they amount to locking one to another.
 

AlbertHall

Joined Jun 4, 2014
11,505
For the phase between two sine wave generators not to slip in phase with respect to each other their outputs much be identical at all times. The only way to achieve that is to lock one generator to the other or supply a single clock to both generators. There are other schemes but they amount to locking one to another.
Yes, if the two signals are being displayed simultaneously but I got the idea from the original post that the two signals were being applied to the same 'scope channel.
Can TS please clarify this.
 

BobTPH

Joined Jun 5, 2013
3,667
My guess is that the outputs are of slightly different magnitude and the trigger is set near the top of the higher one.

Bob
 

Thread Starter

WineJ

Joined Aug 23, 2015
16
In case "TS" is not a familiar term, AlbertHall is asking WineJ to clarity how the two signal were being applies.
The two signal generators are identical but each one works for its own load. The loads are also identical but each system is independent from another. First I have checked the output of sig generator A (with the load connected, then disconnecting it). Then I walked to another control cabinet and checked sig. generator B (with and without load connected to it).
I used the same O-scope, with all the same settings, the same Channel 1.

BobTPH, yes you are right, Output A was 19 Vpp, output B was 9 Vpp. This difference is not critical (per tech manual it can be adjusted from 8 to 30 Vpp)
But why this would make a difference on the o-scope's screen? When I saw the running wave on B, I tried to fiddle with trigger control but it didn't do anything. I will read the O-scope manual trigger section; perhaps you are right. Also, Albert mentioned the dirty signal, so I have to check on that, too.
My original thought was that the sine train (should I use "train" for sine wave?) gets constantly interrupted due to the sig. gen malfunctioning.
 

LesJones

Joined Jan 8, 2017
3,497
If the frequency of the two generators is not EXACTLY the same they will drift in phase. If you check their specifications the frequency accuracy should be specified. You can work out the difference in frequency by measuring the time it takes for the waveform of the unsynchronised waveform to move one cycle with respect to the synchronised one. From that you can see if their frequency difference is greater than it should be if one is on the low side of the specification and the other is on the high side of the specification.

Les.
 

andrewmm

Joined Feb 25, 2011
1,757
Question,
What would you see if one was set at 160.0000 Khz and the other at 160.0001 KHz ?

I dont know wat the spec of each of the sources is , but I bet its worse than that ,
and I 'd say you will see just what you do,

If they are digital based gens, then its all down to the accuracy / stability of the master reference inside the gens.


Now,
if the generators are "digital based", you might have on the back a sync out and sync in BNC.
Nominally 10 MHz.

If you have, and you connect them together, one as master one as slave, and you select them on the front panel, then both gens will have identical master reference, and as they are digital, the output will not drift on to the other.
 

Thread Starter

WineJ

Joined Aug 23, 2015
16
Why would you expect two signal generators to have the same phase when you use one of them to trigger the scope.
This is not exactly the case. I can check 115 VAC outlet at 60.1 Hz and observe a steady sine wave on THS720A screen. Then, if I drive some 15 miles away and do the same on another receptacle (say, it has a different power source, like EDG), I may observe a slightly different sine wave, maybe 117 VAC at 60.3 Hz. The O-scope, provided I still have all the same settings upon turning it on again, would capture it in the same way, and I don't have to press HOLD to "freeze" running sine wave. Time/div, and trigger control don't do magic, the wave is constantly running, and the only way to stabilize it is to press HOLD/RUN button.
 

BobTPH

Joined Jun 5, 2013
3,667
The trigger level controls at what voltage the left side of the trace starts. If the trigger level is set too high, it will not synchronize and you get a running wave. You should be able to stabilize the trace by lowering the trigger voltage. On a digital scope, the level is shown by a dotted line and once it enters the waveform, it should stabilize. DSO's can be a bitch to use, there are so many settings. You need to look at the trigger settings and mess with them until you get it to work.

Bob
 

LesJones

Joined Jan 8, 2017
3,497
Re post #13 In both cases you are triggering on the waveform you are viewing. When you are viewing you two nominally 160 Khz waveforms you are only triggering from one of them. So let's assume you are triggering on channel 1. The displayed channel 1 waveform will be steady but channel 2 will drift, (Because you are NOT triggering on channel 2) If you now change the trigger source setting to channel 2 you will find channel 2 is steady but channel 1 is drifting. The difference will be if channel two was drifting right channel 1 will now drift to the left.

Les.
 

BobaMosfet

Joined Jul 1, 2009
1,850
I hope I can get the answer to my question. Using Tektronix THS720A I was checking two identical 160 kHz outputs of two identical controllers. With all settings on O-scope being the same, I have on the screen a steady sign wave from controller A. When I check output from controller B, the same exactly sine wave is constantly "running" throughout the screen (sort of on the old analog O-scopes, where you have to use time/div controls to make it steady). I tried to use time/div adjustment of THS720A to "froze" it, but the only effect is that the wave shape is being stretched or compressed. The only way to capture it steady for controller B is to use RUN/HOLD control in HOLD mode.
Both wave shapes are exactly the same (an approximated sine wave). Both controller A and B were checked with their respective loads disconnected. Would it be 160 kHz generating circuit of controller B a potential problem? It appears as the sine wave is being produced, then there is an instant interruption, and when new wave starts going out again, it's thrown of phase; then same thing happens over and over again, keeping the wave shape on the screen constantly "running"?
What you are referring to is called 'drift'. One signal generated is more accurate than the other- period. The signal that keeps drifting in the non-triggered state is 'off' some small fraction, which is why you don't see it staying in place, non-triggered. The o-Scope is doing its job. If it displays a frequency value on the screen, it will not be able to show you enough resolution (digits/precision) to show you the error that is there- it could be 10 digits to the right of the decimal, for example.

Normally, signal generators need calibrated against a 'Reference'. It seems to me that your 'good' generator is 'reference grade', and your 'bad' generator has slipped with age. Either an oscillator or some other aspect of internal circuits have degraded slightly.

There should be a means of calibration of your 'bad' generator, and perhaps you can use your good generator as the reference to bring it back to where you want it.
 

LesJones

Joined Jan 8, 2017
3,497
I also missed that point. The only thing I can think of now is trying hf reject or noise filter on the triggering. (If the scope has those settings. I think now there is a problem with one of the signal sources.

Les.
 
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