# Question about Oscilloscope Trigger: 3 Signals, only 2 are moving or v.v.

#### Boplogz

Joined Oct 17, 2022
22
Hi everyone,

Im a student and currently working on our prelim requirement. Im using 4 channels digital oscilloscope and 2 function generators, and I need to output 3 signals with their phase difference coming from the function generators. Two of the signals are from the 1st function generator, and the last is from the 2nd function generator. They are all in 50khz.

My question is, why does the 1 signal constantly move even though I set the trigger to 0v? And if Change the trigger channel, the 2 signals move while the other is not moving.

Is there any problem with my settings?

#### WBahn

Joined Mar 31, 2012
29,828
What is the likelihood that the signals from your two function generators are EXACTLY the same frequency?

#### Boplogz

Joined Oct 17, 2022
22
What is the likelihood that the signals from your two function generators are EXACTLY the same frequency?
Hi WBahn, thank you for the response!

I was thinking of the frequency also, I was just assuming that if I set them both for 50khz, that's okay.

But looking at the oscilloscope reading, they have this slight frequency difference.

Does this mean that this is the reason for this? Because they have different actual frequencies (from the function generator) - they not moving simultaneously?

#### WBahn

Joined Mar 31, 2012
29,828
Hi WBahn, thank you for the response!

I was thinking of the frequency also, I was just assuming that if I set them both for 50khz, that's okay.

But looking at the oscilloscope reading, they have this slight frequency difference.

Does this mean that this is the reason for this? Because they have different actual frequencies (from the function generator) - they not moving simultaneously?
Yep.

For instance, let's say that one is at 50.000 kHz and the other is at 50.001 kHz. That's pretty darn close to the same frequency. But if you are locked onto one of them, the other will drift at the rate of one cycle per second, which is very easily observable on a scope. Even an order of magnitude closer and you will still see a drift of one cycle every ten seconds, which is readily apparent.

#### Boplogz

Joined Oct 17, 2022
22
Yep.

For instance, let's say that one is at 50.000 kHz and the other is at 50.001 kHz. That's pretty darn close to the same frequency. But if you are locked onto one of them, the other will drift at the rate of one cycle per second, which is very easily observable on a scope. Even an order of magnitude closer and you will still see a drift of one cycle every ten seconds, which is readily apparent.
Thank you so much for giving your answer. Now I know why the 3 signals are not triggering simultaneously.

Now, Im thinking about how I can make these 3 signals be triggered simultaneously. By the way, is this possible?

#### WBahn

Joined Mar 31, 2012
29,828
Thank you so much for giving your answer. Now I know why the 3 signals are not triggering simultaneously.

Now, Im thinking about how I can make these 3 signals be triggered simultaneously. By the way, is this possible?
What to you mean by "triggered simultaneously"?

When the trigger conditions that you have set up are met, ALL of the input channels on the scope are triggered at the same time. How is that not simultaneous triggering?

#### crutschow

Joined Mar 14, 2008
33,969
Is there a sync input on either of the generators?

#### Boplogz

Joined Oct 17, 2022
22
What to you mean by "triggered simultaneously"?

When the trigger conditions that you have set up are met, ALL of the input channels on the scope are triggered at the same time. How is that not simultaneous triggering?
My apology, the thing is, Im still talking about the source of the 3 signals in two generators. When I referred to triggering, I was referring to the moving signals. Sorry if I cannot describe it properly.

#### WBahn

Joined Mar 31, 2012
29,828
My apology, the thing is, Im still talking about the source of the 3 signals in two generators. When I referred to triggering, I was referring to the moving signals. Sorry if I cannot describe it properly.
So some signal generators have a sync, or trigger, input that will cause them to start generating the output at a particular time. But that only means that they will start off synchronized. If they are not set to exactly the same frequency (which is pretty much impossible), then they will drive from that point on. By resyncing them fast enough, you can keep the drift from getting too much (with "too much" being defined by your needs).

#### Boplogz

Joined Oct 17, 2022
22
So some signal generators have a sync, or trigger, input that will cause them to start generating the output at a particular time. But that only means that they will start off synchronized. If they are not set to exactly the same frequency (which is pretty much impossible), then they will drive from that point on. By resyncing them fast enough, you can keep the drift from getting too much (with "too much" being defined by your needs).
This is the sample output of the 2 signals from the two function generators. I cannot make them move at the same time.

#### WBahn

Joined Mar 31, 2012
29,828
This is the sample output of the 2 signals from the two function generators. I cannot make them move at the same time.
It appears that they are drifting one cycle in about five seconds, so that means that the two frequencies are the same within about 0.2 Hz. If these are 50 kHz signals, that means that they match within about 2 ppm of the average, which is pretty damn good. Do either of them have a fine-adjust on the frequency control? If so, you MIGHT be able to get them better, but it won't last for long as both generators will have a temperature dependence to their output frequency. In fact, if you CAN get them to the same frequency, you may well find that you can control the drift, back and forth, by blowing on one of them or covering up the chassis vents.

To do much better, you need to be able to trigger one of them with the other (or trigger both of them from the same signal). Some function generators have this ability and some don't.

#### MisterBill2

Joined Jan 23, 2018
17,729
To display all of the signals in a stable manner you would need a multiple-sweep oscilloscope, that can provide a different sweep speed for each displayed trace. Not many models like that are available, and they cost a lot more.

The previous comments about frequencies are correct. If they are not exactly the same frequency, they will be different! and the non-synchronized trace display will move.

#### crutschow

Joined Mar 14, 2008
33,969
What is the make and model of the function generators?

#### Boplogz

Joined Oct 17, 2022
22
Hi everyone,

Apology for super late reply on the thread I was on my province and internet is not good.

I was able to synch all 3 signals using the Burst Mode on the function gen, where the trigger of the 2nd function gen comes from the 1 function gen.

Unfortunately it gives rise to another problem on the circuit I'm testing (breadboard and function gen) (see image below, as the schematic)

Whenever I connected the 3rd signal to the IRF520, the voltage drop on the external power supply drops to 0 and my connecting wires in the output of the IRF520 are starting to smoke and melt. Im not sure whats the problem. I thinking prolly of the high voltage being amplified.

#### WBahn

Joined Mar 31, 2012
29,828
What would you expect would happen if you replaced the MOSFET with a switch and closed it, thereby placing 18 V DC across your 220 mH inductor?

#### MisterBill2

Joined Jan 23, 2018
17,729
"Voltage" is not being amplified, but instead a transistor is being switched into full conduction.
And now a question that MUST be answered: What is the purpose of this circuit??? It is not a regular summing circuit, and it is not a mixing circuit, and with a normal scope it is not required as an amplifier circuit.
So please explain the intended purpose of the circuit shown.

#### Boplogz

Joined Oct 17, 2022
22
What would you expect would happen if you replaced the MOSFET with a switch and closed it, thereby placing 18 V DC across your 220 mH inductor?
It store the energy again and again, and build up increasing voltage output?

"Voltage" is not being amplified, but instead a transistor is being switched into full conduction.
And now a question that MUST be answered: What is the purpose of this circuit??? It is not a regular summing circuit, and it is not a mixing circuit, and with a normal scope it is not required as an amplifier circuit.
So please explain the intended purpose of the circuit shown.
We have mist generator thats needs to be driven in sequence. The driver has a phase difference of 60, 120 upon start.

I wonder also why it does not burn the circuit when my function gen is not configured into burst mode.

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#### WBahn

Joined Mar 31, 2012
29,828
You appear to have some fundamental misunderstandings of basic circuit elements and their behavior.

An inductor, when you apply a DC voltage to it, looks like a short circuit (or the resistance of the windings that make it up), so you will get a very high current from your supply with the typical result being a lot of heat and burned up wires and/or components.

#### MisterBill2

Joined Jan 23, 2018
17,729
It store the energy again and again, and build up increasing voltage output?

We have mist generator thats needs to be driven in sequence. The driver has a phase difference of 60, 120 upon start.

I wonder also why it does not burn the circuit when my function gen is not configured into burst mode.
The application of the circuit would have been very helpful in suggesting a method of producing the driver signal for said "Mist Generator." And certainly in common application it would not be driven by three function generators.
Presenting the requirements for an application instead of asking for why a scope display is not as wanted is not likely to get you any useful assistance.
What sort of three phase circuit is being driven? What are the required voltages, and what is the driving power requirement?? Is this a mechanical device being driven, or is it a piezoelectric device?

#### Boplogz

Joined Oct 17, 2022
22
You appear to have some fundamental misunderstandings of basic circuit elements and their behavior.

An inductor, when you apply a DC voltage to it, looks like a short circuit (or the resistance of the windings that make it up), so you will get a very high current from your supply with the typical result being a lot of heat and burned up wires and/or components.
I apology for my ignorance, but you are right, I did not think of that. Now, I think I've should look for a way how to prevent this instead.

The application of the circuit would have been very helpful in suggesting a method of producing the driver signal for said "Mist Generator." And certainly in common application it would not be driven by three function generators.
Presenting the requirements for an application instead of asking for why a scope display is not as wanted is not likely to get you any useful assistance.
What sort of three phase circuit is being driven? What are the required voltages, and what is the driving power requirement?? Is this a mechanical device being driven, or is it a piezoelectric device?
Thank you for your patience, I tried to understand it part by part before implementing it. Actually I have some separate threads on generating the main sine wave and was planning to connect to an RC shift to achieve the requirements.

The entire circuit was used to drive a piezo atomizer to generate a mist. The requirement is that the input to the 3 piezo should be a sine wave with a phase difference of 0, 60, and 120. Vpp is 9v, and the frequency is 110khz. The mist was designed in such a way that it could maintain the mist with a particular movement in the area.

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