Quick question on setting duty cycle and PRF with generator using Period, No. of cycles etc.

Thread Starter

RichW

Joined Nov 8, 2019
40
Hi all,

I was wondering if anyone here could please help me do a quick conversion, so I can figure out how to set the right duty cycle and pulse repetition frequency on my generator. I've watched a few videos, but they just confuse me, and I keep going around in circles. I'm really looking to put things in terms of formulas that I can just use, then figure out the relationships by repeated use. I'm probably overcomplicating things.

Anyway, on my function generator I can set Period, Cycles, and Burst Period.

1. How would I set, for example, 30% duty cycle (30% on, 70% off) and 100 Hz pulse repetition frequency (100 pulses per second).

2. What if I want to fix the pulse on time at say 12 ms, but then vary the pulse off time between 90 and 500 ms?

For 1. I can set the burst period, but since the frequency is set, then doesn't that fix the number of cycles in that burst period? So how do I set the "off" time to 70%? Also, if I have 100 pulses per second then is that 100 bursts per second?

Getting a little confused here, any simple formula / example calculation would really be appreciated.
 

Thread Starter

RichW

Joined Nov 8, 2019
40
Hi Eric,

Thanks very much, I am trying to understand your comment and diagrams. I think they are a little beyond me at present. Are you saying to simply measure from the oscilloscope screen? I think I did not explain very well, unless I missed something here. Please see below photo, the squred red values are the ones I can change to obtain my required duty cycle and PRF. But I am unsure how to calculate the values to set.

Picture2.png
 

Thread Starter

RichW

Joined Nov 8, 2019
40
hi Rich,
I read your post as you were wanting to build a generator, sorry about that.:(

Do you have the manual, Page 145.?
E
View attachment 260783
No problem at all, I appreciate any help to learn this. How does PM relate to duty cycle and PRF? There is another generator that I could use that has a duty cycle option, but in the event I can't use it, I need to know how to set the duty cycle and PRF using the Period, Cycles, and Burst Period settings. Sorry if I missed your point and thanks again.
 

Ya’akov

Joined Jan 27, 2019
5,660
No problem at all, I appreciate any help to learn this. How does PM relate to duty cycle and PRF? There is another generator that I could use that has a duty cycle option, but in the event I can't use it, I need to know how to set the duty cycle and PRF using the Period, Cycles, and Burst Period settings. Sorry if I missed your point and thanks again.
If that is a Siglent, which it certainly looks like, then look at the soft button in the red box below. It is on the first Pulse waveform setting screen.

1645040143717.png
 

Thread Starter

RichW

Joined Nov 8, 2019
40
If that is a Siglent, which it certainly looks like, then look at the soft button in the red box below. It is on the first Pulse waveform setting screen.

Thanks Yaakov, I will take a look, perhaps I have missed the option somewhere.

So, it is not possible to determine a duty cycle and PRF using only the Period, Cycles, and Burst Period values?
 

Ya’akov

Joined Jan 27, 2019
5,660
Thanks Yaakov, I will take a look, perhaps I have missed the option somewhere.

So, it is not possible to determine a duty cycle and PRF using only the Period, Cycles, and Burst Period values?
First, forget burst, it's orthogonal to duty cycle. If you do ignore burst you should be able to derive the formula for duty cycle yourself if you think for a moment.

Both period and pulse width are times.
The duty cycle is the percentage of time a pulse waveform spends high, that is at the particular peak voltage of the signal.
The period is the time it takes for one cycle, so for example 500Hz is 2ms.
The pulse width is how long the signal is high, also in seconds

So if I wanted ti have a 50% duty cycle at 500Hz I would set the pulse with to 1ms, which is 50% of 2ms, which is 500Hz.

\[ period=\frac{1}{f}\therefore \frac{1}{500} = 0.002s = 2ms \]

So to calculate the period for a particular duty cycle it follows

\[ duty\ cycle\ seconds = \frac{duty\ cycle\ \%}{100}\times period \\ \frac{50}{100}\times 2ms = 1ms \]

i hope that's clear, it's trivial if you understand the meanings of period and pulse width.
 

Ya’akov

Joined Jan 27, 2019
5,660
Reading your original question, I see you were trying to use the burst mode to vary the period between fixed duty cycle waveforms. You can do that, but, I don't understand how you want to choose the repetition rate.

Basically, you have to have a trigger source for the burst that is independent of the channel generating the pulse waveform. You can use CH2 on the AWG, with sweep, to have a periodic variation for example.

What do you want to have controlling the repetition rate?
 

Thread Starter

RichW

Joined Nov 8, 2019
40
Reading your original question, I see you were trying to use the burst mode to vary the period between fixed duty cycle waveforms. You can do that, but, I don't understand how you want to choose the repetition rate.

Basically, you have to have a trigger source for the burst that is independent of the channel generating the pulse waveform. You can use CH2 on the AWG, with sweep, to have a periodic variation for example.

What do you want to have controlling the repetition rate?
Thanks for that Yaakov, it is extremely helpful and much easier to understand than some of the videos on Youtube.

I think I just posted that screenshot to identify the generator. I originally wanted to set 1.1 MHz with 10 cycles and 10 ms burst period. But I struggled to relate these settings to duty cycle. I triggered using the external trigger of the generator. The trigger did work, but for some reason it kept going, please see attached image. One day I will figure this out!

Many thanks again Yaakov, I'll work through your post.
 

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Thread Starter

RichW

Joined Nov 8, 2019
40
Ok, I'm nearly there with this, I think it's all the different terms for the same things thats confusing me. Anyway, I made this diagram:

Picture2.png

So let's just say that I have 1 MHz signal, 30% DC PRF=100Hz. Let's also say I want to find the number of cycles per burst. So for 1 MHz, I have period (T) of 1 ms. PRF is one hundred pulses / bursts per second. In that second, 30% of the time it is on and 70% of the time it is off. So how to figure out the number of cycles per burst / PW?
 

Ya’akov

Joined Jan 27, 2019
5,660
Ok, I'm nearly there with this, I think it's all the different terms for the same things thats confusing me. Anyway, I made this diagram:

View attachment 261203

So let's just say that I have 1 MHz signal, 30% DC PRF=100Hz. Let's also say I want to find the number of cycles per burst. So for 1 MHz, I have period (T) of 1 ms. PRF is one hundred pulses / bursts per second. In that second, 30% of the time it is on and 70% of the time it is off. So how to figure out the number of cycles per burst / PW?
I think your confusion lies in the meaning of burst . (and possibly what a PWM waveform looks is). A burst is a single or multiple repetitions of the waveform at some interval. The waveform, in your case, will be a 1MHz PWM signal with a 30% duty cycle. Assuming you choose just one repetition, then if you trigger it, there will be a 1/1,000,000 second waveform with 30% of that time on and the rest off.

You can't see the off, you would only know about it if another cycle of the waveform followed it. Then you wold be able to measure the start of the first cycle to the start of the second and see the timing.

Here are some screenshots. First is the 1MHz, 30% duy cycle PWM waveform. Between the cursors is one repetition. Ignoring the less than ideal rise and fall times, you can see that a PWM waveform is a square wave not a sine wave. The note the cursor measurements: period 1μs, frequency 1.0000MHz. This is the waveform that burst will produce.

SDS00004.png
Second is the result of using burst. I have set the burst to be triggered externally, NCycle with 1 cycle selected.In order to provide a useful display, the trigger (which is CH2 of the generator patched into the trigger input) is set to 1KHz rather the 1Hz but it works exactly the same way. Each of the purple lines is one of what is between the cursors above.

SDS00005.png
Here's a third, I hope to make it even more clear. In this one, the view below is zoomed into the first pulse on the top. The cursors are set to the distance between the two pulses. You can see the period is 1ms and the frequency 1kHz. If the trigger was 1Hz it would look exactly the same, except the distance between the pulses would be 1000 times larger. The pulse below is precisely the size of the pulses in the first image.

SDS00007.png
Finally, this is using burst with a sine wave. I've set the number of cycles for the burst to 10. This is still 1MHz, but I increased the trigger waveform to 17KHz to fit ore than one on the display. If you need this, then you don't want PWM.

SDS00008.png
One more thing, in case it is a cause of confusion, here is the setting for burst mode that you need to select. If you choose "infinite" the first trigger will leave it to free run. Choosing the number of cycles from 1 up, will decide how many repetitions of your wave form will happen on each trigger. The first is for pulse and the second for sine.

1645442641299.png1645442695092.png
 
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