Triggering scope on a 40MHz signal modulated with a square wave

[petert]

Hello,

I am using a Rigol DS1054Z and feed it a 40MHz signal from a signal generator (SDG 2042X), that is modulated with a square wave.
The square wave consists of a sequence of four long pulses followed by several short pulses.
(See attached screenshot of the scope.)

I'd like to trigger at the beginning of the four long pulse sequence. For that I used edge triggering and a hold off of 8ms, which is the length of the 4 long pulses. But that is not enough to properly trigger, since it will also trigger at the beginning of a longer sequence of short pulses.

I read through all the documentation of the DS1054Z triggering functions, but found no match. There are a lot of triggering functions that handle square waves, but since mine is not "pure", but modulated onto a ~40MHz carrier, they will detect many rising/falling edges and wont work.

Is there a solution?

Edit: Attached a scope screenshot of the triggering I want to achieve while avoiding the one in the first screenshot.

[tautech]

Triggering on modulated waveforms is nearly always best near the peak of amplitude.

[Fungus]

I am using a Rigol DS1054Z and feed it a 40MHz signal from a signal generator (SDG 2042X), that is modulated with a square wave.
The square wave consists of a sequence of four long pulses followed by several short pulses.

If the number of short pulses is fixed then the holdoff needs to be the time of long+short.

[tom66]

If your DS1054Z has the enhanced trigger package, you may be able to use the timeout trigger.

Another option is to pass it through an external envelope circuit and trigger on pulse width.

Difficult problem!

[petert]

Triggering on modulated waveforms is nearly always best near the peak of amplitude.

Tried that, but not sure why it would matter? It does trigger on long and short pulses, just not on the sequence of 4 long pulses in its entirety, if that makes sense.

I am using a Rigol DS1054Z and feed it a 40MHz signal from a signal generator (SDG 2042X), that is modulated with a square wave.
The square wave consists of a sequence of four long pulses followed by several short pulses.

If the number of short pulses is fixed then the holdoff needs to be the time of long+short.

The sequence of short pulses vary, only the long pulses are a fixed count. That's why I don't want to include the short pulses in the holdoff.

[petert]

If your DS1054Z has the enhanced trigger package, you may be able to use the timeout trigger.

I have the fully unlocked DS1054Z. What would you set TimeOut to for it to work?

The documentation (page 102) says:

Trigger when the time interval (△T) from when the rising edge (or falling edge) of the input signal passes through the trigger level to when the neighbouring falling edge (or   rising edge) passes through the trigger level is greater than the timeout time set, as shown in the figure below.

Since it has to be a neighbouring edge, I assume it might be a problem.

[TurboTom]

The only trigger option that i can think of that may get you half-way there is the Nth edge trigger. Set the idle time to slightly less then the pause between the pulses and the edge count a little higher than the number of carrier periods in the short modulation packets. This should permit a stable trigger on somewhere amidst of the long packets. With the right time base setting, you may be able to achieve a stable display. Yet, no way to trigger on the start of the long packet that I could imagine.

[Fungus]

The sequence of short pulses vary, only the long pulses are a fixed count. That's why I don't want to include the short pulses in the holdoff.

If there's absolutely nothing to distinguish the start of a long packet from the start of a short packet then there's nothing you can do.

Can the signal generator output a trigger signal on the other channel?

[Mechatrommer]

have you tried pulse triggering (low pulse with duration longer than 200us, set trigger timeout accordingly. you wont get perfect trigger for the long pulses everytime, but should increase your chance of getting one. what do you want actually? analyzing whats inside the long pulses? you want to zoom in because you want maximized sampling rate? or what? because your screenshots show you got the long pulses already so whats the problem? for special task, say i want to maximize to 1Gsa/s and full 24Mpts capture, analyzing every bits of it i will download to PC using my VisaDSO and do post-analysis or post-processing.

[petert]

Thanks for all the replies. Yes I wanted to analyze the length of short pulse sequences following the long ones. But also know if there was an option in general I was missing.

Apparently, not.

I assume recording a longer stream then processing it on a PC is the easiest option.

Would be cool if you could define your own trigger algorithm, but I assume it's done in the FPGA.

[Fungus]

...I assume it's done in the FPGA.

Yep.

[JohnPi]

If you run the signal through an R.C filter, you can easily remove the 40 MHz component, leaving the modulation envelope -- then you can trigger on the 1st 'wide' pulse.

[vk6zgo]

If you run the signal through an R.C filter, you can easily remove the 40 MHz component, leaving the modulation envelope -- then you can trigger on the 1st 'wide' pulse.

As far as an RC filter is concerned, it is just a high frequency signal, & the modulation envelope has no effect.
You need to demodulate it first.

[rf-loop]

If you run the signal through an R.C filter, you can easily remove the 40 MHz component, leaving the modulation envelope -- then you can trigger on the 1st 'wide' pulse.

Envelope is only in your eyes when you look signal on screen.

There is only pulse modulated 40MHz in this case here. If you attenuate this 40MHz there is - "nearly nothing".




Same if there is 40MHz AM modulated say with 1kHz. Scope screen you may see nice envelope, this "classic modulation image".
Amplitude of 40MHz is changing.
But there is not this 1kHz even when your eyes see detect its shape on scope screen.

It is perhaps more easy or other way to understand it if you look it in frequency domain instead of scope time domain.

Look it with spectrum analyzer. What you see. 1kHz and 40MHz... really. No not at all.
You see 39.999MHz signal, 40MHz(carrier) signal and 40.001MHz signal.
What you think you see if you set some low pass filter for remove this 40M rf.
Nothing, or more or less attenuated these same rf signals.

After modulating for get back this modulation signal you need demodulate. After demodulation you get back this 1kHz.

Simple AM demodulator is so simple as one diode + low pass and this scope trigger engine do not have it.

[TurboTom]

Even with a demodulator or "math trigger" (square, low path, trigger), it wouldn't be possible to trigger reliably on (or close to) the beginning of the first long packet. It would be good to have something like this anyway... 

[Mechatrommer]

Envelope is only in your eyes when you look signal on screen.
There is only pulse modulated 40MHz in this case here. If you attenuate this 40MHz there is - "nearly nothing".

i think he's talking simple rc demodulator (see attached, just as sample 1-2MHz LPF on 40MHz carrier) which is imho doable using pulse length trigger. another simple demodulator..
https://www.multisim.com/content/zCXpWsqBjUaFwxZR8Wtz5X/diode-rc-am-demodulator/
https://www.allaboutcircuits.com/textbook/radio-frequency-analysis-design/radio-frequency-demodulation/how-to-demodulate-an-am-waveform/

[MarkL]

You could also use a spectrum analyzer in zero-span mode to do the demodulation and let the scope do the pulse length trigger on the output.