Electronics > Beginners
Triggering on AM modulated signal
aneevuser:
I have a DS1054Z and an FY6800 sig. gen.
I'm trying and failing to get the scope to trigger reliably on an AM modulated sine wave, in such a way as to let me see the modulation envelope. It seems that the problem is that neither an edge nor a slope trigger has fine enough resolution to distinguish between an edge that occurs when the envelope is large and one when it is small.
Occasionally I've been able to get a more-or-less stable display, by playing around with various combinations of carrier and modulation frequencies, but this is definitely more luck than judgement (I think it may be most favourable when the carrier freq. is some nice multiple of the modulation freq. or something like that). The most useful approach so far has been to simply pause the scope and look at what's in the memory, but I'd really prefer a real time display.
So, two questions:
1) Does anyone have any tips as to how to do this, in particular with a DS1054Z?
2) Is this a problem specific to digital scopes? I vaguely seem to recall looking at such signals without problem on analogue scopes but that was many years ago so my memory may be faulty - maybe it was a hassle then too.
capt bullshot:
As you said, it's a hassle in general. With an analog scope, one tries to set the trigger to a as high as possible level so the scope triggers on a modulation signal maximum level.
Way more simple, if you have access to the baseband signal, is to trigger on the demodulated signal using another channel or ext. trigger input.
Ian.M:
.... and if you don't have the baseband signal available its time to build an AM demodulating RF probe. Use the classic diode charge pump circuit with a BAT54S for the pair of diodes, 22pF input capacitor, around 200pF for the smoothing capacitor (including capacitance of the coax to the scope) and a 100K load resistor. Keep the connections to the common pin of the BAT54S and the 22pF capacitor as short as possible. It may be worth connecting the tip to the 22pF capacitor va an individual pin of a turned pin socket so you can insert a piece of stiff wire for low level signals or a resistor to attenuate high level signals and reduce loading on the D.U.T. The ground lead at the D.U.T. should be very short - its probably best to use a springy sharp solid wire rather than a flexible lead+ clip.
aneevuser:
--- Quote from: capt bullshot on November 28, 2018, 09:31:34 am ---Way more simple, if you have access to the baseband signal, is to trigger on the demodulated signal using another channel or ext. trigger input.
--- End quote ---
Right. This is blindingly obvious in retrospect, and works perfectly, within the slightly clunky limits of the UI of the FY6800.
Thanks.
For those who are interested:
The FY6800 has two o/p channels, and allows modulation of CH1 by CH2 (or by an external signal). So you can use CH1 as carrier and CH2 as baseband.
However, you can only adjust the modulation index in real-time while seeing the changes on the scope; if you want to change either CH1 or CH2 freq., the device stops generating the modulated signal, but reverts to the control page for CH1 or CH2 - this means that you have to change those frequencies blindly, and go back to the modulation page to see the effect.
[edit: oops! I got this wrong - the modulation page in fact allows you to modify the freq. or amplitude of the CH1 signal - you can't however modify the CH2 signal on that page, AFAICS)]
I assume the fancy Rigol and Siglent generators are more flexible that way, but for the price of the FY6800, I wouldn't want to complain (too much...)
aneevuser:
--- Quote from: Ian.M on November 28, 2018, 09:52:34 am ---.... and if you don't have the baseband signal available its time to build an AM demodulating RF probe.
--- End quote ---
Interesting suggestion, which I hadn't considered at all.
In fact, one of my reasons for playing around with modulated signals is to try to overcome my previous negative experiences of RF electronics (in brief: nothing I made ever worked properly, AFAIR), so this is something that I will definitely be trying at some point, regardless of the fact that the "trigger on CH2" technique works fine.
Navigation
[0] Message Index
[#] Next page
Go to full version