EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: MrCRC on May 05, 2015, 11:17:38 pm
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Hi:
Sometimes I wish to trigger a scope when the period of a constant frequency signal deviates from it's proper value by some amount (1). Most scopes from Agilent, Tek, and LeCroy cannot do this, though I think some of the high end ones can do it using "virtual" triggering, ie. software search for events after acquiring. But this isn't satisfactory since, I am doing this at home where I have only an Agilent DSO-X3024A (and an older Tek DPO3014). Plus, I want to do it real time.
I'm quite surprised by the omission of a period trigger capability in these scopes. Yes, they have pulse width trigger and other modes, none of which equate to period triggering. There really should be a proper period/interval trigger that looks between edges 0 and N of the signal on one (or more) channel(s), and lets you select a condition such as <, >, or inside/outside of a time window.
Alas, I discovered last night that Picoscope does have this, tempting me to consider a 5000 series, since I also wouldn't mind a high resolution scope.
Also, it seems that Siglent has this. I haven't looked at Rigol yet.
I will probably try to persuade Agilent (Keysight), and maybe also Tek to add this feature. Agilent listened to me once before to add Relative Std. Dev., and trend plotting capabilities to the 3000X series. But I'm not sure if the people I was in contact with before are still available.
I also evaluated a Tek MDO3000 for a week, and came up with pages and pages of little deficiencies, bugs, and oversights. Most of that I sent to them, and they have turned some of my concerns into Engineering Action Items, or whatever they call them.
In the mean time, since I have a long list of other gripes about current Agilent, Tek, and LeCroy scopes, I'm really inclining toward making my next scope a Rigol, Siglent, or perhaps a PicoTech. I used to not like the idea of a USB scope, since I often need the high waveform update rate. But PicoTech has improved that aspect to "good enough."
NOTES:
1. At present, I am trying to figure out why a uC based digital clock (the kind that tells you the time) is running fast. It takes a 32768Hz signal from a TCXO in on a pin meant for a crystal, prescales it down to 128Hz interrupts on an ATmega8L T2 async. timer. In the ISR, it calls the function to update the time by one second every 128 events. It emits a high level diagnostic pulse on a GPIO pin when entering the T2 OVF ISR, and ends the pulse when finished. Thus, the high pulse width is variable. So I can't use pulse width triggering (on the low part) to approximate the effect of a period trigger.
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If I am understanding you correctly, it _appears_ that my Rigol DS1054z can do this, using the Trigger Type: Delay setting. The user is able to set two Source edges, which can be rising or falling edges on the same or different channels, and choose "delay type" from >,<,<>,>< . Then depending on the delay type chosen, you can set upper, lower, or upper and lower time limits.
I have just confirmed this using a constant-amplitude PLL signal whose frequency can vary slightly according to the distance of my hand wrt a resonating coil. The scope remains untriggered (in "wait" state) until the frequency changes enough so that the trigger criteria are met. It's a very sensitive "hand position" detector... :)
So this would appear to function as a sensitive "period" trigger like you are looking for.
I'm still exploring the capabilities of this scope. But it looks like this is a powerful feature that can indeed serve as a "period" trigger.
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Yeah, that's it!
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You could approximate this on the DSO-X if you have the mask test function. It's not a trigger condition, but you could get an indication if the rising edge was ever not in the expected place (and statistics on how often that happens).
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You could approximate this on the DSO-X if you have the mask test function. It's not a trigger condition, but you could get an indication if the rising edge was ever not in the expected place (and statistics on how often that happens).
Hmm, stats would be nice.
I'm going to try running the pulses into the trigger input of a 33520B, to generate constant width + pulses. Then I can trigger on - pulse width violations. It's a hack, but will work. Won't get me stats though. Maybe I could send Trigger_out to another scope on a slow time scale. In cases with higher frequencies and/or stricter jitter requirements, adding the extra pulse generator might not be appropriate.
So I'm still peeved that Keysight omits this capability on a mid-end or "high end economy" scope.
I'm going to either buy a Picoscope or Rigol next...
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You'd probably find that in many brands an enhanced Trigger suite is a purchasable option.
Yes the Siglent range has the capability you seek, for example the SDS2000 series also has a full decode trigger suite as standard, the decoding as the option not the triggering.
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You could approximate this on the DSO-X if you have the mask test function. It's not a trigger condition, but you could get an indication if the rising edge was ever not in the expected place (and statistics on how often that happens).
Hmm, stats would be nice.
I'm going to try running the pulses into the trigger input of a 33520B, to generate constant width + pulses. Then I can trigger on - pulse width violations. It's a hack, but will work. Won't get me stats though. Maybe I could send Trigger_out to another scope on a slow time scale. In cases with higher frequencies and/or stricter jitter requirements, adding the extra pulse generator might not be appropriate.
So I'm still peeved that Keysight omits this capability on a mid-end or "high end economy" scope.
I'm going to either buy a Picoscope or Rigol next...
You should send their support a question and ask for the feature to be added, they have been very responsive to requests. For now the mask testing is the only way to do it (and a paid option) by triggering; you can setup the mask to trigger a stop, or save the violating capture, or accumulate stats on the violations.
How infrequent is the violation? and what are you trying to measure? Segmented captures could let you capture a small and detailed window around each event but there is no automated search on the segments for secondary trigger conditions, or you could capture several events across the screen and use the measurement trend of period to quickly see the variations. The edge then edge trigger could trigger on a single side of the variations (longer periods).