Does anyone know anything about the HP 3456A External Trigger Latency? - Page 1
Forum section: electronics / testgear

author: montemcguire
author_rank: Regular Contributor
timestamp: December 18, 2015, 07:24:40 am
content: Hi Folks!Does anyone know the latency or timing jitter of the external (BNC) trigger signal sent to an HP 3456A, relative to when the actual measurement takes place?I'm trying to use an HP 3456A to make accurate AC gain measurements of a circuit using a synchronous AC voltage, rather than just using the standard AC voltage function with an unsynchronized, random AC test signal. Even with far more modern meters than the 3456A, the resolution and repeatability of the ACV function is poor, which I imagine to be because the AC stimulus is not synchronized to the meter's sampling interval. A 3458A or Fluke 8508A would be nice, but I'm hoping not to have to spend that much money.To avoid the typically poor ACV function, I plan to use a two channel waveform synthesizer-type device to generate an AC test signal on one channel, to be sent to the DUT, and to also generate a trigger signal on the other channel, to be sent to the 3456A external trigger input. Since this device will generate the trigger synchronously with the AC test signal, the hope is that the meter will consistently sample the AC test signal from the DUT at a repeatable section of its waveform, and hopefully end up with a somewhat accurate (or at least repeatable) AC voltage measurement, unlike what one gets with an asynchronous test signal.I'm not wedded to the 3456A, but given that it, and more modern meters, have such high resolution and basic stability, but really poor ACV measurement accuracy, some means of synchronous sampling might push this error down a lot.Yes, even if the trigger latency is some non-zero value, repeatability might not be difficult if I just ignore it and forge ahead. Still, if I knew the latency, I could arrange for the trigger signal plus latency to end up at the top of a sine wave where the slope is near zero, further minimizing the effects of trigger uncertainty. Using the ACV function on most meters gets you pretty awful accuracy, at least according to the spec sheets. It's also not quite practical to use DC measurements, since the DUT rejects DC. I'd like repeatability to 50-100 ppm, which should not be hard to do even with an ancient 3456A, which can do 8 ppm DCV drift over 24 hours.I suppose I could just try it out with something like a triangle wave, and slip the trigger offset until I trigger on the peak, but I thought I'd ask first, on the off chance that someone knows the answer already.I'm also open to any other good ideas! I actually want to make an accurate AC gain measurement, measuring the input and output of an attenuator, and that might point to a different, perhaps better approach.Thanks in advance,Monte McGuire

author: MadTux
author_rank: Frequent Contributor
timestamp: December 18, 2015, 12:18:48 pm
content: I doubt you will get good results that way. Look at the settling times to 0.1% accuracy in HP 3456A specs, it's 60ms with filter off and 800ms with filter enabled. Since you want to sample a sine signal, you'll need an external sampling/hold stage, because the 3456A needs 60 or 800ms of constant signal on input before it can make accurate voltage readings.AC input will go to AC=>DC converter before it goes to ADC. Thereby the input will only see DC this way and doesn't need all that much bandwitdth or fast settling time.You probably get better results if you build your own DIY diode sampling bridge and read the results from it with 3456A. Or by using a well calibrated audio spectrum analyzer / sound card.

author: MarkL
author_rank: Unknown
timestamp: December 18, 2015, 02:29:01 pm
content: A look through the schematic says that the trigger input is being polled by the processor (U16 pin 16). So, the sample jitter is going to be at least on the order of the poll rate, which is unknown. The firmware will likely have other non-deterministic delays. And we're not talking about a fast processor in these old meters.The 3456A has a small amount of memory where it can do 350 samples in a burst. I would expect those to be evenly spaced so perhaps you could sample your waveform using DC mode in one shot. The fastest rate is around 300 rdgs/sec, with tradeoffs in accuracy. There's a detailed discussion on this method in the manual. It claims the meter can digitize sine waves up to about 100Hz with "fairly good accuracy" (pgs. 3-12 to 3-13).You didn't say what your test frequency was, so I'm not sure if this will work for you. Just an idea.

author: macboy
author_rank: Super Contributor
timestamp: December 18, 2015, 02:40:55 pm
content: Measuring AC is far more difficult than measuring DC, so to get better than 100 ppm as you want, you will have a difficult time with direct measurements using any multimeter.In virtually all AC voltmeters, the AC signal is fed to a RMS-to-DC converter which may use various tricks to get a DC signal proportional to the RMS value of the AC input. Then this DC is measured. But the key is understanding RMS : Root Mean Square. The "Mean" part means average, so by the nature of taking an RMS reading, you need to average the signal over some period of time. Most meters need to be able to measure 50 Hz mains, so this averaging window is long, some significant portion of a second. That is why meters have a relatively long settling time spec for reading ACV. This also means that any trigger latency has virtually no effect. What's a few more microseconds or even milliseconds?Actually, the resolution and repeatability of ACV measurements on good gear is very good, even though overall accuracy is poor. Getting much better than 0.1% overall accuracy is very difficult and only high end bench meters do it. But if you measure the same signal now and one minute from now, the two results will match much, much better than 0.1%, probably just a few ppm off at most. And if you attenuate a signal by 100 ppm, you will easily see that difference, even though the difference itself falls well into the overall "error" portion of measurement.You need to think about whether you actually want an ACV RMS measurement or not. Do you actually want the instantaneous value of the waveform at some moment in time? Then you need to take DC measurements with a short integration/sampling time, not AC at all. Do you understand why? If not then you need to step back and think. Think about what a digital oscilloscope does... it is used to visualize AC signals, but is it taking "AC" measurements? No, it is taking many successiveDCmeasurements. Ponder that.You are talking about measuring gain of some system, but you did not mention the frequency range of that system? 10 Hz or 10 kHz or 10 MHz? The solution to your problem is highly dependent on things like this. I'll take a guess that you want to measure the gain of this system with an arbitrary AC input signal, like audio for instance. One thing you cold try is to attenuate the output of the system with a precision attenuator, then compare this to the input signal, and adjust the attenuation until you get a minimum between them. At that point, the amount of attenuation is equal to your gain. This will become less effective at high frequencies because of phase shift and non-flat frequency response of the attenuator.I have to ask, you said "synchronous AC voltage, rather than ... unsynchronized, random AC test signal". What does "synchronous AC" or "unsynchronized, random AC" mean to you?

author: montemcguire
author_rank: Regular Contributor
timestamp: December 20, 2015, 01:03:42 am
content: Many thanks to all who've answered here - some great stuff, as I'd expected!I wasn't clear in the original post, but I want to use the DCV function to sample a synchronous AC waveform. While the 3456A is slow, my understanding is that it can be made to sample a waveform (using the DCV function) when the external trigger is used, and hopefully return an accurate and repeatable value for the amplitude of some tiny part of an AC waveform.macboy rightly pointed out that the nature of the test signal is not clear. I only care about characterizing the AC gain of this DUT at audio frequencies, probably with a sine wave, around 1 kHz or so, but I could use something slower like 100Hz, perhaps to reduce errors from trigger jitter, or to allow a longer integration time. Absolute AC voltage measurements with absolute accuracy are not important here, only the ratio of two measurements, which can be taken one after another, and thus with little relative drift. Your comments about sampling an AC signal are spot on - see below.As pointed out by MadTux, the 3456A ACV function is not going to be fruitful, and thanks for the taxonomy of how it's going to fail, which also points to a way to make it not fail: use the DCV function to read exactly the right part of the waveform, either sampled by the meter itself, or by an external sample and hold circuit. From what I can tell, the 3456A's multislope converter will integrate the input signal during the runup period, set to some number of power line cycles. If the right trigger timing is worked out, a repeatable (centered around peak) portion of the waveform from the DUT will be integrated and measured using the DCV function, which will be proportional to the AC waveform's amplitude. By measuring both the input and output of the DUT in this way, its gain should be able to be determined accurately, as long as the timing and triggering is stable.MarkL cautioned about the processor polling rate, and other non-deterministic delays that might also be added in. I've looked at every reference I could find, but neither HP nor anyone else has defined the latency of the 3456A trigger input, and perhaps it's because that number is not a constant. My application can't tolerate a spuriously inaccurate result, which could happen if the DUT output waveform were sampled at the wrong time. So, even though the external trigger jack is "hardware", it is still polled by a software loop, and might have non-deterministic latency.Thanks again, and I'll post anything good that I come up with.Regards,Monte McGuire