Some Important Points to Remember when Evaluating Waveform Update Rates

[jpb]

The thing that struck me was that this very long discharge time  will mean that in the example given the next waveform cannot be displayed for 5 microseconds so
the waveform update rate will be limited to 200,000. OK that is quite a high upper limit but it is only 1/5 of Agilent's 3000X series. Also at slower sample rates the
charging time and thus discharging time would correspondingly increase.

Interesting. I wonder if the InfiniiVision X series use a digital trigger - I did a quick Googling but couldn't find a definitive answer - but I would guess they probably do. I know the Rigol UltraVision line does. According to this Rohde & Schwarz paper, one of the advantages of their digital trigger is "No Masking of Trigger Event":

"An analog trigger requires some time after a trigger decision to rearm the trigger circuitry before they can trigger again. During this rearm time, the oscilloscopes cannot respond to new trigger events - trigger events occurring during the rearm time are masked.
In contrast the digital trigger system of the R&S RTO oscilloscopes can evaluate individual trigger events with the Time-to-Digital-Converters (TDC) within 400 ps intervals (Figure 12) with a resolution of 250 fs."

A 400ps interval would put the limit of trigger events at 2.5 meg.

The analog trigger rearm rate depends on the sweep reset time and not the trigger itself except insofar as it has to detect the other slope before arming.  It is quite practical to have triggers which operate into the 250 MHz+ range and some oscilloscopes did this because the trigger was used for other things as well like to drive a frequency counter.

Digital triggers are just a variation of a transition midpoint timing TDC.

The LeCroy literature that I originally quoted from was for analogue triggers but in digital scopes. The trigger could be made faster by having a shorter discharge time but this would then make the time measurement less accurate.

Such an interpolation approach makes sense if the sample points are comparatively widely spaced and you're trying to catch a fast rising edge. With a 350MHz scope (1nsec rise time) and with the sampling rate at 1GS/s I can see that digital interpolation could be quite inaccurate compared to using an analogue trigger (which my scope does) but it does limit the screen update rate (my WaveJet's update rate is only a few thousand wfms/s).

[David Hess]

The analog trigger rearm rate depends on the sweep reset time and not the trigger itself except insofar as it has to detect the other slope before arming.  It is quite practical to have triggers which operate into the 250 MHz+ range and some oscilloscopes did this because the trigger was used for other things as well like to drive a frequency counter.

Digital triggers are just a variation of a transition midpoint timing TDC.

The LeCroy literature that I originally quoted from was for analogue triggers but in digital scopes. The trigger could be made faster by having a shorter discharge time but this would then make the time measurement less accurate.

Such an interpolation approach makes sense if the sample points are comparatively widely spaced and you're trying to catch a fast rising edge. With a 350MHz scope (1nsec rise time) and with the sampling rate at 1GS/s I can see that digital interpolation could be quite inaccurate compared to using an analogue trigger (which my scope does) but it does limit the screen update rate (my WaveJet's update rate is only a few thousand wfms/s).

I remember when LeCroy started advertising their digital triggering.  They had some pretty impressive videos of them in action.

There are lots of ways to implement a TDC.  Dual slope implementations have the limitation you identify and are common on low acquisition rate DSOs simply because a faster TDC would not make them any faster because their trigger rearm time is limited by how quickly they can transfer the current waveform record to the processor.  It took oscilloscopes with segmented memory or DPO functionality to require faster TDCs and even then, directly digitizing the output of the TDC (time to amplitude conversion) instead of deintegrating it will get you to at least 200k waveforms per second.

http://www.ko4bb.com/~bruce/TDC.html

Some form of TDC is needed to support equivalent time sampling.  Many oscilloscopes lack this now simply because they have high enough real time sample rates but in the past equivalent time sampling was the rule.

I have wondered about aliasing with digital triggers.  Jitter and nonlinearity in the digitizer will be exasperated by interleaving and the resulting aliasing from the interpolation will corrupt the trigger position but I assume the effect is small enough compared to the supported equivalent time sample rate to be ignored.

http://cp.literature.agilent.com/litweb/pdf/5989-5732EN.pdf

[jpb]

Some form of TDC is needed to support equivalent time sampling.  Many oscilloscopes lack this now simply because they have high enough real time sample rates but in the past equivalent time sampling was the rule.

I have wondered about aliasing with digital triggers.  Jitter and nonlinearity in the digitizer will be exasperated by interleaving and the resulting aliasing from the interpolation will corrupt the trigger position but I assume the effect is small enough compared to the supported equivalent time sample rate to be ignored.

My WaveJet does have equivalent time sampling (to 100GS/s) and I find it useful. As I don't spend a lot of time glitch hunting I have not noticed the 4k waveforms/s rate as a problem.

The waveform per sec rate was pushed to the top of the spec sheet, mainly by Agilent marketing I suspect, but it is a rather ephemeral figure as what ever rate you have there are always a lot of potential glitches that you'll miss. Even at 1Million wfms/s there is still a lot of blind time at the faster time bases. The marketing literature always picks an example that fits in with the waveforms/sec rate of the scope being sold (e.g. Agilent will use very different examples when selling their 3000 series as to there 2000 series).

Thanks for the links.

[marmad]

The waveform per sec rate was pushed to the top of the spec sheet, mainly by Agilent marketing I suspect, but it is a rather ephemeral figure as what ever rate you have there are always a lot of potential glitches that you'll miss. Even at 1Million wfms/s there is still a lot of blind time at the faster time bases.

Absolutely - and the fact that the rate varies widely across the different time bases is not widely publicized. I'm of the opinion that ALL DSOs should display a blind time percentage meter on the screen at all times - just as many show sample rate and size.

[David Hess]

In general, even analog scopes have dead time when the CRT beam is blanked and moved back to the left side of the CRT in preparation for the next trigger.  What's interesting is that the analog scopes (at least these 2) do not always have lower trigger blind time compared to the digital scope.  In general, the analog scope are better at the faster timebases but start to lose to the digital scope at around 20uS.  Newer analog scope may be quicker but I don't have others to test.

Having a higher blind time at slow sweep speeds on an analog oscilloscope mystified me so I measured my 7904 with 7B92A timebase and at slow sweep speeds, it was always better than 2% at sweep speeds of 200 us/div (2 millisecond sweep) and slower and above that it always measured better than your Rigol.

[Carrington]

Absolutely - and the fact that the rate varies widely across the different time bases is not widely publicized. I'm of the opinion that ALL DSOs should display a blind time percentage meter on the screen at all times - just as many show sample rate and size.

That would be great, should be mandatory.

[David Hess]

Some form of TDC is needed to support equivalent time sampling.  Many oscilloscopes lack this now simply because they have high enough real time sample rates but in the past equivalent time sampling was the rule.

I have wondered about aliasing with digital triggers.  Jitter and nonlinearity in the digitizer will be exasperated by interleaving and the resulting aliasing from the interpolation will corrupt the trigger position but I assume the effect is small enough compared to the supported equivalent time sample rate to be ignored.

My WaveJet does have equivalent time sampling (to 100GS/s) and I find it useful. As I don't spend a lot of time glitch hunting I have not noticed the 4k waveforms/s rate as a problem.

The waveform per sec rate was pushed to the top of the spec sheet, mainly by Agilent marketing I suspect, but it is a rather ephemeral figure as what ever rate you have there are always a lot of potential glitches that you'll miss. Even at 1Million wfms/s there is still a lot of blind time at the faster time bases.

All of my DSOs support equivalent time sampling as well and I have no complaints about it.  They have slower waveform acquisition rates than your WaveJet though so I have a fast analog storage oscilloscope for when I need to find rare glitches.

Absolutely - and the fact that the rate varies widely across the different time bases is not widely publicized. I'm of the opinion that ALL DSOs should display a blind time percentage meter on the screen at all times - just as many show sample rate and size.

I would settle for just specifying the blind time at different sweep speeds or publishing the rearm time which would let it be calculated.

Displaying this information would be nice but I doubt it will happen.  Tektronix at one point considered including a feature on their DSOs which compared the sample rate to the trigger rate (not counting arming) to detect potential aliasing and warn the user but ended up leaving it out because it made their oscilloscopes look bad even though every other manufacturers' DSOs suffered from the same problem.  That is what happens when marketing triumphs over a good idea.