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HP3314A triangle generator circuit, ringing?
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wn1fju:
As an additional data point, my HP 3314A also exhibits no ringing on a (10 MHz) triangle wave.

I have certainly seen many instances of cheap function generators where indeed there is some artifact at the peaks of the triangle waveform where the current in the integrator switches.  I don't recall much ringing, although I probably didn't zoom in that much.  It basically looked like a small voltage spike.  And, of course, with triangle waveforms, one should be concerned with linearity - a place where many vendors fall short.

A minor annoyance that I've found over the years is when you look at a vintage piece, for example from the 1970's, on a modern oscilloscope.  The high bandwidth of the scope will show you all kinds of high frequency junk that you wouldn't have seen 50 years ago on a 50-year old scope.  And turn down the voltage on one of these function generators and your modern scope may have triggering problems since it is seeing the "noise."  It is almost like the designers in that era produced equipment that met specs in the prescribed frequency range and didn't much care if the piece was generating junk tens or hundreds of MHz higher up since few people had the instrumentation to see it.  Hopefully I'm wrong about that!


stephanm:

--- Quote from: wn1fju on June 25, 2022, 11:47:33 pm ---I have certainly seen many instances of cheap function generators where indeed there is some artifact at the peaks of the triangle waveform where the current in the integrator switches.  I don't recall much ringing, although I probably didn't zoom in that much.  It basically looked like a small voltage spike.  And, of course, with triangle waveforms, one should be concerned with linearity - a place where many vendors fall short.

--- End quote ---

Messing up the triangle peaks is indeed easy in a function generator circuit that is based on an integrator. I have seen it both in my simulations and on the breadboard circuits that I have built. You want the peaks to be sharp, this means your current switches must be fast. But then, high speed switching signals can couple into into other parts of the circuit, or the integrator capacitor construction could start ringing. All this introduces artifacts on the triangle peaks. Also, you want to carefully control the amplitude of the peaks to make the instrument produce a signal whose voltage is stable over frequency. And, on top of all this: A common way to generate the sine wave output was to take the triangle signal and run it through a sine shaper circuit. There's a nice patent on such a circuit filed by an HP engineer from Böblingen in 1980, my assumption is that this is what HP actually used in the HP3314A's hybrid sine shaper and amplitude control IC. But... with this circuit, distortion and artifacts of the triangle peaks will lead to distorted peaks of the generated sine wave. So for a function generator like the HP3314A, there was lots of reason for HP to put quite some energy and focus in the triangle wave generator.

Still, it remains unclear to me how they managed to get away without ringing in the integrator when the capacitors are paralleled. In my simulations and breadboard work, I constantly see that it's easier to get away without ringing in the highest frequency range, where only one integrator capacitor is used, not multiple in parallel. That said, waveform photos (or a spectrum analyzer measurement) from the triangle wave below 2MHz would be great.

What I also found is that you can avoid ringing by slowing down the current transition in the integrator. The larger the integration capacitor becomes, the more you need to slow down the transition. The comparator HP implemented in the HP3314A has this property, but, so to speak, only for one half of the transition. The comparator has low gain, it will react as soon as the triangle voltage comes close to the peak, and the slower the triangle voltage approaches it's peak, the earlier (and slower) the comparator will start moving the voltages that control the differential pair current switch (Q208A/B in the circuit diagram I posted.) However, the comparator also requires to provide a bistable behaviour; this is implemented by positive feedback in the comparator. Once the positive feedback kicks in, the comparator will generate steeper slopes for the current switch control voltages. And now I'm back at the place where things start ringing in my simulations ;D
HighPrecision:

--- Quote from: stephanm on June 25, 2022, 05:26:35 pm ---Do you, by accident, have a scope screenshot for e.g. 1MHz or 100kHz? At 1MHz, two timing caps are paralleled, and 3 are paralleled for 100kHz, which gives us the construction that is susceptible to ringing...

Kind regards

Stephan.

--- End quote ---

Hello Stephan,
new images for 100KHz and 1MHz output, no ringing visible, the scope is at max BW (400MHz).

Best, Stephano

stephanm:
Thanks Stephano for providing these images. Very helpful and very interesting! Once again, waveforms look remarkably clean, no indication of artifacts at the peaks.

Congratulations to owning this instrument, it must be a pleasure to work with it :)
nctnico:

--- Quote from: stephanm on June 26, 2022, 10:04:54 am ---Congratulations to owning this instrument, it must be a pleasure to work with it :)

--- End quote ---
Not so much. It is big & loud and prone to failures due to age. There are better tools out there nowadays; I have sold my 3314A a long time ago (after replacing all the reed relays).
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