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| HP3314A triangle generator circuit, ringing? |
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| stephanm:
Disclaimer: I neither own a HP3314A nor have I ever seen one live. It's a function generator from the 1980's, capable of creating sine, square and triangle output up to 20MHz. I spent quite some time studying and understanding the triangle generator circuitry of the HP3314A schematics. Over time, I understood more and more of the issues related to generating fast and clean triangle signals, and what HP did to overcome these issues. It's really an amazing piece of engineering artwork that they did more than 40 years ago! However, there is one thing that keeps puzzling me. For generating the triangle signal, HP basically implemented an integrator where a timing capacitor is alternatively charged by a current +I (for the rising ramp) and -I (for the falling ramp). Frequency is controlled by varying I. The actual circuitry is much more complex, the integrator and much of the other circuitry is working with differential signals, but for the question I have this complexity is not too important I think. For different frequency ranges, the service manual says that the different timing capacitors (C218, C204...C207, C210) which integrate the current are paralleled by the relais K201...K205. So for the 2-20MHz range, only timing capacitor C218 is used, but for e.g. the 200kHz-2MHz range, relais K205 is closed so that C218 + C210 are paralleled. C218 together with some other capacitors and parasitics is supposed to give 27.7pF, and as C210 = 250pF we get C218 + C210 = 277.7pF, which is exactly a 1:10 ratio, resulting in a 1:10 frequency ratio between the two frequency ranges of the instrument. For the lower frequency ranges, it goes on like this, the next capacitor paralleled into the integrator timing capacitance is C207 = 2500pF, so that we get C218 + C210 + C207 = 2777.7pF, again a 1:10 ratio compared to the previous frequency range. Now, with those parallel capacitors, the whole arrangement becomes susceptible to ringing. The smaller capacitors will produce a high frequency resonance with the parasitic inductances of the larger capacitances. Given the photos found in https://www.eevblog.com/forum/testgear/hp-3314a-function-generator-teardown-explanation/ the physical arrangement of the capacitors and relais is somewhat large, so I expect relevant parasitic inductances to be found in this part of the circuit. Since the integrator periodically switches the current through the capacitances from +I to -I and back to +I with a quite impressive slew rate, it is very easy to see this ringing happen it a LTspice simulation with only a few 10s of Nanohenries of parasitics. If this wouldn't be a triangle generator, a set of countermeasures would be available for damping these resonances, but I don't see any of this implemented in the schematics (which makes sense to me, because all countermeasures I can think of would distort the triangle signal even more severely compared to the ringing.) With the parasitics I believe to be reasonable, I would estimate ringing to be in the area just above 100MHz. This should be within the bandwidth of the rest of the circuit (you want high bandwidth for a 20MHz triangle signal, triangle signals have lots of non-negligible harmonics.) Ringing amplitude I get in simulations is small but I somehow cannot imagine that HP engineers just accepted this to happen and found on the instrument's output. What I am curious about is: Does anyone of you have a measurement that shows the triangle signal tips? Do we see high frequency oscillations there on the output? How large is it? Or, if there is no such a thing: How did HP engineers solve this issue, what part of the circuitry is the solution? This keeps bugging me for a really long time already... |
| mawyatt:
Interesting question, don't have this generator so hopefully someone can show these waveforms. Regarding the simulations and actual circuits, suspect the PCB layout and parasitic capacitance, in addition to the parasitic inductance will have a significant effect at the triangle waveform peak where the current direction thru the timing caps is reversed. What speed do your simulations show for the current switching reversal? Best, |
| stephanm:
--- Quote from: mawyatt on June 24, 2022, 12:51:36 pm ---What speed do your simulations show for the current switching reversal? --- End quote --- It's around 2ns, but this might be too optimistic. I only modelled parasitics in the timing capacitor area of the differential integrator so far. As I was actually thinking of (re-)building this circuit with components available today, my simulation just uses many MMBTH10's in places where HP engineers might have spent hours and days to figure out what's the best transistor for the spot. I have also captured the HP3314A differential comparator in my simulation based on NPN+PNP transistor arrays available today. Top speed that I was looking for is 10MHz plus 10% guard band (half of what the HP3314A can do) in the hope that this could ease up some things at least bit. So my timing capacitors are larger, I am running slightly more current through them (the max integrator current in the HP3314A is 2.2mA, I am using 2.5mA), and I increased the triangle p-p voltage a bit (2Vpp differentially for the HP, 2.5Vpp for my attempt.) Oh, and I reduced supply from +/-15V to +/-9V, so my DC level voltage layout is sometimes quite different from where HP has the quiescence voltages throughout the circuit. Still, I did a few checks with adding parasitics in places where I thought it would matter, and I found many things to be surprisingly immune to my attempts to uncover more areas of concern. But that could just mean that I haven't looked deep enough into this. The only notable exception is the differential integrator. |
| HighPrecision:
--- Quote from: stephanm on June 24, 2022, 11:11:32 am ---What I am curious about is: Does anyone of you have a measurement that shows the triangle signal tips? Do we see high frequency oscillations there on the output? How large is it? Or, if there is no such a thing: How did HP engineers solve this issue, what part of the circuitry is the solution? This keeps bugging me for a really long time already... --- End quote --- Hi, below are two images of triangle waveform from my 3314A connected to a 2465B analog scope, output frequency is 10 MHz, in one image delayed trace to zoom details at 'I' change. Best, Stephano |
| stephanm:
Hi Stefano, thanks a lot! The triangle peaks are really remarkably sharp and clean, it's amazing what the HP3314A does. It looks a little as if there would be bumps on the rising and falling slopes, but maybe I'm seeing it wrong. At 10 MHz timing capacitors are not paralleled. 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. |
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