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| DIY Transformer for use with Bode Plots. |
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| RoGeorge:
--- Quote from: mawyatt on June 01, 2022, 10:25:17 pm ---Conclusion is that the Isolation transformer needs to have a much higher Primary inductance to keep the low frequency signal level reasonable for the scope to capture accurately since the high gain feedback loop is effectively nulling the scope Ch1 Vin signal, so we'll be getting a larger core and winding another transformer with larger primary inductance for these low frequency Closed Loop Bode measurements with high gain Op-amps like the OP-07. --- End quote --- About achieving less than 100Hz lower frequencies, are they really needed? Asking that because (in my understanding) the instability of a feedback loop is expected to happen at the frequency where the amplitude of the feedback loop bode plot crosses the open loop amplifier's bode plot. Usually this intersection of the two plots happens at a frequency in the kHz range, at least, so why bother testing at 10Hz? A feedback network designed to work up to only 10Hz will have a bode plot that will not intersect the amplifier's open loop bode plot at all (thinking here the casual opamps with gain bandwidth ~1MHz). Is the sub 100Hz injection useful for "just in case", is it for something else than testing loop stability, or am I missing/misunderstanding something else entirely? |
| 2N3055:
--- Quote from: RoGeorge on June 02, 2022, 06:26:33 am --- --- Quote from: mawyatt on June 01, 2022, 10:25:17 pm ---Conclusion is that the Isolation transformer needs to have a much higher Primary inductance to keep the low frequency signal level reasonable for the scope to capture accurately since the high gain feedback loop is effectively nulling the scope Ch1 Vin signal, so we'll be getting a larger core and winding another transformer with larger primary inductance for these low frequency Closed Loop Bode measurements with high gain Op-amps like the OP-07. --- End quote --- About achieving less than 100Hz lower frequencies, are they really needed? Asking that because (in my understanding) the instability of a feedback loop is expected to happen at the frequency where the amplitude of the feedback loop bode plot crosses the open loop amplifier's bode plot. Usually this intersection of the two plots happens at a frequency in the kHz range, at least, so why bother testing at 10Hz? A feedback network designed to work up to only 10Hz will have a bode plot that will not intersect the amplifier's open loop bode plot at all (thinking here the casual opamps with gain bandwidth ~1MHz). Is the sub 100Hz injection useful for "just in case", is it for something else than testing loop stability, or am I missing/misunderstanding something else entirely? --- End quote --- One area I know that needs it are PFC controllers... sometimes you need to look at frequencies even less than 5-10 Hz... EDIT: My bad I misunderstood the question.... In context of measuring open loop gain of opamp very low frequencies are not very interesting.. |
| Jay_Diddy_B:
Hi, Measuring a control loop is different than measuring the open loop bandwidth of an op-amp. When measuring a control loop the main frequency of interest, as pointed out by RoGeorge, is when the gain is 0dB. By definition the amplitudes of the signals at nodes A and B are equal in amplitude and there is no concern about the signal to noise ratio. When measuring an op-amp open loop gain, the low frequency gain is very high, 1E6 typical, 120dB. The signal on Node B is 1 million times smaller the signal on Node A. Low crossover frequency I have created a model of an amplifier with a dc gain of 10,000 and a Gain Bandwidth of 5Hz. This is G1 and E1 in the model. In the AC analysis the 0db point is 5Hz as designed. The gain at 1Hz is 14dB. In the time domain, at 1 Hz, the signal at node B is only 5x smaller, -14dB, with respect to the signal at node A. With the relatively large signals, there are no SNR challenges. At the 0dB frequency For completeness, at the 0dB frequency, the signals at Nodes A and B are equal in amplitude, and phase shifted by 90 degrees. Jay_Diddy_B |
| mawyatt:
Lots of interesting discussions :-+ Accessing gain and phase margin in op-amp based circuits generally are not a problem for the DSO because the signal levels are sufficient to resolve. However trying to determine overall loop gain and phase at lower frequencies becomes increasingly difficult as the frequency decreases because the signal amplitude decreases due the negative feedback and the isolation transformer lower frequency response. Increasing the signal level available at lower frequencies from the isolation tansformer may help and why we're looking into a higher primary inductance transformer. In a way we are asking a general purpose DSO to perform as a dedicated high input impedance & sensitivity network analyzer and it does a nice job within reason. However, dealing with high loop gain such as op-amp circuits, the available signal level becomes so low at lower frequencies the DSO can't resolve such and the results show. Here's where a dedicated instrument shines, and likely using techniques such as synchronous sampling to help "pull out" the signal from the noise and produce accurate and repeatable results. Most of the time the lower frequency performance can be easily estimated and the lack of performance here isn't a problem, however there are certain case where one needs this lower frequency performance. We ran into one such case a number of years ago when developing a closed loop piezo electric nanometer positioner utilizing "Flexures" as bearings, the control loop for this device was quite complex even at lower frequencies. Anyway, quite pleased that a general purpose DSO can actually perform these type of measurements and produce respectable results within a reasonable range of operating parameters. Now to see if a larger transformer can slightly expand that operating range :) Best, |
| TopQuark:
--- Quote --- --- Quote from: TopQuark on May 22, 2022, 07:45:04 am ---I have been down this path of building wide BW transformers before as well. --- End quote --- Could you change the vertical scale to 0.2dB/div and repost? --- End quote --- Apologies for the delayed response, I've attached the results as per requested. Love your videos btw, keep up the good work. I have received questions regarding the core I used and its availability. I got mine from Taobao which is not very accessible to the rest of the world. If purchasing from Aliexpress is an option to you, you can find cores similar to mine by searching for "nanocrystalline core" at the site. https://www.aliexpress.com/item/4000447631159.html <- This core looks to be the same as the one I have used, although I didn't purchase mine from them. The seller also offer similar cores with other sizes. --- Quote from: RoGeorge on June 02, 2022, 06:26:33 am --- --- Quote from: mawyatt on June 01, 2022, 10:25:17 pm ---Conclusion is that the Isolation transformer needs to have a much higher Primary inductance to keep the low frequency signal level reasonable for the scope to capture accurately since the high gain feedback loop is effectively nulling the scope Ch1 Vin signal, so we'll be getting a larger core and winding another transformer with larger primary inductance for these low frequency Closed Loop Bode measurements with high gain Op-amps like the OP-07. --- End quote --- About achieving less than 100Hz lower frequencies, are they really needed? Asking that because (in my understanding) the instability of a feedback loop is expected to happen at the frequency where the amplitude of the feedback loop bode plot crosses the open loop amplifier's bode plot. Usually this intersection of the two plots happens at a frequency in the kHz range, at least, so why bother testing at 10Hz? A feedback network designed to work up to only 10Hz will have a bode plot that will not intersect the amplifier's open loop bode plot at all (thinking here the casual opamps with gain bandwidth ~1MHz). Is the sub 100Hz injection useful for "just in case", is it for something else than testing loop stability, or am I missing/misunderstanding something else entirely? --- End quote --- I echo the same thoughts, usually I only look at crossover frequency, gain slope at crossover, gain and phase margin for SMPS control loops. I don't deal with PFC much, so if crossover is <100Hz, I probably don't need to run a FRA :P I also agree the bode plot performance issues <100Hz has more to do with the instrument than the transformer. Looking at the videos showing the Bode 100 at work, it is obvious that whatever hardware/software they are using is way faster, has high dynamic range and noise rejection capabilities than the Siglent scope even at <100Hz, using transformers with similar sizes compared to ours. Looking at the whole picture, I think what the industry can really benefit from is something with Bode 100 speed and accuracy, Cleverscope style isolated DAC/DDS (so no isolation transformer needed), NanoVNA design philosophy and price. Although that's just me daydreaming. :P |
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