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Bode Plot Torture Test

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nctnico:

--- Quote from: mawyatt on November 10, 2022, 04:30:44 pm ---
--- Quote from: nctnico on November 10, 2022, 04:10:54 pm ---IMHO noise is not the main problem for doing network analysis as most of the suitable algorithms get rid of noise anyway. And in some cases noise is good to get some extra bits from an ADC. The worst noise can do is swamp the DSO input and thus making it impossible to switch to a low enough range to acquire meaningfull data. What is more likely to be a problem are interfering signals (= external signals with a fixed frequency), harmonics and DUT behaviour.

It would be interesting to come up with a DUT that really tests the limits of FRA / network analysis.

--- End quote ---

Oscillators and SMPS seem like good candidates!!

BTW you should consider doing the test that rf-loop performed with your DSO, this revels the Frequency Selective nature of the "Black Box" Bode Functions and helps one understand the effects and limitations imposed by such,

--- End quote ---
I re-did the test rf-loop did. Applying a fixed 350kHz sine wave / +2dBm to the input that is supposed to come from the DUT. I tried various points /decade settings and the RBW depends on the number of points (which is logical). This is the result with 500pts/decade.



The result would suggest that frequencies close to the measurement frequency would affect the results but the earlier tests we did don't show that. Maybe that is due to the interfering signal being mixed into what is going into the DUT. Another interesting test would be to mix an interfering signal into the signal coming from the DUT only.

mawyatt:

--- Quote from: switchabl on November 10, 2022, 11:13:41 pm ---
--- Quote from: Someone on November 10, 2022, 10:00:10 pm ---This is interesting as it shows the detector being frequency selective, but having zero/no rejection of external signal. Which is the opposite of the example in the first post (external signal rejected). Do you know why this is so different? What options/methods/parameters can a user control to make such a difference?

--- End quote ---

Unless I missed something, in the OP the spurious tone is simply added to the input. Its frequency is also conveniently located on a sweep point, where it will basically just increase stimulus power and maybe shift the phase (both of which is accounted for through the reference channel). And the step size is probably larger than the receiver BW, so adjacent points are hardly affected.

--- End quote ---

We had some time and repeated some tests using a "spurious tone" at 10KHz with a linear sweep of 500 points from 5KHz to 15KHz . Setup was 2 separate AWGs and DSO set to 10X probes and 20MHz BW. AWG spurious tone set to 10VPP and Stimulus AWG set at 1VPP. A second DSO was used to monitor the Input and Output waveforms.

First plot shows result with CH1 driven by AWG Stimulus and CH2 driven by Spurious AWG, no connection between CH1 and CH2. The Bode result appears to have a response to the Spurious Tone with a BW of ~ 1KHz.

2nd plot shows CH1 and CH2 now connected by a 1K Resistor. Edit: See 7th for better plot!

3rd shows CH1 disconnected (no Input signal).

4th shows both AWG summed together with 100 ohm resistors and connected to CH1, CH2 and CH1 still connected by 1K Resistor.

5th shows same as 4th but Spurious AWG frequency changed to 10.063KHz & 100 points Bode sweep.

6th shows same as 2nd (CH1 and 2 connected by 1K, Stimulus AWG to CH1, Spurious AWG to CH2) but with Spurious AWG frequency 10.063KHz & 100 point sweep.

Edit: 7th added for better display of 2nd plot!!

This seems to show that the Spurious signal when applied to the Input (CH1) along with the Stimulus doesn't affect the plots, and with the Spurious applied to the output (CH2) does affect the resulting plots. Changing the Spurious frequency to not land on one of the Stimulus points seems to have no effect.

No attempts were made to keep the signals "clean" nor shielded, just a quick clip lead setup with leaded resistors.

Best,

nctnico:
The effect is logical because the input and output signal amplitude & phase are subtracted from eachother. What isn't in the input, is not subtracted from the output so whatever is extra in the output will show up. They question is however whether this has an influence when doing a real sweep instead of using fixed frequencies.

mawyatt:
That makes sense except what amplitude phase are they using since the Spurious and Stimulus are both present, how does the algorithm know which is which? There is some form of frequency selectivity going on that at least allows the separation of the Spurious signal when it's not close to the Stimulus, maybe just a simple FIR bandpass filter than follows the Stimulus. With Synchronous Sampling/Detection the input is frequency converted because of the BiPhase Modulation and the following LPF gets converted into a BPF of twice the LPF BW and centered around the BiPhase Modulation Toggle rate, but I don't think that's what's going on here.

Anyway, looks like your DSO is also doing something similar!!

Best,

Someone:

--- Quote from: switchabl on November 10, 2022, 11:13:41 pm ---
--- Quote from: Someone on November 10, 2022, 10:00:10 pm ---This is interesting as it shows the detector being frequency selective, but having zero/no rejection of external signal. Which is the opposite of the example in the first post (external signal rejected). Do you know why this is so different? What options/methods/parameters can a user control to make such a difference?
--- End quote ---
Unless I missed something, in the OP the spurious tone is simply added to the input. Its frequency is also conveniently located on a sweep point, where it will basically just increase stimulus power and maybe shift the phase (both of which is accounted for through the reference channel). And the step size is probably larger than the receiver BW, so adjacent points are hardly affected.

--- End quote ---
In which case adding noise to the stimulus should have minimal effect as a) noise has shown to be supressed by the frequency selective detector b) the detector for both sides of the measurement should be equally affected
So the original post may be mostly about the amplitude of the measurement on each side, which could be reproduced by adjusting the vertical gains.

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