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Oscilloscope Bode Plot Speed Comparison
nctnico:
--- Quote from: Berni on June 23, 2023, 05:42:40 am ---
--- Quote from: Fungus on June 22, 2023, 10:55:01 pm ---
--- Quote from: jjoonathan on June 22, 2023, 08:24:36 pm ---
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I was told very sternly by forum members that a Bode Plot should show phase as well, otherwise I can just feed some white noise into my 'scope and do an FFT to get a real time display of frequency response.
--- End quote ---
The Agilent 89410A will do a bode plot just as fast as that and can display phase or group delay too.
On top of that it will also do 85dB of dynamic range. See signals down to -140dBm and will also compensate for the DUT loading it down the signal source as it has 2 inputs and plots the difference between the two inputs.
It really is a bode plotting machine. Tho due to this thing being from the 90s it has taken a lot of hardware to pull this off, making it a huge 21kg boat anchor and draws 100s of watts of power.
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The MS4630B can do all that (and much more as it is a network analyser) but jjoonathan just didn't show it.
tautech:
--- Quote from: nctnico on June 23, 2023, 10:09:44 am ---The MS4630B can do all that (and much more as it is a network analyser) but jjoonathan just didn't show it.
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Just as the 4 port 4.5 GHz SNA5004A then hacked to 8.5 GHz I had could do too but where does one stop ?
I believe it would be useful for the OP to add 'Oscilloscope' to the topic title of this thread.
Berni:
Tho now that i look back at this, most of what makes the Agilent 89410A so fast could be done on any modern scope too.
The 89410A has a weird semi digital architecture that actually has quite a bit in common with digital oscilloscopes. The architecture of it is basically a SDR, the analog side is just an analog front end feeding a ADC. All the rest happens in the digital domain where it has dedicated DSP hardware for processing all that data in real time as it streams out of the ADC. All the spectral analysis features are actually done in the digital domain.
Sure the ADC it uses has more bits (14bit i think) and the front end is lower noise, but apart from that it is basically the same as a scope. With how much more powerful CPUs have gotten since the 90s we could probably run all the processing in software now.
The way it does bode plots is by generating a sine sweep waveform and feeding it into the arbitrary signal source. Then it synchronizes the acquisition with the source and does FFT on what it got back. So the sweep is basically just a more predictable replacement for 'white noise' in other to get a broad frequency signal to excite the DUT. This means it only needs to do one FFT per sweep, so it can do multiple sweeps per second.
I don't see why oscilloscopes don't do the same thing given that they have ARB signal generators and already have FFT capability. All of the scopes with bode plot functionality seam to do the thing where they step trough frequencies one by one and do a one shot amplitude/phase measurement at each one, this is obviously very very slow.
jasonRF:
Unless you know the gory details of how the scope is doing the analysis, these will all be apples to emu comparisons.
For example, using my picoscope 5244B with the FRA4Picoscope app I can change the time by orders of magnitude by adjusting the parameters. If I assume it is a low-noise environment and ask for at least 16 cycles of the waveform, then it takes about 24 seconds. On the other hand, if I assume a high-noise environment and ask for at least 10 Hz resolution bandwidth, I get just a couple of samples per second even at the highest frequencies. Of course at those high frequencies it needs to collect (and transfer over usb 2) huge numbers of samples. I'm sure if I go to even narrower resolution it will slow down accordingly.
I have never dug into the details of how that app works, so don't even know if it does averaging. In principle, this is another knob that can be adjusted (if it isn't already). And then the app has an adaptive mode where it adjusts the signal amplitude, in order to use it with SMPS where too large of a signal can of course cause problems. I have never worked with that, but presumably that can make it even slower.
When writing my own code to do FRA with simple, shallow-memory scopes (Owon vds1022 and Pico 2204a), I usually assumed a low-noise environment so did no averaging. I still get ~70 dB dynamic range (~40 from the scope, ~30 from the several-kSample FFT). In this case at low frequencies I just wanted X cycles of the waveform and maximized the sample rate given the number of samples available; but at high frequencies where I need to use the max sample rate, I end up with a 'constant resolution bandwidth' approach since I don't want use smaller FFTs at the higher frequencies. If you go down to, say, 1 Hz, then collecting data and needing to do things like adjust the vertical scale just slows things down a ton, so I always start with the highest frequencies and work backwards to minimize those low-frequency vertical scale adjustments. The AD2 has the advantage of not needing any such adjustments, which can help speed it up.
jason
mawyatt:
--- Quote from: tautech on June 23, 2023, 10:25:32 am ---I believe it would be useful for the OP to add 'Oscilloscope' to the topic title of this thread.
--- End quote ---
Agree, this should be about Scope Bode Function, FRA or whatever one wishes to call this capability, not all inclusive of other instruments. We all know, or should know, that the dedicated instruments mentioned (likely many more) are going to perform better than the DSO under Bode display & analysis.
A DSO is a Time Domain instrument, designed as such, however does respectable service (at least some do) when asked to perform some service in the Frequency Domain (FFT or Bode).
To put this in perspective, would anyone consider comparing a SA, VNA, Frequency Response Analyzer, Bode Analyzer to display Time Domain waveforms and expect it to perform on par with a modern DSO :o
Best,
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