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Oscilloscope Bode Plot Speed Comparison

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thm_w:

--- Quote from: nctnico on June 22, 2023, 11:49:01 pm ---No, it is not simple as that  ;D The software needs to apply a steep filter (single tone FFT / Goertzel filter) to retrieve the transmitted tone from the signal and determine magnitude & phase from that. Otherwise the result is affected by noise, auxilary frequencies (switching power supplies for examples) and harmonics from non-linear DUTs. It is possible the tone that is measured is swamped by a tone at a different frequency.

--- End quote ---

Is that confirmed on all scopes though? There is a post from Someone here stating cursor measurements may be used: https://www.eevblog.com/forum/testgear/bode-plot-computational-time-for-various-dsos/msg4510168/#msg4510168

For the ones that use FFT, a comparison might mostly depend on whatever memory depth/timing parameters the manufacturer decided to go with. Although R&S did have a fast FFT so it could help.

Anthocyanina:
the AD2 does it in under 20 seconds. this is an active low pass filter, the same test conditions, 200Hz to 10MHz, 105 points per decade. It shows the bandwidth warning, and in the yellow trace you can see the measured wavegen gain compared to what it is supposed to be, it goes to -3db at around 9MHz and remains flat up to about 2.2MHz, but still, that doesn't affect the bode plot speed, only its accuracy. I'll do keysight 1102g next and report!  :-/O



1102G takes 5 minutes 24 seconds, and things get very weird near -18dB

Berni:

--- Quote from: Fungus on June 22, 2023, 10:55:01 pm ---
--- Quote from: jjoonathan on June 22, 2023, 08:24:36 pm ---

--- End quote ---

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.


skander36:

--- Quote from: nctnico on June 22, 2023, 10:21:28 pm ---
... I doubt the RTB2004 is slower as the basic platform is mostly the same...



--- End quote ---

Right! RTB2K 74.1 s for 200Hz-10MHz, 105 points, low pass filter.
Rigol 5K - 5min 45s same parameters.

rf-loop:
Some common comments about FRA aka BodePlot. Bit OT because this do not include speed test but some comments relative to this and specially relative to comparing.



There are different ways to do it in normal oscilloscope.  And when do compare then need carefully think what we compare... ok now speed but without  caring any other things...


But also there are many differences in performance and example in speed. 
Different oscilloscopes may use very different way to do BodePlot what leads then also very different performance. .

Also user can set most optimal settings for different cases (after he well know how it works and how to setup)




Some tiny examples.



Full dynamic range when Channel Gain = Auto 
From displayed noise floor to +40dBm. Yes it can do whole range (up to 140dB)  in single sweep. (when 1M input or External 50 ohm load what can handle these max powers)








Example: one 455kHz IF filter (lot of  better than in normal cheap radio)
There are many oscilloscopes what have BP... many of them can not do this, not even close.
Also this Siglent oscilloscope FRA can not measure where is real stop band... because oscilloscope noise level is far too high. But with this we still know stop band is least what is displayed.







Simple example: One 84.050kHz resonator. Even in this simple example dynamic range is over 100dB
(in this picture one div is 5 Hz)
There are many oscilloscopes what have BP... many of them can not do this, not even close. IN many oscilloscopes BodePlot is just for checkbox feature for marketing. They can do some RLC filter etc very simple cases, lot of fun but in bit more challenging cases they fails.  Of course there are also oscilloscopes what have good FRA also for bit more difficult things than just for easy playing... also example some Nnao like VNA's are out as...snow man in summer.




Then what it mean it is based to frequency selective receiver...
Systems what do not have frequency selectivity can not do good job when there exist strong harmonics or spurs what also DUT can produce and specially in many cases when we do control loop analysis is some PSU etc.



Some random example about Siglent FRA frequency selectvity when it sweeps.
Keep in mind that this is not normal BodePlot sweep when BodePlot control Generator.
Here BodePlot "sweep" over 200kHz to 700kHz frequency band, but input to Channel 4  is 455kHz fixed frequency signal so we can see BodePlot selectivity figure in this point.





About speed.

It is good to remember that with Channel Gain Auto sweep is much slow than Ghannel Gain Hold.
Also speed may be different depending input levels when Channel Gain Auto (some may call it as AGC).
Also speed may be different depending communication between oscilloscope and generator.

It is good to realize that after every frequency hop first it need solve level and adjust optimal V/div using input channels fine steps. After every step it can do full adjust for all channels in use (500uV to 1V/div when 50ohm and 500uV - 10V/div when 1Mohm)
Naturally when AGC is off (HOLD) measurement dynamic range is reduced. Still enough for most cases (but it need also that user carefully set first this optimal settings for input channels so that input signals do not clip in any case.)

Also for normal passive low order filters we really do not need so many steps as is used in OP test example (naturally this is ok for speed test purpose as used well ok)

With low frequencies this is still slow. It simply can not do it faster as long as we keep same measurement performance because this IS time domain oscilloscope and all is still based to it.

Example for low frequencies (10Hz  - 100Hz)
100ms/div horizontal speed takes 100ms/div. 10 div take 1s and then... whole this 100kpts need FFT  and repeating... repeating until enough for solve level and specially phase, and then go to next frequency step. Higher frequencies naturally t/dif is faster and FRA steps are then faster.
If reduce this, also performance is reduced. 
Example some AD2 can do it much faster. AFAIK it do not work as frequency selective sweeping receicver. If there is some other signals, say example harmonics or non harmonic spurs it also listen these.

Naturally there can do quite fast in oscilloscope BodePlot  if use more or less different method, and reducing reducing performance. Also Siglent can do. But 100dB or max 140dB dynamic range (with also good harmonics and spurs attenuation) is not so easy peace of cake.

So, When compare do not compare oranges and shoes. Not even apples and oranges or let us also then know all about performance differences. Speed is just one thing.


Then as @Berni well said... some real "bodeplotters", as example Hewlett-Packard 89410A (Sadly Lady Fiorina destroyed Hewlett-Packard, later known as Agilent and Keyshitg)
These instruments are very extremely different "animals" and expesive and they do just this but, and well, but not lot of other things. These BodePlot performance is really in different level.. as also price.

I remeber in old times I have some R&S dedicated "bode plotter". One was heavy and expensive R&S SWOB IV. Fast and "nice" to use for some filters etc. But not nice to move (heavy).

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