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Cheap bode plot device...

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Fungus:

--- Quote from: bsas on June 25, 2022, 06:27:39 am ---To be fully honest I am still quite a beginner and learning. So, doing those stuff "by hand" or doing some cleaver stuff like the Youtube tutorial gets me more confused than I would like to admit :(... I know it is my fault, but, well, I need to be honest with myself :D

--- End quote ---

If you put a sine wave on screen then sweep the frequency around manually you can easily see when the amplitude changes.

Another trick is to feed it white noise then look at the spectrum - all the frequencies sumiltaneously!

Warpspeed:
I have always wished to have something to accurately measure relative amplitude and phase that did not cost a king's ransom to take Bode plots from an actual real working circuit.

I bought myself an ancient HP3575A phase meter a long time ago. These are now getting rather old, but still excellent, and I picked one up from e-bay for a couple of hundred dollars which is typical.  The specifications are pretty good 3Hz to 11Mhz, phase to 0.1 degrees resolution, absolute and relative amplitude measurement to 0.1db resolution.
And a full service and repair manual is available free from the internet.

If you are only interested in analog circuitry, amplifiers, filters and such, its probably all you need, along with a suitable function generator of course.

Sadly it will not work with switching power supplies, it needs reasonably clean signals. Trying to resolve millivolt signals below volts of switching spikes and a very noisy ground is just not practical with either an oscilloscope or something like the HP3575A.

So at the moment I am building a very narrow dual channel tracking filter for it.  Basically two channels tunable from a very few Hz to 40Khz with only about 3Hz bandwidth.  That should block the wideband switching noise and allow only the specific frequency of interest to reach the HP3575A.
Still working on this right now and so far its looking quite encouraging.

Just discovered this thread while surfing the internet looking to see what other people are doing to take Bode plots on real functioning equipment, and not just playing with pie in the sky circuit simulations.

Simulations are wonderful things, a world where noise problems do not exist, there is infinite dynamic range, and components are perfect. 
The real world is rather more hostile...

jasonRF:

--- Quote from: Warpspeed on August 24, 2022, 08:59:30 am ---I have always wished to have something to accurately measure relative amplitude and phase that did not cost a king's ransom to take Bode plots from an actual real working circuit.

I bought myself an ancient HP3575A phase meter a long time ago. These are now getting rather old, but still excellent, and I picked one up from e-bay for a couple of hundred dollars which is typical.  The specifications are pretty good 3Hz to 11Mhz, phase to 0.1 degrees resolution, absolute and relative amplitude measurement to 0.1db resolution.
And a full service and repair manual is available free from the internet.

If you are only interested in analog circuitry, amplifiers, filters and such, its probably all you need, along with a suitable function generator of course.

Sadly it will not work with switching power supplies, it needs reasonably clean signals. Trying to resolve millivolt signals below volts of switching spikes and a very noisy ground is just not practical with either an oscilloscope or something like the HP3575A.

So at the moment I am building a very narrow dual channel tracking filter for it.  Basically two channels tunable from a very few Hz to 40Khz with only about 3Hz bandwidth.  That should block the wideband switching noise and allow only the specific frequency of interest to reach the HP3575A.
Still working on this right now and so far its looking quite encouraging.

Just discovered this thread while surfing the internet looking to see what other people are doing to take Bode plots on real functioning equipment, and not just playing with pie in the sky circuit simulations.

Simulations are wonderful things, a world where noise problems do not exist, there is infinite dynamic range, and components are perfect. 
The real world is rather more hostile...

--- End quote ---
If you have a digital scope that can save waveforms, then you can import the data into your favorite software (I use Octave) and use FFTs to conceptually do the same thing.  The longer your waveform captures, the narrower the effective filter bandwidth is, and the more gain you get on the sinusoid of interest to pull it out of the noise.  The signal phase shift is the same as the relative FFT phase at the frequency of interest.  Don't forget to use a window so the sidelobes of the noise don't swamp your signal of interest. 

The simple way of doing this does require saving one file for each frequency of interest.  So it isn't fast, but it can work.  I have never tried it with super noisy data, but I have done it using a square-wave source and just wanted to use the fundamental to measure frequency response. 

jason

mawyatt:
If you search using Bode Plot, you'll find all sorts of good information on Bode Plots. Some of which are related to the Siglent DSO which host a nice Bode Function that employs a "Frequency Selective" feature to help out of band signals & noise. These also use Dynamic Input scaling to achieve large effective Dynamic Range performance. Here's a few we've been associated with:


https://www.eevblog.com/forum/testgear/diy-transformer-for-use-with-bode-plots/msg4182022/#msg4182022
https://www.eevblog.com/forum/projects/things-coming-together-bode-plot-diy-isolation-transformer-peltz-oscillator/msg4288363/#msg4288363
https://www.eevblog.com/forum/projects/bode-routine-shows-op-amp-output-z/msg4307539/#msg4307539
https://www.eevblog.com/forum/testgear/capacitive-impedance-plots-with-sds2104x-plus-bode-function/msg4335745/#msg4335745

Best,

jasonRF:
Those Siglent results look very nice!   Channel separation on that scope is excellent.  Perhaps unless someone writes their own code to control instruments, I suspect that is the most cost-effective way to get automated Bode plots up to those high frequencies. 

I have a couple of Picoscopes, and the free Bode plot app also does the dynamic scaling.   My lowly 2204a has a siggen that is limited to 100 kHz, and the combination of low siggen output voltage (2V peak) and scope sensitivity (10 mV/div) limit dynamic range to 70 dB or so.  I posted an example here
https://www.eevblog.com/forum/testgear/picoscope-2000/msg4129702/#msg4129702

When that was my only scope, I used my slow method and an external siggen (that only has front-panel controls) to get Bode plots up to 12 MHz. Of course my slow way also has dynamic scaling, since the user needs to adjust the scope before saving each waveform. 

Jason

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