EEVblog #396 – Bode Plotting on Your OsciloscopePosted on December 8th, 2012 16 comments
Dave shows you a neat trick on how to get a real time frequency response bode plot on your oscilloscope using your function generator. Useful for filter or system response characterisation.
Forum Topic HERE
Spectrum Analyser Video
You should teach electronics at a University… if you won’t directly, I’ll use you myself
This is not a bode plot. A bode plot also includes phase.
Alternatively, you can use a white noise gen and FFT of your scope, pretty darn near to a scalar network analyzer
As we say where I’m living: “Knowlege is like jam: the less you have, the more you spread it”.
That’s why some advanced signal generators actually have marker outputs. My Agilent 3325 can sweep from 0hz to 22MHz and generate up tot 40vpp it has several outputs you connect to a scope.
There is a sweep start output that you connect to the trigger input of the scope. this makes sure the scope runs in sync with the sweep. there is a marker output that you connect to channel two of the scope. this is a 5 volts pp signal and you put the scope at 0.5 volts/div vertical and set the trace in such a way you only see the vertical portion of that pulse.
The generator is programmed to swepp and by pressing the ‘Marker’ button on the generator , whenever the internal sweep counter crosses the marker value the immediate fruquency is latched in the generator.
in essense where that vertical line of channel 2 is , is whatever frequency is dispayed on the function generator. the generator changes its display to show you what frequency corresponds to the marker. as the sweep is a pure digital affair the frequency indication is absolute.
this not only allows yo to bode pots but it also allows you to have a marke ron the screen an know the exact frequency the generator is at. so you could find the -3db point ( point where only 70.7% of the amplitude is left ) by hitting the marker left/right buttons an reading the amplitude on the scope. once found you look at the generator and you know the frequency.
these older synthesizers were built especially to do this kind of stuff as spectrum analysers invariably do not have the dynamic range required and DSA’s are bloody expensive. I have another synthesized source that is very ‘pure’ as in no harmonics and has differential output capability including floating above ground.
The 3325 does is not an ARB but a real sine oscillator steered by a DDS circuit driving a pll. So it has excellent linearity at its output.
We frequently use it to find the resonance point of the voice coil for example. This is a tuned LC network but as the resonance point changes depending on the position of the arm you need to measure it at multiple positions to model it.
To do the active filters in the positional servo control loop of the harddisk we have a few of those Stanford SRS756 DSA machines.
Dave, with your skills you could design a small moderately high speed multichannel ADC external card to do that stuff on a PC (uADC?). That would become an instant purchase for those people who need a good quality analog (AC&DC) input and don’t want to shell out the money needed for an industrial DAQ or a professional multichannel audio card.
Should it have a good bandwidth, most HAM radio operators would also find it immensely useful for I/Q SDR decoding.
(minor typo in the title: “osciloscope”)
Use a Cleverscope – 80 dB dynamic range! Not a lousy 8 bit ADC – but 14 bit! Bode plot built in. And you can copy and paste the bode plot into your word doc – and all nicely labelled. But I know Dave doesn’t like PC scopes, ah well. And you get Gain and Phase, pretty good for stability etc. Check it out!
By the way all those exclamation marks are to be said in Dave’s unique style!
most function gens can output the control voltage for the VFO. Mine does
You could build up a opamp bridge-rectifier plus a logarithmic output stage. That way you can have the response and the frequency axis to be logarithmic if fed into the scope running in x/y mode. Btw. this should also help to overcome the dynamic issues with the 8-Bit adc resolution.
Nice Blog. Took me hours to watch and keep up.
Very neat trick! I just recreated this here at my desk. Just wanted to point out that the 3000 X-Series you use here has a built in function generator, so this could potentially be done with just one box. Only difference is that there is no direct sweep mode, but there is FM modulation (simulated with a 50kHz sine, ramp modulation of 50kHz). Thanks as always for the entertaining videos!
Dave, please pronounce the surname “Bode” properly:
Scraping from Wikipedia…
Hendrik Wade Bode (pronounced Boh-dee in English, Boh-dah in Dutch).
Born 24 December 1905, Madison, Wisconsin, Died 21 June 1982 (aged 76) Cambridge, Massachusetts
“Something should be said about his name. To his colleagues at Bell Laboratories and the generations of engineers that have followed, the pronunciation is boh-dee. The Bode family preferred that the original Dutch be used as boh-dah.”
Dave, why didn’t you use your scope in X/Y mode and use the sweep output of your generator for the Y axis?
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