Bode ploting

I am a Portuguese science student and although I am a little ashamed to be doing this question, I really need to do it as I have a practical exame this week.

One of the tasks we have to do is to make a frequency analysis of a circuit in order to get a bode plot. I know how to do it making manual increments of frequency and registering the amplitude. However, I have to do it automatically and without using multisim. Somewhere in my notes I have written that it has something to do with the SWP mode of the function generator but I still don't know how to do this kind of plot automatically using only a function generator and an oscilloscope.

I wonder if you could help me.

Sorry for the trouble.


Reply to
Loading thread data ...

Your function generator needs to have a sync output of some sort, which you use as the trigger input for the scope. Even if there is not a separate pulse output for this, there is likely a VCO control voltage. It will be a slow sawtooth that rises in voltage as the function generator rises in frequency. If you trigger on the

*falling* edge of this, at the end of a sweep, the scope trace will start at the next sweep start.

You will of course need to set the scope sweep very slow, similar to the function generator sweep time.

Alternatively, you could put the scope in X-Y mode and use the VCO signal as the scope sweep itself.

If you view the circuit response with the scope, you won't get a single-line trace like the typical Bode plot. Instead, you will get an envelope whose upper edge is the same as the Bode plot. There will be a symmetrical negative edge on the envelope, and between them will be more-or-less solid light.

If you want a conventional single-line plot, you will need to rectify and filter the circuit output before feeding it to the (DC coupled) scope input. It won't be a conventional log-log Bode plot... for that you'd need to add log converters.

Though you may need to use the class-supplied scope and function generator for this assignment, note that for audio frequencies you can do everything with a computer and sound card. It can generate the sweep and plot the response, including showing it in log-log format.

Best of all, it can use faster (and often better) methods than a simple sweep. See the Frequency Response Measurement topic for my Daqarta software at:

Best regards,

Bob Masta DAQARTA v5.10 Data AcQuisition And Real-Time Analysis

formatting link
Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI DaqMusic - FREE MUSIC, Forever! (Some assembly required) Science (and fun!) with your sound card!

Reply to
Bob Masta

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.