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Bode plots with an oscilloscope

Our group needed a good microphone for our weekly conference calls as a part of us are in Portugal and another in Glasgow. The >100 GBP camera microphone which we had did not provide satisfactory results, none at all. This is the reason why I decided to build a microphone preamplifier myself, which supposedly had to perform better and replace the camera mic. The design and soldering process however, provoked an idea for a more esoteric measurement setup.

I saw the idea implemented originally by Dave Jones in his EEVBlog. I had a look at his video again but a 25 minute ranting video seemed way too long for me to watch, so I decided to squeeze the general idea into five minutes. Here is a basic explanation and demonstration of how to do a Bode plot with a scope.

The circuit I used for the test is a non-inverting amplifier, realized with an opamp (TL072) with having an f-dependent feedback, effectively forming a high-pass filter. Practically we need some sort of DC reject filter as to avoid opamp saturation. Here is my circuit:

And two screenshots of a linear and a log sweep:

A linear sweep from 10Hz to 5MHz
A logarithmic sweep from 10Hz to 5MHz

There is some noise in the system and performing averating to filter out the noise is not an option here as one can never achieve the same phase on every sweep/acquisition. Turning on averaging just in my case just distorted the picture even more.

It would be fun to find out how to do a phase plot on the scope, so that a full picture of the transfer characteristics of our circuit can be acquired.

Date:Tue Oct 27 23:09:02 GMT 2014


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