Signal Generator for Free; Notch Filter Progress

So I’m trying to put together an analog filter to cut the mains hum from a ELF/VLF radio receiver.
The sig gen built into the BitScope DSO is very limited, only offering certain fixed frequencies. This isn’t a huge amount of use for evaluating a notch filter circuit. Neither was the use of a white noise source as input to the circuit with a freq domain trace at the output, the notch wasn’t visible (had a bit more smoothing been available in BitScope DSO, this should have worked).

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I thought my easiest next step would be to put together an analog sweepable oscillator on the breadboard. But then, while falling asleep, I had a Eureka moment: the laptop I’m using for the DSO display has a soundcard! I’m running Ubuntu on the lappie, and the Audacity audio recording/processing tool has the facility for generating swept-frequency signals. So I attached jumper leads to a mini jack plug and stuck that in the headphone socket, bingo!

By generating a 1 second sweep, 20Hz-200Hz, in Audacity and loop-playing it, I got a reasonably respectable view of the filter response in the freq domain display of the DSO.

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Next up I just need to tweak the resistor values to focus the notch bang on 50Hz. Putting pots in parallel with the current values will allow me to find these values. However, silly me, I don’t have a multimeter to check the pot values. I did see one in a hardware shop for AUS$11 the other day, and given that’s about the price of a pint of beer here, I guess I have no excuses.

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Author: Danny Ayers

Web research and development, music geek, woodcarver. Originally from rural northern England, now based in rural northern Italy.

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