[sdiy] filter response on a scope

Tom Wiltshire tom at electricdruid.net
Mon Oct 18 01:50:13 CEST 2010

I've wondered about this as a good PIC project.
You could easily generate a good quality sine wave with the required sweep, and output the sweep and the sine ready for patching to the filter/scope.
Perhaps you could also feed the filter output back into the PICs ADC, so that you can record the frequency response curve for signals that are too slow to see easily on a 'scope.


On 17 Oct 2010, at 21:37, Scott Juskiw wrote:

> Use XY mode on your scope. Y is driven by a sawtooth wave (upward ramp) that also controls an oscillator's pitch. Feed the oscillator (sine wave output) into your filter's audio input. X is driven by the output from your filter. Set the filter cutoff frequency as appropriate to get a nice display.
> On 2010-10-17, at 2:02 PM, David G. Dixon wrote:
>> Hey Team,
>> Lastnight I designed a board to extract 18 additional filter responses from
>> my 2164 4P LPF, thus bringing the total number of filter responses from the
>> module to 22 (4 LP, 4 HP, 6 BP, 2 N, 4 AP, and 2 more as yet unspecified).
>> It will involve a 1U panel with 18 jacks on it sitting next to the 3U filter
>> panel (hence my last query about PCB mounting), or it could just be a 4U
>> panel.
>> I'd like to be able to look at the filter responses on my old Tektronix
>> scope, but I'm not sure how.  I'm thinking a sawtooth sweep of expo FM on a
>> sine-generating VCO into the filter at constant cutoff, and then perhaps
>> taking the filter output through an envelope follower and then into the
>> scope to see just the positive outline of the response.  However, if anyone
>> has a nice method for doing this, I'd love to hear it.
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