On Sun, 9 Jun 2002, Dave Tweed wrote:

> > Input is via a series .02uF cap to the base of a 2N2222a, 330K resistor
> > to Vcc, 56K resistor to ground, PIC input is from the collector with a
> > 4.7K pullup, emitter to ground.  It does what it's supposed to
> > (amplify). I get a nice pulse to ground at the primary frequency of the
> > string...  but I'm also getting what looks like some harmonics (or maybe
> > just noise) that are apparently getting amped enough to trigger the PIC
> > input (Schmitt trigger, A0 on a 12CE674).
>
> The input cap, along with the input resistance of the amplifier (bias
> network in parallel with the transistor itself), is creating a high-pass
> filter with a cutoff of a few hundred Hz, so you're already emphasizing
> harmonics over the fundamental.

As others have also noted.  I just followed the original design to the
extent possible...  except looking at it again, I think I misplaced a
decimal point.  The original used a 100nF cap, I tried 10 and 20.  Duh.

> First step would be to increase this
> cap to to 0.1 uF or even 0.47 uF. It might also help to raise the input
> impedance of the amplifier by putting about 100 ohms or so in series with
> the emitter, although this will reduce the gain somewhat.
>
> Another thing to try would be to add some higher-frequency rolloff to help
> kill off harmonics and noise. This could be done by adding either a
> capacitor from the collector to ground (about 0.15 uF for a cutoff
> frequency of 225 Hz) or a capacitor from the collector to the base
> (negative feedback - about 0.02 uF would do it here).

I'll try that, thanks.

Dale

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