On Wed, 29 Mar 2000, Andy Kelley wrote:

> On Wed, 29 Mar 2000 06:12:33 -0600 Scott Dattalo <scott@DATTALO.COM>
> writes:
> > On Tue, 28 Mar 2000, Andy Kelley wrote:
> >
> > > Anyone have a cheap ( also real-estate is at a premium ) method
> > for
> > > detecting wheter or not ANY tone is present?
> >
> > Andy,
> >
> > It's hard to answer your question optimally without addition
> > information.
> >
> > 1) What's the frequency of the tone?
>
> Variable.  I  am using the circuit to decode cw and would like to be able
> to vary the tone.
>
> > 2) What's the amplitude?
>
> A nice sinewave.

Nice 1mV sine wave or nice 100V sine wave?

>
> > 3) Is there any additional signal present along with the tone?
>
> static...
>

Spehro's circuit, or a variation there of, sounds like the ideal solution. His
circuit is essentially a high pass filter followed by an amplifier. As it
stands, the circuit requires a minimum amount of voltage to excite it, but
grounding the emitter gives it really high gain. This is good and bad. It's good
in that it thresholds your signal. In other words, the 'static' to which you
allude would have to be very large (one good vague adjective deserves
another) to trigger the signal. It's bad if the "nice sinewave" is too
small. Another minor problem is that Spehro's circuit's threshold/sensitivity
has a strong temperature bias - but this IS a minor issue. Some of these effects
can be mitigated by biasing the transistor. However, if you add an emitter
resistor then the minimum drive level of the transistor will be affected.

Scott