A trick I used years ago for audio FSK RTTY on two meters was to put a
frequency divider behind the 555. Generating the 2125 and 2295 frequencies for
transmitting was done at 10 times the freq in the 555, and a resistor was
switched in the 555 timing circuit to shift the frequency. Doing it this way
allowed the non-synchronous RTTY signal to modulate the tone frequency without
generating excessive clicks or transients in the audio output. A good sine
wave output was guaranteed by simply using a low pass filter on the output of
the square wave generated from this circuit. A single transition would be of a
different frequency somewhere between 2125 and 2295, depending on just where
the RTTY signal went from mark to space.
73 - Jim W5ZIT
skipp025 <[EMAIL PROTECTED]> wrote:
> W.r.t. early homebrew encoders, I never really liked the 555
version
> because of the non-sinewave output. Since the encoder need to cover
> more than an octave, fixed filtering of any kind couldn't be used to
> clean it up.
You can find the 555 used as a pretty good tone generator in many a
circuit, including a fair number of dtmf pads. If you look at the
circuit on the sonic web page you'll notice the low pass filter,
which works pretty well. Even with the values shown I found the
described circuit puts out a lot more audio than required for a
typical radio so after all the filtering you still had a high value
resistor in series to knock the level down.
> So I went with the XR2206 function generator which put out a nice
> clean sine wave. Only other problem was frequency stability:
Advantage to the 555, which by nature of design and operation is
relatively immune to voltage and temp drift with more than a
reasonable amount of change.
> only many years later after spending much time & effort looking
> for the most stable Rs & Cs did I discover when trying to build
> a 1000 Hz sine generator that the chip temperature affects the
> operating frequency.
Never had much of a a problem with the 555 and the circuit is dirt
cheap to build.
cheers,
s.
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