Instruction manual

STC1• 6
When this voltage on pin 5 of U2:B goes higher than 5 volts of the supply on pin
6 of U2:B, the output of U2:B goes high, thus lighting the LED clip indicator.
The other part of the signal goes to U3, a switched capacitor lowpass 8th order
Butterworth filter. These filters are really neat since they don’t need any high
accuracy frequency dependent parts, and don’t require pancakes. All that is
needed is a good steady TTL clock signal to set the cutoff frequency. Internally
these chips have a divide by 100 cutoff in relation to the clock frequency, so to
get a cutoff frequency of 15KHz, we needed a clock frequency of 1.5MHz.
Since 555 timers don’t like to run at this speed, a stable source was needed
that was better than a CMOS oscillator. Well, a little overkill never hurt anyone,
so now there is a crystal oscillator running at 6MHz, and divided by 4 by U6:A
and U6:B to give us 1.5MHz. Now we have a cutoff frequency right at 15kHz
with very little drifting in frequency.
There is also an internal opamp inside of U3, which allows us to filter the clock
frequency out of the audio signal before going out to J2 and then on to your
transmitter. R14,16,17, C9, and C16 are the parts included for this filter (Notice
similar part values around U1:A)
Well, that about sums it all up. Now we will get on to the fun stuff, and make
ourselves a really great kit that we understand!
NOTE TO NEWCOMERS: If you are a first time kit builder you may find this
manual easier to understand than you may have expected. Each part in the kit
is checked off as you go, while a detailed description of each part is given. If
you follow each step in the manual in order, and practice good soldering and kit
building skills, the kit is next to fail-safe. If a problem does occur, the manual
will lead you through step by step in the troubleshooting guide until you find the
problem and are able to correct it.