Ten-Tec SSB-CW-AM Instruction Manual page 106

only one digit is actually lit at a time. The processor continuously
scans through all six digits very rapidly however so they appear
to be lit simultaneously. As each digit is enabled by the high
current driver IC U5, the processor feeds a BCD output to the 7
segment decoder U4. This chip then lights the individual
segments needed to form each number. A separate transistor,
Q12, operates the decimal point.
Let's go back now and discuss the details of generating the first
LO signal. As we have mentioned, this oscillator is phase-locked
for accuracy and stability. The circuitry responsible for this task
is mostly contained in the phase-locked-loop chip U3. This IC
contains a reference crystal oscillator, programmable dividers
and phase detector. The major sections of the synthesizer are
shown in block diagram form in Figure 4R.
Fig. 4R: Synthesizer Block Diagram
The reference oscillator determines the overall accuracy of the receiver
and for this reason we have included a trimmer capacitor, C2, for setting
this frequency. Specifically, we are using a 3.579545 MHz crystal which is
a standard part used in television sets. The trimmer is used to set the
crystal slightly off frequency, to 3.580 MHz. The oscillator is then divided
down by a factor of 1432 to provide a 2.500 kHz signal to one side of
1254 - Reference 13