Ramsey Electronics FX-440 Manual page 105

of multi-engine planes (or boats) make slight RPM adjustments to get all
engines in phase. If you have two audio oscillators handy, listen carefully
as you adjust both to EXACTLY the same frequency. Another analogy is
the process of tuning a BFO equipped receiver to a perfect null or "zero
beat.• If you are exactly on frequency, you hear nothing, even though the
BFO and incoming signal are both fully present. In a PLL, we want the
"beat note" to be a perfect zero or constant direct current (DC), and this
DC controls the VCO.
What breaks the circle? What UN-locks the loop, the PLL? Let's think
and make a list of possibilities:
• VCO will not tune in proper frequency range
• Bad Reference Oscillator (IC, crystal, component?)
• Invalid • N • number?
• A physical break in the loop?
Caused by (what else?):
• Bad solder connections
• Incorrect components
• Defective components (rare)
• Incorrect programming
The preceding is about as untechnical as we know how to get in giving a
hint of how the PLL Frequency Synthesizer portion of your transceiver
functions. The Glossary of Terms in this book might give some additional
help, but there's really no substitute for just exploring the knowhow of
modern communications technology by whatever learning method works
best for you.
Now, let's take another look at the whole circuit in a more formal way.
The PLL Frequency Synthesizer IC:
The MC145152 IC incorporates the equivalent of 8000 individual
transistors and contains the following circuits:
o
A crystal reference oscillator governed by
Y~.
12. 80 MHz.
I
o
A counter or "frequency divider" circuit set externally to divide
the crystal oscillator output by 1024, for a Reference
Frequency output of 12.5 KHz.
o
A second counter or frequency divider that divides the frequency
from the Prescaler (U3) by the externally programmed
number that we call "N".
o
A third frequency divider ("A") also used for programming
o
Control logic circuitry which permit the • N • and •A• counters to
work together for channel programming.
o
The Phase Detector (or "phase corrector") which compares the
12.5 KHz Reference Frequency with the "intended" 12.5
KHz output of the divide by N counter and sends
correcting pulses to the VCO to keep the output of the N
divider right at 12.5 KHz.
o
A "lock detect signal" circuit.
FX-440
Assm. 45