Ramsey Electronics FX-440 Manual page 104

DIVIDE
BY
1024
DIVIDE BY N
PROGRAMMING
PHASE
DETECTOR
U5
LOOP
FILTER
DIVIDE BY
64165
PRESCALER
U3
07
vco
SYNTHESIZED
OUTPUT
FREQUENCY
Inserted into this circle is the frequency synthesizer (U6 with U3), which
compares the output of the VCO with the output of its own 12.80 MHz
Reference Oscillator (see Y2) and also with the frequency programming
which you have set up. After making these comparisons, it gives an
instruction to the VCO in the form of a precision voltage applied to the
varactor diodes, and the cycle repeats itself at lightning speed.
I
Try dividing 12800 KHz by 1024 on your calculator. If you get an answer
of 12.5 (KHz) you are correct and perhaps on your way to seeing how the
FX transceiver can be programmed in 12.5 KHz steps! If you are curious
about U3, its job is to divide the VCO's VHF output down so that U6
need only analyze ("count") 1 /64 of it in order to send voltage
adjustments back to the VCO. This prescaling keeps the input frequency
to U6 within specifications.
"Will the circle stay unbroken?" asks an old folksong. If the PLL's circle
of activity is broken by incorrect phasing, the loop is said to be
•unlocked.•. ff all is going well, the loop is locked. The loop is UN-locked
only when the phase detector gets so unhappy that what is being fed into
its two inputs is so far •out of whack" that it cannot make corrections. If
the loop 1s unlocked, no useful control voltage reaches the VCO. ff the
VCO operates at all, it will be at a wrong or unknown frequency.
The phase "detector• can also be called a phase "corrector." It does
more than just detect or look at the two incoming 12.5 KHz signals. It
sends correcting pulses to the VCO to keep it in phase with the 12.5 KHz
output of the crystal reference oscillator.
A good word for understanding "phase" is synchronization or "sync." The
process of tuning a musical instrument is a good illustration of sync or
phase. When two guitar strings are very close but not exactly on the
same note, there is a discernible third sound, a sort of rumble that
disappears as soon as both strings are exactly on the same frequency.
By tuning the one string to the exact pitch of the other, you have
corrected the phasing of the two independent frequency sources. Pilots
Assm. 44 FX-440