The VCO requires a very pure source of well-filtered DC, free of AC hum,
alternator whine or other disturbance. R19 and the 47 uf C40 form a basic
low pass filter. Transistor Q4 serves as an electronic capacitance multiplier.
The actual effect of the filter is that the beta of Q4 multiplies the 47 uf for a
virtual capacitance effect of a much larger device.
Stage G: The FX- Transceiver Synthesizer PLL
The MC145152 IC incorporates the equivalent of 8000
individual transistors and contains the following
circuits:
• A crystal reference oscillator governed by Y2, 10.24 MHz.
• A counter or "frequency divider" circuit set externally to divide the
crystal oscillator output by 2048, for a Reference Frequency output
of 5 KHz.
• A second counter or frequency divider that divides the frequency
from the Prescaler (U3) by the externally programmed number that
we call "N".
• A third frequency divider ("A") also used for programming
• Control logic circuitry which permit the "N" and "A" counters to
work together for channel programming.
• The Phase Detector (or "phase corrector") which compares the 5
KHz Reference Frequency with the "intended" 5 KHz output of the
N-divider and sends correcting pulses to the VCO to keep the
output of the N-divider right at 5 KHz.
• A "lock detect signal" circuit. The reference oscillator is internal to
U6, governed by Y2.
The precision of the 10.240 MHz reference oscillator can be adjusted by
trimmer C81. The R divider feeds 5 KHz to the phase detector section of U6
(10240 KHz divided by 2048).
The output of the UPB569, Ã64/65 prescaler U3 is AC coupled via C57 to
pin 1. U3 is a dual modulus prescaler, controlled by pin 9 of U6. The
prescaled output of the VCO is fed to the A and N counters. The "N" number
programmed on the diode matrix is predetermined to divide this frequency
down to 5 KHz for phase comparison with the 5 KHz output of the crystal
controlled reference divider. Maximum "N" is 65,535, achieved by switching
FREQ. N = BINARY PROGRAMMING VALUES
222.000 44,400
225.000 29,600
1010 1101 0111 0000
1010 1111 1100 1000
223500 (KHz)
5 (KHz)
To Next Page
Loading...