Ramsey Electronics FX-440 Manual page 24

05 is a common base buffer which affords good isolation, low input
impedance and broadband characteristics. The buffered output from 05
is fed into U3, UPB569, a dual modulus
+
64/65 prescaler, the output of
which is fed to the A and N counters in U6. The output is further buffered
and amplified by 016, the VCO buffer which feeds T1 through C35 for
receive, and 01 O
through C56 for transmit.
The VCO is is frequency modulated by microphone amplifier U4. D5 and
R31 perform an interesting function. Remember that the VCO control
voltage has a range of about 1.0 volts DC (low frequency) to 7.0 volts
(high frequency). Therefore, more modulation voltage is needed at the
higher frequencies. As the VCO control voltage increases, D5 turns on
and places R31 in parallel with R33, reducing the resistance in the line
to half and thereby increasing available modulation voltage.
The VCO requires a very pure source of well-filtered DC, free of AC hum,
alternator whine or other disturbahce. R19 and the 47 uf C40 form a
basic low pass filter: Transistor 04 serves as an electronic capacitance
multiplier. The actual effect of the filter is that the beta of 04 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, 12.8 MHz.
• 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.
• 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") compares the 12.5 KHz
Reference Frequency with the "intended" 12.5 KHz output of the
N-divider and sends correcting pulses to the VCO to keep the
output of the N-divider right at 12.5 KHz.
• A "lock detect signal" circuit.
The reference oscillator is internal to U6, governed by Y2. The precision
of the 12. 8 MHz reference oscillator can be adjusted by trimmer C81.
The R divider feeds 12.5 KHz to the phase detector section of U6 (12800
KHz divided by 1024).
The output of the UPB569,
+
64/65 prescaler, U3 is AC coupled via C57
to pin 1. The dual modulus prescaler, is 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
Ref. 22
FX-440