CCWN 75:32
                                                   
         The frequency stabilizer was designed to enable CCW
operation on 3550 Khz for transmitting and receiving, while
retaining the capability for non-coherent operation over the
normal operational range of the TEN-TEC. The CCW operation of the
PM-1 requires frequency stabilized local oscillators at 3550 Khz
on transmit, and either 3551 or 3549 Khz for receiving to present
the required 1 Khz audio input to the coherent filter. The
circuit is shown in schematic 3.           A sample of VO1 module
(VFO) output is squared up by Q1 and Ula, and is fed to the input
of a type D flip flop. The TTL output of the standard is rich in
harmonics, so there is appreciable 3500 Khz energy available at
the clock input of the same D flip-flop. This digital mixer will
then provide the difference between the output of the VFO and the
3500 Khz harmonic of the standard, namely 50 Khz plus or minus
undesired drift. A repetition of the process in an exclusive OR
gate (U3a) comparing this to the 50 Khz standard will result in
no voltage offset being applied to the varactor diode if there is
no difference in the sample and the standard frequency signal. A
difference, however, will create a voltage offset to the varicap
diode which will pull the VFO back to the desired frequency.      
   A similar process is employed in the receive chain, only this
time 1 Khz is used in the exclusive OR gate comparator cirCuit to
lock up the receive local oscillator for the direct conversion
type receiver on either 3551 or 3549 Khz.          The stabilizer
board is pictured in photo 5. It is installed in its own minibox,
and powered from the +5 volt regulated source available in the
frequency standard. RF chokes and bypassing are included in both
the receive and transmit offset control lines, to minimize the
TTL interference leak-through.          Modification of the
TEN-TEC VFO board is divided into three areas:       (1)
Attempting to remove some of the worst mechanical instabilities   
           (2) Modification of the switching system to direct the
proper offset control voltages to the varactor diode from the
transmit or receive comparator.       (3) Installation of the
varactor diode assembly.                Stabilization is
accomplished by both mechanical and electrical means. The
oscillator board should be removed from the unit, and a small
DPDT should be added using solid wire with direct leads to wire
in the 80/40 meter switching. The existing stranded wire 80/40
meter leads should be removed, freeing the panel mounted DPDT
switch for later use. A small piece of printed circuit board is
cemented between the VFO tuning capacitor and the coil shield to
minimize mechanical vibration, and the VFO board is then
reinstalled. Three photo sockets are mounted in the rear of the
PM-1 case, to provide tie points for cable connections for the
sampling, receive offset and transmitter offset lines. The
varactor diode and its isolation capacitor are coupled to the VFO
at terminals Gl/Al, and control lines run to the panel mounted
DPDT switch, for use in switching the receive offset (RO) or
transmit offset (TO) lines to the varactor diode as desired.
Connection of the output terminal of the VFO through a 10 Pf
capacitor to the VFO sampling phono connector (SI) completes the
modification of the unit.