CCWN 75:60
                                                   
STABILIZATION OF THE SB-303 RECEIVER FOR CCW BY CHAS. WOODSON

          For CCW reception the receiver must be tuned to the
desired frequency with an accuracy and stability of a Hz or two.
For CCW reception with the Petit filter(CCwN 75:17), the receiver
must be tuned such that the audio output for the desired signal
is 1000 Hz, plus or minus 1 or 2 Hz.            The Heath SB-303
is a double conversion solid state receiver. Mine was assembled
by my father, K0ESE, and has given very good service on the air
and for a variety of CCW experiments. I found I could set my
SB-303 to within 200 cycles of the desired signal and that it was
stable within 10 or 20 cycles for the period of a QSO. From my
experience, these values are above average for quality receivers,
but far from adequate for CCW.            I have used a number of
procedures to stabilize my SB-303. The one I have found most
effective for CCW is to mix the HFO, VFO, and BFO to get an
oscillator-derived frequency 1000 Hz higher than the desired
signal frequency. This oscillator derived frequency is then
locked to a reference from the station standard by a phase lock
loop which controls the VFO. If the HFO, VFO, or BFO shift in
frequency, the VFO is automatically shifted to compensate for the
change, thereby keeping the receiver tuned to the desired
frequency.            Heath Company kindly provided a handy way
to control the VFO more precisely than the customary tuned dial.
The FSK capability was designed to shift the VFO for keying but I
found I could use it to control the VFO frequency. I ran a wire
from the FSK (brown wire) connection on the VFO to one of the
unused phone jacks on the back of the SB-303. This was the only
modification of the receiver necessary for CCW. The circuit
internal to the VFO is apparently something like the following,
and such could be added to any receiver VFO.               When
the FSK connection is shunted to ground with a variable resistor
(50k), the voltage at the FSK point can be shifted. A variation
from 10 to 2 volts shifts the VFO about 2000 Hz.           
Figure 1 describes the overall connection of the elements to
receive a 14,050,000 signal. The HFO (about 22,895,000 Hz) is
mixed with the VFO (about 5,450,400 Hz), to get about 17,443,100
Hz. This signal is then mixed with the BFO (about 3,392,110 Hz)
to get about 14,051,000 Hz which is supplied to the phase lock
loop.