reference to a zero level signal at an output of the Mixer. Since this corresponds
with a –10dBV signal within the Mixer the actual range of input levels to the
correlator unit is –50dBV to –70dBV.
12.18.2
Circuit
The circuit diagram is shown on Drawing No. DE221.
Board B94/1 carries the circuitry for Forward Path A and Low Level Meter A.
Board B95/2 carries the circuitry for Forward Path B and Low Level Meter B.
Board B96/1 carries the Inhibitor circuitry.
Board B97/2 carries the circuitry of the gates and for the correlator meter.
12.18.3
Circuit Description
12.18.3.1
Forward Path A
VT1 and VT2 are a d.c. coupled low gain (about 6dB) pair which provide a
high input impedance (more than 2M
Ω
) and have a considerable overload margin
(more than 20dB) so that the high input impedance is maintained at high input
levels. R1 and R2 provide a potential of –10.3V to which the base resistor R3 is
connected. The common point of these three resistors is boot-strapped to the
emitter of VT1 by C2 so as to raise the input impedance.
The collector of VT2 is coupled by C3 to the circuit R7 and R8, D1, VT3. The base
of VT4 receives the potential across D1 which at low signal levels is non-
conducting. As the signal level is increased D1 limits the negative excursions of the
base of VT4 to about 0.5V, and on positive excursions VT3 is turned on so these
are also limited to about the same voltage.
When VT3 is off, current flows from the +20V line through R10 and R9 in
series to charge C12 in the inhibitor section.
VT4 provides a gain of about 18dB for low level signals. As the signal from
VT4 reaches about 0.5V D2 limits the negative swing and VT5 limits the positive
swing, VT5 also switching current to the inhibitor similarly to VT3.
VT6, VT7 and D3 and associated components provide another stage
identical with the previous one.
VT8, VT9 and D4 and associated components provide a further very similar
stage. The nominal gain is 16dB but it is adjusted by selection of AOT8 so that the
gain from the input to the collector of VT8 is 58dB.
The collector of VT9 is connected through D5 to the base of VT10. The
diode is included to prevent reverse voltage breakdown of VT10. The emitter is
connected to the mid point of a potential divider R29, R30 across the positive supply
and the collector is connected to a negative point provided by R31, R32 across the
negative supply. A connection is taken from this collector through terminal 2 of this
board to terminal 2 of the gates board.
12.18.3.2
Low Level Meter A
The circuit for driving this meter is tapped from Forward Path A by connecting
the emitter of VT8 to the base of VT11. At this point the nominal gain from the input
