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5.4. BEAT FREQUENCY OSCILLATOR SECTION (B.F.O.)
A block diagram of the B.F.O. section is shown in Fig.5.10.
Regulating
From Fixed Amplifier
Oscillator L---r---'
1.2 MHz
Amplifier
L....-----1
+
Rectifier
>
Compressor
Input
Fig.5.10.
B.F.O. section block diagram
Attenuator
B.F.O.
Output
1711.11
The generator section is comprised of a mixer which receives two fre-
quencies:
a) f
0
-a fixed frequency of 1.2 MHz from the fixed frequency section,
'
and
b) fv -a variable frequency (between 1.0 and 1.2 MHz) from the voltage
contra lied osci II at or.
The B.F.O. output frequency is fs where fs
=
f
0 -
fv· This frequency is
displayed on the Nixie tube display and also indicated by the frequency
scale pointer and scale range selector.
Before the mixing process, the 1.2 MHz fixed frequency signal is passed
through a variable gain amplifier. The gain of the amplifier is controlled by a
compression signal. The compression signal enters the 2010 at the socket
marked COMPRESSOR INPUT. A variable potentiometer (COMPRESSOR
VOLTAGE) controls the level of the compression signal, which is then
passed through a full-wave rectifier. The integration time constant of the
rectifying circuit is determined by the setting of the COMPRESSOR SPEED
selector. Compression speeds of 3, 10, 30, 100, 300 and 1000 dB/sec are
55
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