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of this circuit is to control automatically the output of the Beat Frequency
Oscillator by applying an external voltage. The external control voltage
should be fed into the screened jack marked COMPRESSOR INPUT on the
front panel of the Oscillator.
A variable potentiometer, marked COMPRESSOR VOLTAGE is inserted in the
input circuit of the regulating amplifier and can be used as level control for
the output from the Oscillator, when automatic regulation is employed. The
regulating amplifier has a linear frequency characteristic from 200 to 200ooo
Hz and should have an input signal of approximately 3 volts on the grid
of the amplifier tube for full regulation. The input impedance, measured
across the terminals of the jack marked COMPRESSOR INPUT is approx
-
imately 10 kohms, and the obtainable range of regulation is at least 45 dB.
The amplified control voltage is rectified in a full-wave double-diode
rectifier, designed to give a DC output voltage proportional to the average
value of the control voltage.
The speed of regulation can be varied with the switch marked COMPRESSOR
SPEED on the front panel of the Oscillator. Regulation speeds of 30- 100-
300 or 1ooo dB/sec may be chosen by changing the value of the capacitor in
the R-C filtering network for the rectified control voltage. When the switch
COMPRESSOR SPEED is in position COMP. OFF the output from the recti-
fier is short-circuited thereby disconnecting the automatic regulation.
It is also possible to obtain different regulation characteristics dependent
on the position of the potentiometer marked OUTPUT LEVEL. This can be
seen from Fig. 1.2.
To obtain good regulation even when maximum output power is required
from the Oscillator, the level of the high frequency voltage from the 1.2 MHz
fixed Oscillator is raised approximately 10 dB when the automatic regulation
is switched in.
The anode-circuit of the pentode in the variable
,
a -amplifier is tuned to
1.2 MHz, thereby forming a band-pass filter, the output of which is fed to
the mixer.
The frequency of the variable oscillator can be continuously altered from
1.2 MHz to 1.0 MHz by means of a specially designed variable capacitor.
This capacitor is made with a high degree of accuracy and a maximum
deviation of o. 7 degrees from a logarithmic frequency curve is obtained.
The Frequency Main Scale can be remotely scanned via a mechanical drive.
The arrangement is intended to be driven from the B
&
K Level Recorder
Type 2305 via the Flexible Shaft UB 0041 (1 meter). In this manner the
frequency scan of the B.F.O. can be driven in synchronism with the pre-print
of the recording paper of the Level Recorder. The built-in worm gear, having
a ratio of 50 :1, causes a scale pointer movement corresponding to one
octave, when the input drive of the B.F.O. turns through three revolutions
.
The worm gear can be set and released by a magnetic slutch which is
operated from a switch (AUTOMATIC SCANNING) on the front panel of the
6
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