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By means of a level regulator circuit (COMPRESSOR) which can be con-
trolled from an external voltage, it is possible to keep the voltage current, or
sound pressure constant during measurements, when the oscillator is being
used as a power source,
(LOAD).
Oscillator and Mixer Section.
Fig. 1.1 shows a block diagram of the complete Oscillator. The diagram is
divided by means of a dotted line showing the oscillator and mixer section,
and the amplifier section respectively.
Fig. 1.1. Block diagram of the BFO 1013.
The fixed oscillator is tuned to
1.2·
MHz and can be frequency modulated
by means of an arrangement also shown in Fig.
1.1.
The reactance tube circuit
acts as a variable inductance and the modulation swing can be continuously
varied from 0 to
±
2ooo Hz by means of a potentiometer on the front panel
of the
apparatus,
marked FREQUENCY DEVIATION.
By means of the switch marked MODULATION FREQUENCY the frequency
of the built-in saw-tooth oscillator may be chosen. Frequencies of 2-4-8-
16-32 and 64 Hz are available. The oscillator is a blocking type, tuned to
approximately 7 MHz, and the frequency of the saw-tooth oscillations is
selected by the changing of the grid resistor.
Provision is made for external modulation, for which the external generator
should be connected to two terminals of the jack on the front plate marked
REMOTE CONTROL. For external modulation a voltage of approximately
5 volts is necessary when a modulation swing of
.±
2ooo Hz is required.
The impedance of the external generator must be low
(approximately
1 k.Q).
When external modulation is employed the switch marked MODULATION
FREQUENCY must not be in position
"Mod.
Off" as in this position of the
switch the reactance tube is cut off.
A variable capacitor of 60 pF, inserted in the tuned circuit of the fixed
oscillator, and operated by the knob marked FREQUENCY INCREMENT,
4
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