BRUEL & KJAER 1022 Instructions And Applications page 11

The reactance circuit is needed to control the actual frequency deviation
(modulation swing) of the fixed oscillator by producing an inductive react-
ance across the oscillator's tuned collector. The deviation can be varied
linearly from 0 to
±
250 Hz where the sensitivity is approximately
5 mV/Hz.
A saw-tooth generator determines the frequency of modulation i.e. the
rate of change of the frequency swing of the oscillator. Frequencies of
1-1.6-2.5-4-6.3-10-16-25 Hz are available. Provision is also made for
modulation by an external generator. Alternative modulation frequencies or
wave shapes can then be chosen and frequency swings of up to
±
300 Hz
obtained (but the modulation sensitivity varies).
The signal from the fixed oscillator is passed to a regulating amplifier
which controls the output level. The regulating amplifier is used to perform
the oscillator stop and dead zone blocking. A compressor amplifier can be
switched in to control the regulating amplifier so that constant output level
is obtained. When the instrument is being used for instance to power a
loudspeaker, the compressor circuit can be used with a microphone to main-
tain a constant sound pressure level.
The compressor circuit consists of an amplifying stage and a full-wave
averaging rectifier stage. The signal from the rectifying stage is then used to
vary the gain of the regulating amplifier. A variable potentiometer (Com-
pressor Voltage) in the input circuit of the regulating amplifier can be used
to control the output power from the instrument when automatic regula-
tion is used.
The speed with which the variation in output level is regulated back to
normal depends on the setting of the Compressor Speed knob, but also
depends to a certain extent on the amount of the deviation from the normal
level. Compressor speeds of 30 - 100 - 300 - 1000 dB/sec. are available
and are determined by the integration time constant of the rectifying cir-
cuit.
The input impedance of the Compressor Input is approximately 25 kQ
and the maximum range of regulation is 50 dB. Regulation characteristics
for different positions of the Output Level potentiometer are shown in
Fig.3.2.
The variable oscillator is of similar design to the fixed one except that it
has a variable capacitor in its tuned circuit to vary the oscillation frequency
between 100 and 120 kHz. A worm gear, connected to the capacitor
11