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signal is then fed into the second mixer via a 1.2 MHz band pass filter. The
centre frequency of the signal is transformed to 30kHz at the second mixer
by mixing with a 1.23 MHz signal. The resulting 30kHz signal is fed to the
30 kHz centre frequency band pass filter where the 1000 Hz and 316 Hz
bandwidths of the input signal are obtained. From this stage the signal is fed
to the selector circuit.
If a bandwidth in the range 3.16 to 100 Hz is required, the signal from
the 1.2 MHz band pass filter is fed to the third mixer. In the third mixer,
the signal frequency is transformed to 750Hz, by mixing with a 30.75 kHz
signal from the fixed frequency section. The 750 Hz signal is fed to the
750 Hz centre frequency band pass filter where the selected bandwidth is
obtained.
The signal from this final mixer is then fed to the output section via the
selector circuit.
The selector circuit feeds the output section amplifier with either: the
filtered signal from the second or third mixer, with or without bandwidth
compensation, or: the signal issued from the input section amplifier, con-
ditioned by the internal linear or standard ABCD weighting networks or by
an external filter.
The signal from the output amplifier is fed to the RMS rectifier, where
the averaging time can be selected. The level of the signal is indicated on the
meter scale. The signal is also present at the recorder output. The read-out
selector offers the choice of AC, linear or logarithmic DC signals.
The analyzer section and the beat frequency oscillator (B.F.O.) section
are supplied by the same voltage controlled oscillator (V.C.O.). The signal
from the V.C.O. which is fed to the first mixer is also fed to the B.F.O.
where it is mixed with a 1.2 MHz signal to give a sinusoidal output of
frequency corresponding exactly to the tuning frequency of the analyzer.
The level of the B.F
.0.
signal can be regulated by means of the com-
pressor circuit.
5.2. AMPLIFIER SECTION
5.2.1. Input Section
A block diagram of the input section is shown in Fig.5.2.
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