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If the INPUT OVERLOAD light flashes, turn the INPUT SECTION ATTENUATOR
switch to a higher range.
If the OUTPUT OVERLOAD light flashes, turn the OUTPUT SECTION ATTENUATOR
switch to a higher range.
Further further details of frequency analysis with the 2010, see chapter 4.
3.7. USE OF THE BFO SECTION
34
The BFO (Beat Frequency Oscillator) section incorporates a compressor, the main func-
tion of which is to compensate for frequency-dependent components in the measure-
ment system when swept-frequency measurements are being taken.
The compressor may be disabled by switching COMPRESSOR SPEED to "Comp. Off".
With the compressor off, the BFO OUTPUT is completely under the control of BFO OUT-
PUT VOLTAGE and BFO ATTENUATOR. The BFO output voltage (before the attenuator)
may be read off the meter by selecting "BFO" on the METER AND RECORDER switch.
The reading (and the BFO output) will be unaffected by the settings of the INPUT SEC-
TION ATTENUATOR and OUTPUT SECTION ATTENUATOR.
When the compressor is used, its action is such as to reduce electronically the BFO out-
put in order to keep constant the voltage at the COMPRESSOR INPUT. This voltage is at-
tenuated by the logarithmic COMPRESSOR VOLTAGE potentiometer before being recti-
fied, smoothed and compared with a fixed reference equivalent to 0,5 V RMS. Thus to
increase the demanded voltage at the COMPRESSOR INPUT, the COMPRESSOR VOL-
TAGE control is turned anticlockwise. In this instance the numbers round he knob are
for reference only. 0,5 V, the minimum voltage with which the compressor will regulate,
corresponds to "1 0"
.
The voltage fed to the COMPRESSOR INPUT need not be the same frequency as the
BFO, for the compressor will operate on all frequency components between 2Hz and
200kHz.
The choice of COMPRESSOR SPEED determines the time-constant of the smoothing fil-
ter which follows the COMPRESSOR INPUT rectifier. Too high a speed results in ripple
from the rectifier modulating the BFO output waveform (see Fig.3.4); too low a speed re-
sults in a failure of the compressor to track resonances in the test object in swept-fre-
quency analysis.
To regulate the BFO output signal, the procedure is as follows:
1
.
Turn the COMPRESSOR VOLTAGE knob fully clockwise.
2.
Turn the BFO OUTPUT VOLTAGE knob fully anticlockwise.
3.
Connect the feedback signal to the COMPRESSOR
INPUT.
4.
Select a suitable regulation speed using the COMPRESSOR SPEED switch.
5
.
Turn the BFO OUTPUT VOLTAGE knob slowly clockwise to the maximum position,
checking that the compressor circuit works.
6.
Turn the COMPRESSOR VOLTAGE knob anticlockwise until the desired level is ob-
tained.
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