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compressor circuit of the B.F.O. is used. In this instance the pen-lifting
arrangement of the Level Recorder can be controlled from the frequency
blocking circuit by making the appropriate connections to the REMOTE
CONTROL jack of the B.F.O. In cases where the entire frequency range
(200-200ooo Hz) of the B.F.O. is utilized, the normal frequency blocking,
which functions outside the scale graduation, should be set out of operation.
The writing pen of the Level Recorder
can
now be lifted from the paper
outside the frequency range of interest and a proper working of the com-
pressor also at the initial frequency
(2QO
Hz) is ensured during the automatic
scan.
If
the described methods is not utilized, the following would take place:
No signal will be present in the range 200ooo Hz to 200 Hz
(outside
the scale
graduation), i.e. the compressor of the B.F.O. will be in such a condition to
give full output signal of the B.F.O. Consequently, when the scale pointer
goes inside the scale graduation
(200
Hz) full output level will be transmitted
at 200Hz, and after the chosen time delay
(COMPRESSOR
SPEED) the signal
level will be compressed to the proper
(preset)
value. A deflection on the
recording paper which is not a response of the measured object would thus
be recorded.
Output Amplifier Section.
The voltage from the low-pass filter is fed to the control grid of the first
tube in the two-stage output amplifier via a
variable
potentiometer. This
potentiometer is operated by the knob marked OUTPUT LEVEL on the
front panel of the Oscillator and is used for continuous adjustment of the
output power. The
gain
of the amplifier is stabilized by negative voltage
feedback. The anode circuit of the output tube is coupled to the impedance
matching circuit.
Four different output impedances are available and can be chosen by the
switch on the front panel marked MATCHING IMPEDANCE. The different
positions of the switch are indicated by 6, 60, 600 and 6ooo ohms respectively
and the output voltage is taken from the terminals marked LOAD.
It
should
be noted that the output impedance of the Oscillator is only approximately
20
%
of the indicated values, but with correct loading maximum power output
is obtained with minimum harmonic distortion
(Fig.
1.3). In addition, correct
loading ensures the output voltage to be independent of the frequency to
within
±
1 dB for frequencies between 200 and 200ooo Hz. A fifth position
of the switch MATCHING IMPEDANCE is marked "Att." and connects the
output transformer to an attenuator,
variable
in steps of 10 dB from
400 fA-VOlts to 12.5 volts. The attenuator is operated by the switch marked
ATTENUATOR on the front panel. In this position of the impedance switch
the output circuit is connected to the screened jack on top of the front panel.
The output impedance is constant and approximately 50 ohms. The accuracy
of the attenuator is better than
±
2
%.
The voltage on the output terminals is indicated by a vacuum-tube voltmeter
8
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