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selected digit (3rd, 4th or 5th digit from the left) can be recorded using the
event marker pen on the B & K Level Recorder Type 2305 or 2307. The
digit which operates the marker relay is selected by a screwdriver operated
switch situated on the rear panel. The cable (AO 0027) for connection of
the frequency marking socket on the 2010 and the event marker socket on
the Level Recorder Type 2305 is supplied with the 2010.
The cable (AO 0035) for connection of the frequency marking socket on
the 2010 and the event marker socket on the Type 2307 is not supplied, but
is available upon request.
A short pulse is supplied to the event marker pen every time the selected
digit changes. When the selected digit becomes zero, a continous signal is
provided to the marker pen until the digit changes again.
5.3.4. Mixer Section
1st Mixer
The input signal, f
5
,
is mixed with a signal fv from the voltage controlled
oscillator. If the tuned frequency of the analyzer is f
5
,
the frequency of the
V.C.O. is:
Where f
0
is a fixed frequency of 1.2 MHz. Hence the signal emerging
from the first mixer is always of frequency 1.2 MHz. The modulator is
balanced in order to suppress the frequency fv which may be present, in the
resulting signal. Instructions for balancing the modulator may be found in
section 3.1.3. The modulator is followed by a bandpass filter in order to
suppress the image frequency of 1.26 MHz signal which may be present.
This frequency if mixed with the 1.23 MHz signal may produce an un-
wanted 30kHz signal. The band pass filter suppresses the 1.26 MHz signal
by at least 85 dB.
·
2nd Mixer
The signal from the first mixer is fed into a balanced modulator which
also receives a 1.23 MHz signal from the fixed frequency section. A signal of
centre frequency 30kHz is produced together with some unwanted high
frequency components. The high frequency components are removed by a
49
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