3.3.5.4.
Adjusting overcritical coupled band filters
The mostly overcritical coupled band filters in the FM-.IF amplifier must be damped and adjusted
alternately. The connection wires of the damping resistor should be
as
short
as
possible.
Indicator
Fig. 3.3-1 1
Adjusting overcritical coupled
IF band filters
Repeat the adjustment of a band filter several times.
3.3.5.5.
Symmetry of a ratio detector
The S-shaped demodulator characteristic (fig. 3.3-17) should be linear and symmetric to the zero-axis
crossing with normal control. For determining the symmetry,
it is convenient to adjust the
ratio
detector in amplitude limitation
.
The rectified voltage
at
the limiter capacitor C2 is halved in the vol-
age divider and is symmetricalwith refence to the LF demodulator output (fig. 3.3-12). For recording
the s-shaped demodulator characteristic, supply the signal to the base of the
last
IF stage.
R
1
U
n
R
6
10k
Y
n
Fig. 3.3-12
Adjusting the symmetry of
a
ratio detector
3.3.6.
Examples of wobbling
3.3.6.1.
Test setup for checking and adjusting an
FM
receiver
see
fig. 3.3-13. Examples of oscillograms are shown in figs.
5
and
6.
The centre frequency in the FM-IF
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