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3.9.
AUTOMATIC FREQUENCY CONTROL (AFC)
36
The AFC circuit enables the VCO to be locked onto resonance peaks. The AFC is acti-
vated by depressing the AFC push button on the front panel.
The AFC only operates in the four lower bandwidths (3, 16Hz to 100Hz). The AFC con-
sists of a phase detector which compares the phase between a point in the third mixer
and the output from the output amplifier of the mixer, and produces a DC voltage de-
pending on the phase difference. At a resonance point, the phase difference is 90° and
no DC voltage is produced. At a point away from the resonance, the DC voltage pro-
duced is fed back as a control signal to the VCO until the resonance frequency is again
reached.
The pull-in capability of the AFC circuit depends on the scale range and the chosen band-
widths. The pull-in ranges are shown in Table 3.3. The pull-in or tracking speed of the
AFC circuit also depends on the scale range and bandwith. The pull-in speeds are
shown in Table 3.4.
~
3,16Hz
e
10Hz
31,6 Hz
100Hz
X
0,1
10
-
-
-
X
1
10
100
100
-
x10
10
100
1000
1000
740079
Table 3. 3. Pull-in range of the AFC
(Hz)
~
3,16 Hz
e
10Hz
31,6 Hz
100Hz
X
0,1
1
-
-
-
X
1
1
20
20
-
x10
1
20
200
400
740080
Table 3. 4. Pull-in speed of the AFC
(Hzs -t)
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