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IF Gain Control (6~
The IF Gain Control interfaces with A4A5 Step Gain, the Log Amplifiers in A4A2, A4A3, and
Bandwidth Control @
Bandwidths from 3 MHz to 100 kHz (BW 2 +14.8V)
These bandwidths are produced in the 21.4 MHz IF by four parallel tank circuits (two in
determines bandwidth) is controlled by PIN diodes used as variable resistors. The resistance
of these PIN diodes is determined by the average current through them, which is generated
by Q9. Either R60, R61, or R62 is switched in by U2, depending on which bandwidth had
been selected. These potentiometers (3 MHz, 1 MHz, and 300 kHz) determine the amount of
current sunk by Q9, which in turn determines the bandwidth. If no current is sunk by Q9,
the filters go to their highest Q (determined by factory-selected resistors), which yields the
100 kHz bandwidth.
Bandwidths from 30 kHz to 3 kHz (BW7 = -0.6V)
These bandwidths are produced in the 21.4 MHz IF by five crystal filters (two in A4A8 and
three in A4A4). Their Q (which determines bandwidth) is controlled by PIN diodes, the same
as for the wider bandwidths. Refer to the preceding discussion of the wider bandwidths. QlO
is the current sink for bandwidths from 30 kHz to 3 kHz. The amount of current it sinks is
selected by U2 and adjusted by potentiometers R65 and R66 (10 kHz and 3 kHz), depending
on which bandwidth is selected. If no current is sunk by &lo, the filters go to their lowest Q
(determined by factory-selected resistors), which yields the 30 kHz bandwidth. Ql should be
off except for the 3 MHz to 100 kHz bandwidths.
Bandwidths from 1 kHz to 10 Hz (SWITCH = OV)
These bandwidths are produced in the 3 MHz IF (A4A7) by five crystal filters. The Q of
these filters is controlled by resistors which are switched in by diodes. These, in turn, are
controlled by Q25, Q26, Q28, and Q29. The transistors switch lines which are named after the
bandwidths they produce. Q27 is on (saturated) only for bandwidths 1 kHz through 10 Hz.
2 A4A9
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