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signals in either a linear or log mode. In log mode the calibrated display range is 90 dB. A4A3
contains the first five of nine log stages, and A4A2 contains the last four. A4A3 also contains
circuitry for using one of the log stages as a switchable 10 dB gain step. This is used only in
linear mode.
After being processed by the first five log amplifier stages, the 21.4 MHz IF signal is applied
to the log amplifier filter, the bandwidth of which is either narrow (approximately 1 MHz)
or wide (>20 MHz). The filter is in narrow mode for resolution bandwidths of 130 kHz to
reduce noise generated in the log amplifier stages.
Control lines from A4A9 IF Control drive circuits of A4A3 which control the log amplifier
filter, log/linear switching, and 10 dB linear gain step.
Log Amplifier Stages @
In LOG mode, the five log amplifier stages in A4A3 and the four log amplifier stages in A4A2
limit their gain in sequence with increasing signal level to provide 90 dB of log display range.
In LINEAR mode, signal-level-dependent components are switched out of the signal path, and
linear display is provided.
a
LOG Mode
A simplified schematic of a log amplifier stage is shown in Figure 1. In LOG mode, the
LOG/LIN control line is high (about +7 V), Ql6 is on, forward biasing log diodes CR25,
CR26, and all other log diodes in this assembly. CR25 and CR26 are Schottky diodes
with a forward bias voltage of approximately +0.4 V. Qll is an emitter follower used as a
voltage source which drives Q12 through the resistance of CR25 and CR26. Q12 forms a
common-base amplifier tuned to approximately 21.4 MHz. The gain of the amplifier is set by
the ratio of R48 to the total resistance RT between the emitters of Qll and Q12 (primarily
the resistance of CR25 and CR26). The formula for computing gain is:
stage (about 10 dB) is set by the dc bias current through the log diodes. As the signal level
at the Qll emitter increases, signal current cancels bias current in the log diodes, increasing
becomes very large. Signal current flow through R49 keeps the stage gain slightly above unity
to compensate for the input loading of the following stage.
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