Log Amplifier-Filter, Circuit Description
Log Amplifier-Filter, with
Log Amplifier-Detector, provides the ability to display
signals in either a linear or log mode. In log mode the calibrated display range is 90
contains the first five of nine log stages, and
contains the last four.
also contains
circuitry for using one of the log stages as a switchable 10 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
MHz). The filter is in narrow mode for resolution bandwidths of
to
reduce noise generated in the log amplifier stages.
Control lines from
IF Control drive circuits of
which control the log amplifier
filter, log/linear switching, and 10 linear gain step.
Log Amplifier Stages
In LOG mode, the five log amplifier stages in
and the four log amplifier stages in
limit their gain in sequence with increasing signal level to provide 90 of log display range.
In LINEAR mode, signal-level-dependent components are switched out of the signal path, and
a
linear display is provided.
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
V),
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
V.
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
between the emitters of
and Q12 (primarily
the resistance of CR25 and CR26). The formula for computing gain is:
is at a minimum (approximately 150 for small signals. The small-signal gain of the
stage (about 10
is set by the dc bias current through the log diodes. As the signal level
at the
emitter increases, signal current cancels bias current in the log diodes, increasing
The gain of the stage for large signals is reduced to nearly unity (0
because
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.
1
Содержание 85662A
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