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WJ-8718-iy/rt. Mr
KLUCIVCK.
U1KUU1T DESCRIPTION
Buffer amplifier U1A isolates C4 from the following circuits.
Q2 is a
threshold blocking AGC action for weak signals.
The base of Q2 is
biased to
approximately +0.2 V.
If the emitter of Q2 is lower than about +0.8 V, Q2 will be
turned off and no AGC action can occur.
When the output of U1A is greater than
+0.8 V, Q2 conducts and a gain control voltage appears across R13.
When the Manual
gain mode is selected, Q3 and Q6 will be turned on and will clamp the voltage on R13
to ground, and +5 V will be applied to the RF Gain potentiometer on the front panel.
Inverting summing amplifier U2B combines the voltage at R13 (which will be zero in
Manual gain mode) and the voltage on the RF GAIN control (which will be zero in
Fast or Slow AGC modes).
The output of summing amplifier U2B is buffered by OP AMP UID and
fed to the 455 kHz amplifier on A4A7.
Zero volts from UID allows the 455 kHz
amplifier to operate at maximum gain while a negative output from UID causes the
gain of the IF amp to be reduced.
A sample of the IF gain control voltage from U2B is also applied to RF
AGC threshold detector Q5. This threshold detector causes the gain reduction to occur
only in the 3rd IF amplifier, unless the signal at the RF input of the receiver and in
the early stages of the receiver is great enough to ensure a good signal-to-noise ratio
even in the early stages.
The operation of the threshold, detector is the same as that
of Q2, except with polarities reversed to allow for the inversion which occurs in U2B.
The base of Q5 is biased around -2.7 V so the IF gain control voltage must be more
negative than -3.3 V for Q5 to conduct.
To achieve the desired relationship between AM Detector output and RF
gain reduction requires that the control diode current rise slowly at first, then more
rapidly as the received signal strength increases further (exponentially).
This
current/voltage relationship is obtained through a shaping network comprised of U2D,
R47, R48, CR5, and R31. The actual current for the control diode is supplied by buffer
U2A.
In the Manual gain mode, this voltage is proportional to the RF input signal
voltage.
Its polarity is inverted by OP AMP U1C and it is applied through R49 and
front panel switches A10A1S1B and S2C to the meter.
This allows the receiver to act
as a tuned voltmeter whose calibration depends on the setting of the RF GAIN control.
In the AGC modes, the voltage out of U1A increases approximately
linearly with signal voltage up to the AGC threshold level of 3 V (RF input).
Above
this level the U1A output is compressed by AGC action to be nearly proportional to
the logarithm of the RF input voltage. By using a shaping network composed of R41,
R50, R51, CR6, CR7, and CR8 to suitably compress the output of U1C at low signal
levels, the signal strength meter is made to be approximately linear in dB over a
greater than 100 dB range.
Resistors R50 and R51 control the amount of compression
and the exact fit of the meter scale with signal strength.
If an accurate source of
variable signal level is available, these fixed resistors may be replaced with variable
ones which may be adjusted for best tracking of the meter. The variable resistors may
then be removed, measured and replaced with fixed resistors of the same value.
3-39
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