TROUBLESHOOTING AND REPAIR
RF LEVEL/AM
Leveling Loop 6D-5.
The leveling loop controls the 15 to 1056-MHz signal level at the detector diode (CR20)
on the Output PCA; therefore the leveling loop also controls the signal level at the
buffer amplifier (Q7) on the Output PCA. The leveled RF signal is proportional to the
leveling loop control voltage which appears at TP7 on the Modulation Control PCA.
The Schottky detector diode (CR20) generates a temperature-dependent DC voltage.
This is a non-linear function of the applied RF voltage, thus temperature compensation
and linearization are necessary. The detector diode signal is low-pass filtered by L34
and C34, and is offset by the voltage across temperature-compensating diode CR19.
Q1, Q2, and associated components on the Modulation Control PCA form a current
source circuit that provides bias current for CR20 and CR19.
The offset detector diode voltage at U7 pin 3 on the Modulation Control PCA is
linearized by amplifier U7 and its associated feedback components. Potentiometer
R28 provides an adjustment for best detector linearity at low RF levels. Thus, the
voltage at U7 pin 6 (TP2) is proportional to the level of the RF signal incident on the
detector diode CR20 on the Output PCA.
This voltage is applied to pin 2 of the loop-integrator/summing amplifier, U41. The
leveling loop control voltage (plus any AM) is applied to pin 3 of U41. U41 drives the
leveling/AM modulator through U14 and U15 and circuits that compensate for
modulator non-linearity. R35, R36, CR5, and CR6 form an additional linearizing
network that acts on the control signal. Amplitude modulation is achieved by
summing an appropriately scaled modulation signal with the DC leveling loop control
voltage.
The amplitude modulator on the Output PCA consists of PIN diodes CR27 through
CR33 and associated components. Attenuation through the modulator is a function of
bias current through these PIN diodes. This current is provided by the modulator-
linearizer circuit on the Modulation Control PCA. U14 and associated components
provide modulator series diode current, while U15 and associated components provide
shunt diode current.
Modulator attenuation is approximately proportional to the modulator control
voltage on TP8. Proportionality is required to maintain constant leveling loop
bandwidth as modulator attenuation varies. Minimum attenuation is obtained with a
modulator control voltage of 10V, while maximum attenuation is obtained with 0V.
Comparator U10 and associated components form an unleveled indicator circuit. The
comparator senses the modulator control voltage at TP8. This voltage is normally less
than +11V, and the comparator output is high. If the modulator control voltage
exceeds +11V, the modulator attenuation is at a minimum, and the leveling loop
becomes inoperative (unleveled). This condition could be due to a fault or some
abnormal operation such as overmodulation. In this case, the comparator output
(UNLVLL) goes low. The Controller PCA senses this low and causes the front panel
STATUS indicator to flash and displays an unleveled status code (241) if interrogated.
6D-5
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