Agilent Technologies 8564EC, Service Manual

Page: 349 / 512

454 Chapter 9
IF Section
Log Amplifier (P/O A4 Assembly)
Log Gain Compensation
The gain of A4U508 is nominally 6.8, measuring from pin 3 to pin 8. To
check the log offset/gain compensation circuits inject a +10 dBm signal
into J3 with the 8564EC or 8565EC spectrum analyzer set to log
mode. Measure A4U503 pin 3, V
in
(1) and A4U508 pin 3, V
out
(1) and
record the results. Decrease the input level to
−
40 dBm and make the
same measurements recording V
in
(2) and V
out
(2).
The gain is then:
This gives an offset-independent gain measurement.
Video MUX
The video MUX switches the video output between linear, log and
4.8 kHz IF (for digital RBWs). The demod video is an unused feature.
The easiest way to troubleshoot this circuit is to look for blown FETs.
Bad FETs are characterized by having significant gate current. Only
one of the signal lines LIN_VIDEO, IF_VIDEO or LOG_VIDEO should
be high (+15 V) at any given time. The others should be low (
−
15 V).
Also look for a voltage drop of several volts across the gate resistors
R601, R605, R609, or R613 when in either the off or on state. This
indicates gate current and thus a bad FET.
V
OUT
1
( )
V
OUT
2
( )
–
( )
V
IN
1
( )
V
IN
2
( )
–
( )
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