Agilent Technologies 8564EC, Service Manual

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452 Chapter 9
IF Section
Log Amplifier (P/O A4 Assembly)
The input switch switches between log and linear modes. In addition it
contains a 20 dB attenuator which is used only in digital resolution
bandwidth settings. CR207, CR208, and CR209 form the input switch.
CR205 and CR206 switch in R234 when in linear mode to maintain a
constant impedance at J3. CR210, CR211, CR212, and CR221 switch
the 20 dB attenuator in and out.
LO Switch
Refer to function block F of A4 log amplifier schematic diagram (sheet 2
of 4) in the 8560 E-Series Spectrum Analyzer Component Level
Information.
The LO switch switches the limiter input between the 10.7 MHz path or
the 10.6952 MHz VCXO path.
Synchronous Detector
A wide dynamic range linear detector is realized by the limiter (block
G), the isolation amplifier (block H), the LO amplifier (block I), and the
detector/mixer (block J). The combination of these circuits form what is
commonly known as a synchronous detector.
The input signal is split between two paths. One path flows through the
isolation amplifier and the other path flows through the limiter and LO
amplifier The path flowing through the limiter generates the LO for the
detector/mixer block. The path through the isolation amplifier drives
the RF port.
To troubleshoot this group of circuits set the RBW to 300 kHz. Inject
10.7 MHz at +6 dbm into J3. Probe the gate of A4Q404 or A4Q405 with
a scope. Look for a 0 to
−
3 V square wave. Decrease the input power
from +6 dBm to
−
84 dBm in 10 dB steps. The square wave signal should
remain unchanged. It is normal for the phase of the signal to jitter at
the lowest signal levels.
The signals at the gates of A4Q404 and A4Q405 should be 180 degrees
out of phase from each other. If they are not 180 degrees out of phase or
one of the signals are not present, troubleshoot the LO Amplifier or the
FETs in the mixer. If the signal is not a symmetrical square wave,
troubleshoot the LO amplifier. If the signal drops out prematurely or is
not present at all, troubleshoot the limiter or LO amplifier.
Repeat the procedure for an RBW
≤
100 kHz. If the log amplifier works
in the 300 kHz RBW but not in the narrower RBWs, troubleshoot the
log narrow filter in the limiter or isolation amplifier. A4CR302 and
A4CR303 are varactor diodes in the limiter filter and are used to tune
the filter.