Agilent Technologies 8564EC Service Manual page 275

General Troubleshooting
Block Diagram Description
A13 Second Converter
The A13 second converter down-converts the 3.9107 GHz 1st IF
(low-band and millimeter band) to a 310.7 MHz 2nd IF. In the
microwave band, it passes the 310.7 MHz 1st IF from the A10
YIG-tuned filter/mixer to the A15 RF assembly. The converter
generates a 3.6 GHz second LO by multiplying a 600 MHz reference.
Bandpass filters remove unwanted harmonics of the 600 MHz driving
signal. 1st IF and 2nd LO signals are filtered by cavity filters.
Second IF Amplifier (part of A15)
The second IF amplifier (SIFA) amplifies and filters the second IF.
Access to this pre-filtered signal is available at the rear panel 2ND IF
OUTPUT (Option 001 only).
The external mixing input from the front panel IF INPUT connector is
also directed through the SIFA. A dc bias is placed onto the IF INPUT
line for biasing external mixers.
Third Converter (part of A15)
The third converter down-converts the 310.7 MHz IF to 10.7 MHz. A
PIN-diode switch selects the LO signal used. For normal operation, a
300 MHz LO signal is used. The signal is derived from the 600 MHz
reference PLL. During signal identification (SIG ID ON), the 298 MHz
SIG ID oscillator is fed to the double balanced mixer on alternate
sweeps.
Flatness Compensation Amplifiers (part of A15)
The flatness compensation amplifiers amplify the output of the
double-balanced mixer. The variable gain of the amplifier (0 to 45 dB)
compensates for flatness variations within a band. Band conversion loss
is compensated by step gain amplifiers in the IF section.
Control for the amplifiers originates from two DACs on the A3 Interface
assembly. (DAC values are interpolated approximately every 17 MHz
based on data obtained during the frequency response adjustment.) The
flatness-compensation control circuitry on A15 converts the RF GAIN
voltage, from A3, into two currents: RF GAIN1 and RF GAIN2. These
currents drive PIN diodes in the flatness compensation amplifiers.
356 Chapter 7