Agilent Technologies 8564EC, Service Manual

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Chapter 7 359
General Troubleshooting
Block Diagram Description
Fractional N PLL (part of A14)
The fractional N PLL produces an output of 60 MHz to 96 MHz. This
PLL output serves as the reference frequency for the YTO PLL. A
one-to-one relationship in frequency tracking exists between the
fractional N PLL and the YTO. A change of 1 MHz in the fractional
N PLL will produce a 1 MHz change in the YTO frequency.
IF Section
The IF section processes the 10.7 MHz output of the RF section and
sends the detected video to the ADC/interface section. The following
major assemblies are included in this section:
A3 interface assembly
A4 log amplifier/calibration oscillator assembly
A5 IF assembly
The spectrum analyzer uses trace-data manipulation to generate the
5 dB/DIV scale from the 10 dB/DIV scale. The A3 interface assembly
amplifies and offsets the 10 dB/DIV video to generate the 2 dB/DIV
scale. The 1 dB/DIV scale is generated from the 2 dB/DIV scale through
trace data manipulation. The first 50 dB of IF gain (log and linear
mode) is achieved using the linear step-gain amplifiers on the A5
assembly. The video-offset circuit on the A4 assembly provides the
remaining 60 dB of log mode IF gain. The linear amplifiers on the A4
assembly provide 40 dB of linear mode gain. IF gain steps of less than
10 dB (regardless of the reference level) are accomplished on the A5
assembly.
A4 Log Amplifier/Cal Oscillator Assembly
The A4 log amplifier has separate log and linear amplifier paths. After
amplification, the signal path consists of a linear detector, video log
amp, buffer amplifier, video offset, and video buffer amplifier. Other
auxiliary functions include the frequency counter prescaler/conditioner,
the AM/FM demodulator, and down-conversion to 4.8 kHz for digital
resolution bandwidths of 1 Hz through 100 Hz.
The cal oscillator, which is part of A4, supplies the stimulus signal for
automatic IF adjustments. Normally, the oscillator operates only during
retrace (for a few milliseconds) to adjust part of the IF. (All IF
parameters will be re-adjusted approximately every five minutes.) With
continuous IF adjust on, a group of IF parameters are adjusted during
each retrace period (non-disruptive). If continuous IF adjust is off, the
most recent IF calibration data will be used.
The IF parameters adjusted include step gains, log amplifier gain and
offset, bandwidth centering, 3 dB bandwidth, bandwidth amplitude,
crystal-filter symmetry, and oscillator frequency used in 1 Hz through
100 Hz resolution bandwidths.