Agilent Technologies 8590 Series Service Manual page 362

Assembly Descriptions and Block Diagrams
RF and LO Section for 8590L, 8591E, and 8591C Spectrum Analyzers
The sampling oscillator provides a 279 to 298 MHz driving signal to the
A25A1 sampler. The first LO signal is also applied to A25A1 sampler.
The first LO signal is mixed with a harmonic of the sampling oscillator
signal to generate the sampler IF. This IF is divided by 10 and fed to
one input of the frequency counter. The equation used to produce the
first LO frequency is:
Ist LO Frequency =N x F
N represents the harmonic of the sampling oscillator.
F represents the sampling oscillator frequency
SO
Sampler IF represents the counted sampler IF (may be negative).
The stabilizer mixes the divided sampler IF (nominally 8.25 MHz) with
the 7.5 MHz, and feeds the difference signal into the discriminator. The
discriminator output, DISCRIM, is fed back to the A7 analog interface
assembly to tune the A3A7 YTO precisely to the center frequency.
When the frequency count marker is active, the analyzer pauses at the
marked frequency and counts the first LO as described previously. The
21.4 MHz IF is then divided by four, to a nominal value of 5.35 MHz, fed
to another frequency counter input, and counted. With the second LO,
the third LO, the counted first LO, and the counted final IF all
referenced to the 10 MHz reference, the actual input signal frequency is
calculated.
A25A1 Sampler
The A25A1 sampler mixes the first LO with a harmonic of the sampling
oscillator to produce the 76 MHz to 89 MHz sampler IF output. In the
phase-locking process, the sampler IF is counted and the YTO tune
DACs on the A7 analog interface assembly are adjusted until the actual
sampler IF frequency is equal to the desired sampler IF. In YTO spans
of 10 MHz or less, the sampler IF is also applied to a discriminator. The
output of the discriminator (DISCRIM) fine-tunes the YTO precisely to
the center frequency.
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+ Sampler IF
SO
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Chapter 9