Agilent Technologies 8753ES Option 011 Service Manual page 383

Theory of Operation
Receiver Theory
A4/A5/A6 Sampler/Mixer
The A4, A5, and A6 sampler/mixers all down-convert the RF input signals to fixed 4 kHz
2nd IF signals with amplitude and phase corresponding to the RF input.
The Sampler Circuit in High Band
In high band operation, the sampling rate of the samplers is controlled by the 1st LO from
the A7 pulse generator assembly. The 1st LO is a comb of harmonics produced by a step
recovery diode driven by the fractional-N VCO fundamental signal. One of the harmonic
signals is 1 MHz below the start frequency set at the front panel. The 1st LO is combined
in the samplers with the RF input signal from the source. In analyzers with Option 006,
samplers are additionally capable of recognizing RF input signals from 3 to 6 GHz. The
mixing products are filtered, so that the only remaining response is the difference between
the source frequency and the harmonic 1 MHz below it. This fixed 1 MHz signal is the
1st IF. Part of the 1st IF signal from the R sampler is fed back to the A11 phase lock
assembly. (The RF output must be connected externally to the R input connector for
phase-locked operation.)
The Sampler Circuit in Low Band or Super Low Band
In low band or super low band (in analyzers with Option 006), the sampler diodes are
biased continuously on, so that the RF input signal passes through them unchanged. Thus
the 1st IF is identical to the RF output signal from the source (300 kHz to 16 MHz for
lowband; 10 to 300 kHz for super lowband), and sweeps with it. Part of the 1st IF signal
from the R sampler is fed back to the A11 phase lock assembly.
(Refer to "Source Theory Overview" on page 12-14 for information on high band and low
band operation of the source.)
The 2nd LO Signal
The 2nd LO is obtained from the A12 reference assembly. In high band, the 2nd LO is fixed
at 996 kHz. This is produced by feeding the 39.84 MHz output of a phase-locked oscillator
in the A12 assembly through a divide-by-40 circuit.
In low band, the 2nd LO is a variable frequency produced by mixing the output of the
fractional-N VCO with a fixed 39.996 MHz signal in the A12 assembly. The 2nd LO covers
the range of 0.304 to 16.004 MHz (or 0.014 to 16.004 MHz for Option 006) in two
subsweeps that correspond with the source subsweeps. These subsweeps are 0.304 to
3.304 MHz and 3.304 to 16.004 MHz.
Chapter 12 12-29