Agilent Technologies 8753ES, Service Manual

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Chapter 12 12-29
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 the 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 Sampler Circuit in Low Band or Super Low Band
In low band or super low band, 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 0.014 to 16.004 MHz 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.
The Mixer Circuit
The 1st IF and the 2nd LO are combined in the mixer circuit. The resulting difference
frequency (the 2nd IF) is a constant 4 kHz in both bands, as
Table 12-4 shows.
Table 12-4 High Band Subsweep Frequencies
Band 1st IF 2nd LO 2nd IF
Super Low 0.010 to 0.300 MHz 0.014 to 0.304 MHz 4.0 kHz
Low 0.300 to 16.0 MHz 0.304 to 16.004 MHz 4.0 kHz
High 1.000 MHz 0.996 MHz 4.0 kHz