HP HP 8753E, Руководство по обслуживанию, Страница: 471 / 612

The and sampler/mixers all down-convert the RF input signals
to 4 IF signals with amplitude and phase corresponding to the
RF’ input. The and sampler/mixer assemblies both include an 8 gain
preamplifier in front of the sampler. This improves the noise figure performance
of the analyzer’s receiver channels A and B.
In high band operation, the sampling rate of the samplers is controlled by
the LO from the pulse generator assembly. The 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 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 The mixing
products are so that the only remaining response is the difference
between the source frequency and the harmonic 1 MHz below it. This
back to the All phase lock assembly.
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 IF
is identical to the RF output signal from the source (300 to 16 MHz for
10 to 300 for super lowband), and sweeps with it. Part of the
IF signal from the R sampler is fed back to the All phase lock assembly.
(Refer to “Source Theory Overview” for information on high band and low band
operation of the source.)
The LO is obtained from the reference assembly. In high band, the
LO is at 996 This is produced by feeding the 39.34 MHz output of a
phase-locked oscillator in the assembly through a circuit.
In low band, the is a variable frequency produced by mixing the output
of the fractional-N with a 39.996 MHz signal in the assembly.
The 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.
Theory of Operation