A4/AS/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
A5
and
A6
sampler/mixer assemblies both include an 8
dB
gain
preamplifier in front of the sampler. This improves the noise figure performance
of the analyzer’s receiver channels A and B.
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
flltered,
so that the only remaining response is the difference
between the source frequency and the harmonic 1 MHz below it. This
fixed
1MHzsignalisthelstIF.RartofthelstIFsignalfromtheRsamplerisfed
back to the All phase lock assembly.
The
hnpler
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 All phase lock assembly.
(Refer to “Source Theory Overview” for information on high band and low band
operation of the source.)
The
2nd
LO
Signal
The
2nd
LO is obtained from the
Al2
reference assembly. In high band, the
2nd
LO is llxed at 996
kHz.
This is produced by feeding the 39.34 MHz output of a
phase-locked oscillator in the
Al2
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
VCG
with a fixed 39.996 MHz signal in the
Al2
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.
Theory of Operation
12-29
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