HP 8753D Option 011 Service Manual page 330

The 2nd LO Signal
The 2nd LO is obtained from the A12 reference assembly. In high band, the 2nd LO is xed
at 996 k z. This is produced by feeding the 39.84 M z 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 xed 39.996 M z signal in the A12 assembly. The 2nd LO covers
the range of 0.304 to 16.004 M z (0.014 to 16.004 M z if Option 011 is combined with
Option 006) in two subsweeps that correspond with the source subsweeps. These subsweeps
are 0.304 to 3.304 M z and 3.304 to 16.004 M z.
The Mixer Circuit
The 1st IF and the 2nd LO are combined in the mixer circuit. The resulting di erence
frequency (the 2nd IF) is a constant 4 k z in both bands, as Table 12-4 shows.
Band
Super Low*
Low
High
* This band is present on the HP 8753D Option 011 nly when it is combined with the Option 006.
A10 Digital IF
The three 4 k z 2nd IF signals from the sampler/mixer assemblies are input to the A10
digital IF assembly. These signals are sampled at a 16 k z rate. A fourth input is the analog
bus, which can monitor either an external input at the rear panel AUX IN connector, or
one of 31 internal nodes. A multiplexer sequentially directs each of the signals to the ADC
(analog-to-digital converter). ere they are converted to digital form and sent to the A9 CPU
assembly for processing. Refer to \Digital Control Theory" for more information on signal
processing.
RAFT
3/21/106 15:14
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Table 12-4. Mixer Frequencies
1st I
0.010 to 0.300 MHz
0.300 to 16.0 MHz
1.000 MHz
2nd LO
0.01 to 0.30 MHz
0.30 to 16.00 MHz
0.996 MHz
Theory of Operation 12-21
2nd I
.0 kHz
.0 kHz
.0 kHz