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Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
1) to eliminate IF feedthrough in band 2 and reduce or elimi
nate higher order spurs in bands 3 and 4;
( 2) because of the limited tune range of the 719 MHz LO,
the lower IF cannot be used above band 4;
3) if a measurement band were to include the first interme
diate frequency within its range, it is possible for some input
signals admitted by the preselector to pass through the 1 st
Converter (without conversion), into the 2nd converter at
the 1st intermediate frequency. The resultant spurious out
put will cause the baseline level on the screen to rise, and
could possibly obscure real signals. By using two selectable
2nd Converters, the analyzer can have overlapping mea
surement bands that do not include the first intermediate
frequency, and completely avoid the problem.
The 2072 MHz 2nd converter mixes the 2072 MHz from
the first converter with the output from a cavity oscillator.
This local oscillator is swept over a 7.5 MHz range. At the
converter input, a four-cavity bandpass filter is used to pass
only the 2072 MHz 1st IF signal and prevent unwanted sig
nals generated within the 2nd Converter from passing back
through to the 1 st Converter. A diode mixer is used to mix
the 2072 MHz IF input and the local oscillator signals to
generate the 110 MHz second IF output. The 110 MHz out
put passes through a 110 MHz lowpass filter that blocks
higher frequency signals from the mixer.
The 829 MHz 2nd Converter uses a phase-locked volt
age controlled oscillator to produce the 719 MHz signal that
is mixed with the 829 MHz first IF signal. The swept
2182 MHz 2nd local oscillator is used as a reference for the
719 MHz local oscillator. The 719 MHz oscillator is designed
so that it can be disabled upon command from the
microcomputer in the IF selection process. The phaselock
circuit maintains a constant relationship between the two
local oscillators as the 719 MHz oscillator is swept over a
2.5 MHz range. A four-section coaxial bandpass filter is
used before the mixer to exclude any RF signals other than
the desired 829 MHz first IF. Again, a diode mixer is used to
mix the 829 MHz input and local oscillator signals to
produce the 110 MHz second IF output.
Selection between the two IF signals also takes place
within the 829 MHz converter system. Under command of
the microcomputer (by way of the RF Interface circuits) a
diode selector switching network connects one of the two
110 MHz second IF signals to the output for application to
the 3rd Converter.
T ab le 5-2
2ND CONVERTER IF SELECTION
F r e q u e n c y
B and
C e n te r
F r e q u e n c y
R a n g e
L o cal O s cillato r
F r e q u e n c y (MHz)
R a n g e
C o n v e r te r
S y s te m
IF
1
0—1.8 GHz
2182 ±3.75
2072 MHz
2
1.7—5.5 GHz
719 ±1.25
829 MHz
3
3.0—7.1 GHz
719 ±1.25
829 MHz
4
5.4— 18.0 GHz
719 ±1.25
829 MHz
5
15.0—21.0 GHz
2182 ±3.75
2072 MHz
6
18.0—26.5 GHz
2182 ±3.75
2072 MHz
7
26.5—40.0 GHz
2182 ±3.75
2072 MHz
8
40.0—60.0 GHz
2182 ±3.75
2072 MHz
9
60.0—90.0 GHz
2182 ±3.75
2072 MHz
10
90.0— 140.0 GHz
2182 ±3.75
2072 MHz
11
140.0—220.0 GHz
2182 ±3.75
2072 MHz
REV AUG 1981
5-7
