Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
determine the widest resolution of the analyzer. Another fil
ter function is to provide image rejection (that is, to prevent
the mixer from producing 10 MHz outputs from input signal
of 90 MHz). Still another function is to limit the noise spec
trum appearing at the 10 MHz IF circuits to those frequen
cies at which signals also appear.
Though the filter is a sealed unit, in the interest of system
understanding, the following brief description is provided.
The filter consists of three small helical resonators en
closed in cans and tuned with multi-turn trimmer capacitors.
For purposes of impedance matching, the filter is symmetri
cal. The end resonators are connected to external circuits
by 10 picofarad capacitors attached to taps on the coils.
Coupling between resonators is accomplished through
holes in the resonator cans.
Adjustment of the filter for minimum attenuation is per
formed at calibrati ·η by setting the three trimmer capaci
tors. Insertion loss is on the order of 4 to 4.5 dB.
From the filter, the 110 MHz signal is applied to the 3rd
Converter.
3RD CONVERTER
Refer to Diagram 4 and to the block diagram adjacent to
Diagram 17. The 3rd converter converts the 110 MHz IF
signal to 10 MHz for application to the Variable Resolution
circuits. It also generates the 100 MHz signal for the 3rd
Converter, the front panel CAL OUT signal, and the
110 MHz reference for most of the phaselock loops in the
analyzer. The circuits consist of an oscillator and driver, four
identical reference output amplifiers, a mixer, and a calibra
tor output amplifier.
Refer to Diagram 17 while reading this description.
Oscillator/Driver Circuit
The oscillator Q3041 is of the Colpitts configuration with
a 100 MHz microwave type crystal operating in the series
resonant mode in the feedback loop. That the crystal is a
microwave type indicates that it is not only a high-Q type,
but that it is mounted at three points to alleviate mechanical
vibration problems. (The components inside the dashed line
immediately below crystal Y3036 in Diagram 17 are included
for future use only and are not described here.) Tuning ca
pacitor C3031 in the collector circuit serves to adjust for
maximum output.
From the oscillator collector circuit, the output is RC
coupled to driver stage Q2036. The driver is a feedback
amplifier that provides output power on the order of
+ 10 dBm to drive all of the reference amplifiers plus the
mixer amplifier. The output is transformer coupled from the
collector circuit.
Reference Amplifier Circuits
The reference output circuits consist of four identical low-
gain common emitter amplifiers with relatively high levels of
emitter degeneration. These are transistors Q4018, Q2015,
Q3015, Q2016 and associated components. The primary
purpose of these amplifiers is to provide isolation among the
reference outputs and isolation of those outputs from the
oscillator and mixer circuits. The output of each is approxi
mately 0 dBm.
From amplifier Q4018, the output is applied to the
829 MHz IF circuits through coaxial connector J2013. From
amplifier Q2015, the output is applied to coaxial connector
J2012 and is reserved for future use. From amplifier Q3015,
the output is applied to the Phaselock Synthesizer circuits
through coaxial connector J1023. From amplifier Q2016, the
output is applied to coaxial connector J4027 and is reserved
for future use. The Q2016 output is also coupled to the
Calibrator Output Amplifier.
Mixer Circuit
The mixer circuit combines the 100 MHz oscillator fre
quency with the 110 MHz IF signal from the 110 MHz
Bandpass Filter to produce the 10 MHz IF output signal.
From transformer T2026 of the driver circuit (Q2036), the
100 MHz signal is applied to transformer T2041, which con
verts the single-ended driver output to a balanced signal to
drive the push-pull amplifier that drives the mixer. This am
plifier consists of transistors Q1048 and Q2046 and pro
vides a balanced signal, coupled through transformer T3053
to diode ring mixer CR2054. The signal level of the
100 MHz, applied to the mixer, is approximately 100
milliwatts to provide adequate intermodulation distortion
performance. The 110 MHz IF, through Bandpass Filter
(FL36), is applied through coaxial connector J2058, the im
pedance matching LC circuit that consists of inductor L1055
and capacitor C1056, and transformer T1053, to the mixer.
The 10 MHz output from the center tap of T1053 is ap
plied through a diplexer and coaxial connector J3057 to the
Variable Resolution circuits. Loss through the mixer is typi
cally 9 dB. The input level from the 110 MHz bandpass filter
is nominally —26 dBm and the output to the Variable Reso
lution circuits is nominally —35 dBm.
5-22
REV AUG 1981
