Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
110-MHz IF AMPLIFIER AND 3rd
CONVERTER
The 110 MHz IF Amplifier and 3rd Converter accept the
110 MHz output from the 2nd Converters, amplify and con
vert the signal to a 10 MHz IF signal which is applied to the
resolution circuits in the IF section. The 110 MHz signal is
amplified in a three-stage gain block and applied to a three-
section bandpass filter. This filter uses helical resonators
and has a nominal bandwidth of 1 MHz. From the bandpass
filter, the signal is applied to a mixer and heterodyned with a
100 MHz local oscillator signal to produce a 10 MHz third IF
signal. The resulting signal, nominally at a level of —35 dBm
at the top of the screen, then drives the Variable Resolution
circuits.
Initial gain for the analyzer is provided by the 110 MHz IF
Amplifier. This gain compensates for signal level losses in
the three mixers. Three stages of amplification are used,
plus a pin diode controlled attenuator that allows for adjust
ment of the gain. Typical gain for the amplifier is 21 dB.
From the amplifier, the 110 MHz signal is applied to the 3rd
Converter through a bandpass filter.
The filter is a three section unit using helical resonators.
Its bandwidth of 1 MHz defines the broadest resolution
bandwidth of the analyzer, provides good image rejection,
and limits noise in the frequency spectrum in which desirable
signals appear.
Consisting of a mixer, an oscillator, and various output
amplifiers, the 3rd Converter converts the 110 MHz second
IF signal into the 10 MHz third IF signal. The local oscillator
is a crystal controlled circuit that generates a precise
100 MHz signal. This 100 MHz is applied to the mixer and to
output amplifiers. The 100 MHz signal is used in the 2nd
Converter and the phaselock section. It is also furnished to
a front panel CAL OUT connector for external use.
The mixer is a diode ring type that is fed from balanced
drivers which are driven by the 100 MHz oscillator. From the
mixer, the output signal, at 10 MHz, is applied to the Vari
able Resolution section of the 3rd Converter.
110 MHz IF AMPLIFIER <^>
The 110 MHz IF Amplifier consists of three stages of am
plification and an attenuator. Since the first two mixers in
the RF system offer no high frequency gain, it is important
that, this amplifier exhibit low noise characteristics. Also, it
must be relatively free from third-order intermodulation
distortion.
Signal input to the amplifier is from the 2nd Converter
through coaxial connector P321. This signal is nominally
110 MHz and is applied to an impedance matching
bandpass filter consisting of inductor L2044 and capacitor
C325. The signal is injected into the parallel tuned circuit
through a tap in the inductor and taken out at the high im
pedance side through another variable capacitor, C2047. In
ductive input provides for converting to high impedance
within the tuned circuit; the extra capacitor on the output
provides for converting back to 50 Ω nominal. The primary
tuning capacitor (C325) adjusts the resonant point; the out
put capacitor (C2047) is adjusted in combination with C325
for good impedance match at 110 MHz. This is done using a
return loss bridge. The nominal return loss is 35 dB. The Q
of the input filter is approximately 20.
From the input filter, the signal is applied to Q4053, the
first stage of amplification. This is a broadband feedback
amplifier to provide good input and output impedance and
controlled gain. All feedback is through reactive components
(transformer T3054) not resistive components. Thus, the im
pedance and gain can be controlled without significant noise
problems.
The second amplifier stage, Q4037, is essentially the
same as the first, with only minor bias differences. Gain
through each of these stages is approximately 9 dB. The
output is applied through a 3 dB attenuator, to preserve the
impedance figure, to the bridged T adjustable attenuator.
The 3 dB attenuator consists of resistors R2039, R2038,
and R2043.
From the 3 dB attenuator, the signal is capacitively coup
led through C2037 to the adjustable attenuator. This
attenuator uses two PIN diodes (CR3030 and CR1029) in
the mode in which the resistance to RF signal flow is con
trolled by the current through the diodes. Refer to Fig. 5-8
as an aid in understanding the following description.
With reference to Fig. 5-8, if resistor R1 were set to infi
nite resistance and resistor R2 were set to zero resistance,
the RF signal path would be through R2 to ground, thereby
producing infinite signal attenuation. If resistor R1 were set
5-20
R EV FEB 1983
