Tektronix 492, 492P, Руководство по обслуживанию

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Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
Fig. 5-2. Filter equivalent circuit.
The two inductors and one capacitor at the output of the
mixer form a lowpass filter that passes 110 MHz
unattenuated to the 829 MHz 2nd Converter via coaxial
connector P182. Capacitors at each of the three inputs to
the mixer function as dc blocking capacitors to keep the
diode bias from being impressed upon the RF and local os­
cillator lines.
The bias circuit, which consists of operational amplifier
U1014 and the associated components, establishes the bias
for the mixer diodes and also provides the means for effec­
tively switching the mixer off (under control of the
microcomputer). When the mixer is active, each diode has
approximately 2 mA of forward bias. For this condition, the
IF SELECT signal from the Z Axis/RF Interface circuits (ap­
plied through feedthrough capacitor C182) is low. This
causes the output from U1014A to be at +14 V and the
output from U1014B to be —14 V. Diodes CR1014 and
CR1018 are thereby reverse-biased. Thus, the series resis­
tances of potentiometer R1019 and resistor R1014, and po­
tentiometer R1010 and resistor R1017 provide forward bias
to the diodes. The potentiometers provide for balancing the
bias levels.
In operations in which the mixer is not active, the IF SE­
LECT signal is high. This reverses the states of the U1014
outputs and forward-biases diodes CR1014 and CR1018.
With these diodes conducting, resistors R1014, R1016,
R1017, and R1018 form two voltage dividers that set the
reverse bias to the mixer diodes at 5 V. This effectively
turns the mixer off, and attenuates the 110 MHz signal by
about 55 dB.
Precision External Cables
The external cable that connects the Four-Cavity Filter
output to the mixer circuit and the external cable that con­
nects the cavity 2nd Local Oscillator to the mixer circuit are
both critical length cables. The reasons the length is critical
are as follows:
The 4-Cavity Filter-to-Mixer input cable: Several pro­
ducts and harmonics of the local oscillator and RF input
frequencies are allowed to exit the mixer via the RF input
port of the mixer. Two significant products are the image
(RF input minus the 2nd Local Oscillator) and the sum (RF
input plus the 2nd Local Oscillator). There is enough energy
in these two signals to warrant efforts to recover that
energy.
Only the RF signal at 2072 MHz can pass through the
Four-Cavity Filter. Thus, any other frequency applied to the
REV AUG 1981
5-9