Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
S E R I E S R E S O N A T O R l 0 = 8 2 9 M H z
S E R I E S R E S O N A T O R fo = 8 2 9 M H z
Fig. 5-4. Diplexer sim plified schematic.
attenuator. The overall noise figure is approximately 2.8 dB.
The gain stages are designed as general purpose, uncondi
tionally stable amplifiers for use in a 50 Ω system. Operation
of a stage can be most easily understood if the ac and dc
signal paths are described separately. Refer to Figs. 5-5 and
5-6 for simplified schematic diagrams of the ac and dc signal
paths.
In the ac circuit of Fig. 5-5, capacitor C1 and printed
circuit inductors L1 and L2 form the input matching network.
(In the first stage, inductor L1 is actually the series induc
tance of dc blocking capacitor C1016.) The collector circuit
is matched to 50 Ω by inductor L4 and capacitor C2. Gain is
controlled primarily by printed circuit emitter inductor L3.
High frequency stability is enhanced by resistors R1 and R2.
That is, at frequencies well above 829 MHz, resistor R1 en
sures low common base gain and resistor R2 helps to
dampen the collector circuit.
In the dc circuit of Fig. 5-6, negative fedback through the
voltage divider consisting of resistors R3 and R4 sets the
collector voltage as a fixed proportion of the — 12 volt refer
ence supply. Collector current is determined by resistor R5.
Less current is used in the first stage than in the second
because the first stage requires less intermodulation distor
tion performance. Reverse breakdown of the base-emitter
junction can degrade the transistor performance, so a diode
base clamp is provided in each circuit (CR1013 and
CR1022) for protection in the absence of the +12 volt
supply.
Not shown in Figs. 5-5 and 5-6 are an inductor and a
capacitor in the base circuits (L1014 and C1014 for Q1017;
L1021 and C1023 for Q1025) and a capacitor in the collec
tor circuits (C1013 for Q1017; C1027 for Q1025). These
components perform decoupling functions to isolate the sig
nal path from the bias network.
The 3 dB attenuator assists in maintaining a wideband
50 Ω interface between the second amplifier stage and the
829 MHz bandpass filter. It consists of resistors R1026,
R1027, R1025, and R1029. A test point (J1029) at the out
put of the attenuator is used to verify amplifier performance
and to aid in adjustment of the following 829 MHz bandpass
filter. From the atteunator, the signal is applied to the
829 MHz 2nd Converter Mixer circuit.
829 MHz Mixer Circuits
Refer to Diagram 15. Frequency conversion from
829 MHz to 110 MHz occurs on the 829 MHz 2nd Convert
er board. The board contains a coaxial bandpass filter, a
1.3 GHz lowpass filter, a 3 dB attenuator, and a two-diode,
single-balanced mixer with associated frequency diplexing
circuitry.
5-14
R EV A U G 1981
