CIRCUIT ANALYSIS
PRESELECTOR FILTER
The received RF signal (J2) is routed through the Prese-
lector Filter. This filter provides front end selectivity and
attenuates the potential spurious signals of first conversion.
Typically, the filter has an insertion loss is 3 dB and an opera-
tional bandwidth of 2 MHz. The filter is primarily a five-pole
helical bandpass filter (L1 thru L5) and is tunable in the band
split MHz range.
PREAMPLIFIER
The output from the Preselector is coupled through an
impedance matching network consisting of L6, C2, and DC
blocking capacitor C1 to the base of Preamplifier Q1. Q1 is a
broadband common emitter amplifier. The Preamplifier stage
is supplied by the reg12 Vdc line (VCC1) and draws
about 70 mA through R4. It has a low noise figure and high
Third Order Intercept point. Transistor Q2 provides Q1 with a
constant voltage and current source. The bias on Q1 is moni-
tored by the Fault Detector circuit via R17. Capacitors C20 and
C21 prevent the RF component from entering the fault circuit.
The output signal is coupled to the Image Rejection Filter via
an impedance matching network consisting of C4, L8, and
resistors R5 and R6.
IMAGE REJECTION FILTER
Following the Preamplifier is the Image Rejection Filter.
The Image Rejection Filter is a fixed tuned helical bandpass
filter and can meet the desired image rejection of the frequency
band.
INJECTION AMPLIFIER
The local oscillator input (J3) from the Receiver Synthe-
sizer is coupled through an impedance matching network (C5
and L9) to the base of the Injection Amplifier Q3. Q3 and Q8
are common emitter amplifiers. The output from Q3 is coupled
through an impedance matching network (C6, C7, and L11) to
the base of Q8. The Injection Amplifier, consisting of Q3, Q8,
and associated circuitry, is capable of amplifying the injection
signal from 0 dBm to +25 dBm in the 428 to 449 MHz range
or to +18 dBm in the 446-472 MHz range. The amplifier is
powered by the reg12 Vdc line (VCC1). Transistors
Q4 and Q7 provide Q3 and Q8 with a constant voltage and
current source. The bias on Q3 and Q8 is monitored by the
Fault Detector circuit via R21 and R31, respectively. Capaci-
tors C22, C23 and C26 prevent the RF component from
entering the fault circuit. The output signal is coupled to the
Injection Filter via an impedance matching network consist-
ing of C8, L13, and resistors R15 and R16.
INJECTION FILTER
Following the Injection Amplifier is the Injection Filter
consisting of C9 through C19, L14 through L20, and R30.
Configured as a bandpass filter, the Injection Filter has a
bandwidth of 428 to 450 MHz (G3, G4) or 446 to 472 MHz
(G7) and is used to attenuate the harmonics of the Injection
Amplifier. The filter also has an insertion loss of about 2 dB.
DOUBLE BALANCE MIXER
The Double Balance Mixer (DBM) is a broadband
mixer. It converts an RF signal to the 21.4 MHz first conver-
sion IF frequency. The mixer uses low side (G3, G4) or high
side (G7) injection driven by a local oscillator signal of +20
(G3, G4) or +15 (G7) dBm. The mixer conversion loss is
typically about 6.5 dB. The IF output signal is then routed to
the output connector (J4).
FAULT DETECTOR
The Fault Detector circuit monitors the operation of
preamplifier and injection amplifier devices. Operational
amplifiers U1.1 and U1.2 compare the bias on the Preampli-
fier Q1 to preset levels, while U1.3 and U1.4 compare the
bias levels on Injection Amplifiers Q3 and Q8.
When the bias for Q1, Q3, and Q8 is within the preset
window limits, the output from the comparators is a high
level. This causes Q5 to conduct, turning off Q6 and the fault
indicator, CR2. A high level signal is also sent to the Con-
troller on the FLAG 0 line.
Copyright© July 1992, Ericsson GE Mobile Communications, Inc.
Table 1 - General Specifications
ITEM
SPECIFICATION
FREQUENCY RANGE
450.0 MHz - 470.0 MHz (G3, G4)
425.0 MHz - 470.0 MHz (G7)
IF FREQUENCY
21.4 MHz
3 dB BANDWIDTH
>3 MHz
IMPEDANCE
50 ohms at RF, LO, and IF Ports
CONVERSION LOSS
-2 dB
±
1 dB
NOISE FIGURE (NF)
<7.5 dB
THIRD ORDER INTERCEPT POINT
>+20 dBm (G3, G4)
>+15 dBm (G7)
IMAGE REJECTION
>100dB
INJECTION POWER
+2 dBm
±
2 dB
TEMPERATURE RANGE
-30°C TO +60°C
SUPPLY VOLTAGE
12.0 Vdc
SUPPLY CURRENT
290 mA
±
20 mA typical
230 mA
±
20 mA typical (G3)
Figure 1 - Block Diagram
LBI-38673F
1
Содержание 19D902783G1
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