QUANTUN QP-750, Service Manual

Page: 32 / 79

QP-750 Service Manual
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7.2.2 Receiver Back-End
The receiver back end is a dual conversion design. High IF selectivity is provided by a 49.95 MHz
crystal filter with a minimum 3 dB bandwidth of
±
7.5 kHz, a maximum 35 dB bandwidth of ±25
kHz, and a maximum insertion loss of 4 dB. The output is matched to IF amplifier stage Q1203 by
R1294 and C1225. Q1203 provides 16 dB of gain. The output of Q1203 is applied to the input of
the receiver IFIC IC1204.
The signal from IC1204 is mixed with the second oscillator signal again in IC1204 to create a 455
KHz second IF signal. The second IF signal then passes through a 450KHz ceramic filter
(wideband: CF1201
，
narrowband: CF1202) to eliminate unwanted signals before it is amplified and
detected in IC1204.
7.2.3 Narrowband/Wideband Switch Circuit
Pin WCON and NCON of IC2506 outputs wideband (WCON high level) and narrowband (NCON
high level) controlling signal respectively to turn on corresponding diode-connector, and to choose
ceramic filter CF1201 (wideband) or CF1202 (narrowband) to filter useless spurious signal.
7.3 UHF Transmitter
The transmitter is divided into four major blocks as shown in Figure 7-2.
• AF and Signaling
• RF Power Amplifier
• Antenna Switch and Harmonic Filter
• APC