200 Mhz Receiver Front-End; Receiver Front End - Motorola CDM1550 LS+ Detailed Service Manual

2-18 Theory of Operation
2.4 200 M Hz Receiver Front-End
The 200 MHz receiver design covers the frequency range of 217 to 222 MHz and is separated into
two blocks, the front end and the back end. The overall block diagram of the receiver is shown in
Figure 2-11. Detailed descriptions of these stages are contained in the paragraphs that follow.
RX from
Antenna Switch
--~
Antenna
Filter
First LO
RF
Amp
from Synthesizer - -......
1
RF Amp Crystal IF
Filter 1st Mixer Filter Amp
Cer Fltr Cer Fltr
Ceramic
Resonator
r---
Recovered Audio - - 1 Demodulator
RSSI..
I
Figure 2-7. 200 MHz Receiver Block Diagram
2.4.1 Receiver Front End
=
T
The received signal from the antenna is routed through the harmonic filter and antenna switch and
applied to the antenna filter, consisting of L0320-22 and C3040-47. This fixed-tuned 3-pole
bandpass filter is configured to provide steeper attenuation above the passband for improved
spurious rejection when high-side first injection is used. The output of this filter is coupled to RF
amplifier 03002. Diode CR3003 protects the RF amplifier by limiting excessive RF levels. Current
mirror 03003 maintains constant current operation of the RF amp vs. device and temperature
variations, for optimum dynamic range and noise figure.
The output of the RF amplifier is applied to the RF amp filter comprised of L3023-4, L3041, C3052-
7 and C3359, also a fixed-tuned 3-pole bandpass filter configured to provide steeper high-side
attenuation. The antenna filter and RF amp filter have a combined 3 dB bandwidth of 24 MHz and
utilize 2% inductors and 1% capacitors for accurate and repeatable passband response.
The output of the RF amp filter is routed through a switchable local-distance attenuator consisting of
R3099, D3090, 03090 and associated components. Normally 03090 and D3090 are on and the
receiver sensitivity is at maximum. When 03090 and D3090 are off, the series resistor R3099
reduces receiver sensitivity to minimize intermodulation when operating in the presence strong
undesired signals. The output of the local-distance attenuator is connected to the passive double-
balanced mixer consisting of components T3001, T3002, and CR3001. High-side injection from the
frequency synthesizer is lowpass filtered by C3073-6, C3081, and L3027-8 to remove second
harmonic energy which may degrade half-IF spurious rejection performance, and applied to T3002 _)
at a level of +19 dBm.