Receiver Front-End; Receiver Back-End; Figure 3-6. 200 Mhz Receiver Block Diagram - Motorola HT1250-LS+ Service Manual

Theory of Operation
RX from
Antenna Switch
3.6.1

Receiver Front-End

The received signal from the antenna is routed through the harmonic filter and antenna switch and
applied to the antenna filter, consisting of L3320-22 and C3340-47. 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 Q3302. Diode
CR3303 protects the RF amplifier by limiting excessive RF levels. Current mirror Q3303 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 L3323-4, L3341, C3352-7
and C3359, also a fixed-tuned 3-pole bandpass filter configured to provide steeper high-side
attenuation. Both the antenna filter and RF amp filter have 3 dB bandwidths of 22 MHz and utilize 2%
inductors and 1% capacitors for accurate and repeatable passband response.
The output of the RF amp filter is connected to the passive double-balanced mixer consisting of
components T3301, T3302, and CR3301. High-side injection from the frequency synthesizer is low-
pass filtered by C3373-6, C3381, and L3327-8 to remove second harmonic energy which may
degrade half-IF spurious rejection performance, and applied to T3302 at a level of +5 dBm.
The IF output from T3301 is applied to a diplexer (L3325, C3361, R3332, L3281) which matches the
44.85 MHz IF signal to the crystal filter and terminates the mixer into 50O at all other frequencies.
3.6.2

Receiver Back-End

The receiver back end is a dual conversion design. High IF selectivity is provided by FL3201, a 4-pole
fundamental mode 44.85 MHz crystal filter with a 3 dB bandwidth of 7.5 kHz. The output is fed to IF
amplifier stage Q3200, whose input impedance is adjusted using feedback to provide a proper
terminating impedance for the filter. The output of Q3200 is applied to the input of the receiver IFIC
U3101. Diode CR3200 prevents overdriving the IFIC.
Antenna RF
Filter Amp
First LO
from Synthesizer
Recovered Audio
RSSI

Figure 3-6. 200 MHz Receiver Block Diagram

RF Amp
Filter 1st Mixer
Inj Filter
Ceramic
Resonator
Demodulator
IF
Crystal
Amp
Filter
Cer Fltr
Cer Fltr
6G 4F
3-7