Receiver Front-End; Receiver Back-End - Motorola HT Series Service Manual

6-6
6.3.1

Receiver Front-End

(Refer to VHF Receiver Front-End Schematic Diagram on page 9-92, VHF Receiver Back-End
Schematic Diagram on page 9-93, and VHF Transmitter Schematic Diagram on page 9-96.)
The RF signal is received by the antenna and applied to a low-pass filter. For VHF, the filter consists
of L3531, L3532, C3532 to C3563. The filtered RF signal is passed through the antenna switch. The
antenna switch circuit consists of two PIN diodes (D3521 and D3551) and a pi network (C3531,
L3551 and C3550).The signal is then applied to a varactor tuned bandpass filter. The VHF bandpass
filter comprises of L3301, L3303, C3301 to C3304 and D3301. The bandpass filter is tuned by
applying a control voltage to the varactor diode (D3301) in the filter.
The bandpass filter is electronically tuned by the DACRx from IC404 which is controlled by the
microprocessor. Depending on the carrier frequency, the DACRx will supply the tuned voltage to the
varactor diodes in the filter. Wideband operation of the filter is achieved by shifting the bandpass filter
across the band.
The output of the bandpass filter is coupled to the RF amplifier transistor Q3302 via C3306. After
being amplified by the RF amplifier, the RF signal is further filtered by a second varactor tuned
bandpass filter, consisting of L3305, L3306, C3311 to C3314 and D3302.
Both the pre and post-RF amplifier varactor tuned filters have similar responses. The 3 dB bandwidth
of the filter is about 12 MHz. This enables the filters to be electronically controlled by using a single
control voltage which is DACRx.
The output of the post-RF amplifier filter is connected to the passive double balanced mixer which
consists of T3301, T3302 and CR3301. Matching of the filter to the mixer is provided by C3317,
C3318 and L3308. After mixing with the first LO signal from the voltage controlled oscillator (VCO)
using high side injection, the RF signal is down-converted to the 44.85 MHz IF signal.
The IF signal coming out of the mixer is transferred to the crystal filter (Y3200) through a resistor pad
(R3321 - R3323) and a diplexer (C3320 and L3309). Matching to the input of the crystal filter is
provided by C3201 and L3200. The crystal filter provides the necessary selectivity and
intermodulation protection.
6.3.2

Receiver Back-End

(Refer to VHF Receiver Back-End Schematic Diagram on page 9-93.)
The output of crystal filter Y3200 is matched to the input of IF amplifier transistor Q3200 by L3203.
Voltage supply to the IF amplifier is taken from the receive 5 volts (R5). The IF amplifier Q3200 is
actively biased by a collector base feedback provided by R3202 and R3203. The gain controlled IF
amplifer provides a maximum gain of about 16dB. A dual hot carrier diode (CR3201) limits the filter
output voltage swing to reduce overdrive effects at RF levels above -27dBm. The amplified IF signal
is then coupled into U3220 (pin 1) via L3202, C3207, and C3200 which provides the matching for the
IF amplifier and U3220.
The IF signal applied to pin 1 of U3220 is amplified, down-converted, filtered, and demodulated, to
produce the recovered audio at pin 7 of U3220.
Within U3220, the first IF 44.85 MHz signal mixes with the 44.395 MHz second local oscillator (2nd
LO) to produce the second IF signal at 455 kHz. The 2nd LO signal frequency is determined by
crystal Y3201. The second IF signal (455 kHz) is then filtered by an external ceramic filter Y3205
before being amplified by the second IF amplifier within U3220. Again, the signal is filtered by a
second external ceramic filter Y3203 or Y3204 depending on the selected channel spacing. Y3203 is
used for 20/25 kHz channel spacing whereas Y3204, for 12.5 kHz channel spacing. The simple
circuit consisting of U3221, CR3202, CR3203 and resistors R3209, R3212, R3211 and R3205 divert
the second IF signal according to the BW_SEL line. The filtered output of the second IF signal is
applied to the limiter input pin of U3220 (Pin 14).
VHF Theory of Operation: Receiver (for models with PCB 8486473Z04)