Vhf Receiver; Receiver Front-End; Figure 2-14. Vhf Receiver Block Diagram - Motorola PR860 Detailed Service Manual

Theory of Operation: VHF Receiver

2.13 VHF Receiver

Antenna
Pin Diode
Antenna
Switch
RF Jack

2.13.1 Receiver Front-End

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.
Varactor
RF Amp
Tuned Filter
First LO
from VCO
Control Voltage
from ASFIC
Demodulator
Recovered Audio
RSSI
BW SEL

Figure 2-14. VHF Receiver Block Diagram

Varactor
Mixer
Tuned Filter
Limiter
455kHz
455kHz
Filter
Filter
455kHz
Filter
Crystal
IF Amp
Filter
U3220
Second LO
2-21