Service manual

TP0332/2 3.5
Figure 3.3 Receiver Block Diagram
Second Mixer, Second IF Amplifier and Ceramic Filters
The second IF amplifier is implemented around an SA615 integrated circuit (IC201). The second
local oscillator is provided by a separate VCO (TR209 and associated components), tuned by D219/D220,
and whose frequency is 455kHz above or below the first IF frequency. The choice is made automatically for
each channel to avoid harmonics of the second local oscillator interfering with the required signal.
Dual channel spacing is achieved by using analogue switches, IC208, IC203, IC204 (20/25kHz
channel spacing) and IC209, IC205, IC206 (12,5kHz channel spacing) to select the required ceramic
filters. The switching arrangement is shown in Figure 3.3. The ceramic filters, F203, F202 (20/25kHz) and
F205, F206 (12,5kHz) are supplied through a 6dB splitter and a 4dB pad giving a total loss of 12dB (typical
filter loss is 2dB) for RSSI (Receive Signal Strength Indication) linearity.
In the single channel spacing variant, some of the switches are replaced by capacitors C324, C279
and C280.
Squelch
Squelch is derived from IC201 and fed to active filter (IC202_A and associated components) which
acts as a band-pass filter at approximately 20kHz. Noise picked up at 20kHz is amplified by IC202_B and
rectified by D213. This DC level is compared by IC207, with a DC level on TX_PWR_SQ_SET, from a DAC
(in IC701). This SQUELCH signal is fed back to the control section.
Antenna
First
RF
Amplifier
Low Pass
Low Pass
Filter
Tracking
Band Pass
Filter
Second
Band Pass
Filter
Tracking
First
IF
First
Filter
Switch
Antenna
From
Mixer
Switch
Second
Mixer
Second
Local
Oscillator
VCO
Loop
Filter
From
Frequency
Generation
Switch
Ceramic
Filters
Ceramic
Filters
12,5kHz
20/25kHz
2nd
IF
Limiter
Discriminator
Squelch
Receiver
Audio
Output
Switch
Ceramic
Filters
12,5kHz
Ceramic
Filters
20/25kHz
45MHz/
Filter
21,4MHz
Crystal