Transmitter Power Amplifier (PA) 45 W 2-3
2.4 2nd Intermediate Frequency (IF) and Receiver Back End
The 44.85 MHz 1st IF signal from the second IF amplifier feeds the IF IC (U3101) at pin1. Within the
IF IC, the 44.85 MHz high IF signal mixes with the 44.395 MHz second local oscillator (2nd LO) to
produce the 2nd IF signal at 455 kHz. The 2nd LO frequency is determined by crystal Y3101. The
2nd IF signal is amplified and filtered by an external pair of 455 kHz ceramic filters FL3112, FL3114
for 20/25 kHz channel spacing or FL3111, FL3113/F3115 for 12.5 kHz channel spacing. These pairs
are selectable via BWSELECT. The filtered output from the ceramic filters is applied to the limiter
input pin of the IF IC (pin 14).
The IF IC contains a quadrature detector using a ceramic phase-shift element (Y3102) to provide
audio detection. Internal amplification provides an audio output level of 120 mV rms (at 60%
deviation) from U3101 pin 8 (DISCAUDIO) which is fed to the ASFIC_CMP (U0221) pin 2 (part of the
Controller circuits).
A received signal strength indicator (RSSI) signal is available at U3101, pin 5, having a dynamic
range of 70 dB. The RSSI signal is interpreted by the µP (U0101, pin 63) and in addition is available
at accessory connector J0501-15.
3.0 Transmitter Power Amplifier (PA) 45 W
The radio’s 45 W PA is a four-stage amplifier used to amplify the output from the VCOBIC to the
radio transmit level. The line-up consists of three stages which utilize LDMOS technology, followed
by a final stage using a bipolar device. The gain of the first stage (U3401) is adjustable, controlled by
pin 4 of PCIC (U3501) via Q3501 and Q3502 (VCONT). It is followed by an LDMOS pre-driver stage
(Q3421), an LDMOS driver stage (Q3431) and a bipolar final stage (Q3441).
Figure 2-2 VHF Transmitter Block Diagram
Devices U3401 and Q3421 are surface mounted. The remaining devices are directly attached to the
heat sink.
Antenna
To Microprocessor
PCIC
Pin Diode
Antenna
Switch
RF Jack
Harmonic
Filter
Power
Sense
PA-Final
Stage
PA
Driver
From VCO
Controlled
Stage
Vcontrol
Bias 1
Bias 2
To Microprocessor
Temperature
Sense
SPI BUS
ASFIC_CMP
PA
PWR
SET
To Microprocessor
Pre
Driver
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