Motorola Uhf Manual

Chapter 2
THEORY OF OPERATION
1.0 Introduction
This Chapter provides a detailed theory of operation for the UHF circuits in the radio. For details of 
the theory of operation and trouble shooting for the the associated Controller circuits refer to the 
Controller Section of this manual.
2.0 UHF (403-470MHz) Receiver
2.1 Receiver Front-End
The receiver is able to cover the UHF range from 403 to 470 MHz. It consists of four major blocks: 
front-end bandpass filters and pre-amplifier, first mixer, high-IF,...

2-2THEORY OF OPERATION
There are two 2-pole 44.85 MHz crystal filters in the high-IF section and 2 pairs of 455 kHz ceramic 
filters in the low-IF section to provide the required adjacent channel selectivity .The correct pair of 
ceramic filters for 12.5 or 25KHz channel spacing is selected via control line BWSELECT. The 
second IF at 455 kHz is mixed, amplified and demodulated in the IF IC. The processing of the 
demodulated audio signal is performed by an audio processing IC located in the controller...

UHF (403-470MHz) Transmitter Power Amplifier (PA) 25 W 2-3
produce the low IF signal at 455 kHz. The 2nd LO frequency is determined by crystal Y3101. The 
low 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...

2-4THEORY OF OPERATION
controlled by a DC voltage applied to pin 1 from the op-amp U4402-1, pin 1. The control voltage 
simultaneously varies the bias of two FET stages within U4401. This biasing point determines the 
overall gain of U4401 and therefore its output drive level to Q4421, which in turn controls the output 
power of the PA.
Op-amp U4402-1 monitors the drain current of U4401 via resistor R4444 and adjusts the bias 
voltage of U4401 so that the current remains constant. The PCIC (U4501)...

UHF (403-470MHz) Transmitter Power Amplifier (PA) 25 W 2-5
3.5 Antenna Switch
The antenna switch consists of two PIN diodes, D4471 and D4472. In the receive mode, both diodes 
are off. Signals applied at the antenna jack J4401 are routed, via the harmonic filter, through 
network L4472, C4474 and C4475, to the receiver input. In the transmit mode, K9V1 turns on Q4471 
which enables current sink Q4472, set to 96 mA by R4473 and VR4471. This completes a DC path 
from PASUPVLTG, through L4437, D4471, L4472,...

2-6THEORY OF OPERATION
4.0 UHF (403-470MHz) Frequency Synthesis
The synthesizer subsystem consists of the reference oscillator (Y4261 or Y4262), the Low Voltage 
Fractional-N synthesizer (LVFRAC-N, U4201), and the Voltage Controlled Oscillator VCO.
4.1 Reference Oscillator
The reference oscillator (Y4262) contains a temperature compensated crystal oscillator with a 
frequency of 16.8 MHz. An Analogue to Digital (A/D) converter internal to U4201 (LVFRAC-N) and 
controlled by the microprocessor via serial...

UHF (403-470MHz) Frequency Synthesis2-7
A voltage of 5V applied to the super filter input (U4201 pin 30) supplies an output voltage of 4.5 
VDC(VSF) at pin 28. It supplies the VCO, VCO modulation bias circuit (via R4322) and the 
synthesizer charge pump resistor network (R4251, R4252). The synthesizer supply voltage is 
provided by the 5V regulator U4211.
In order to generate a high voltage to supply the phase detector (charge pump) output stage at pin 
VCP (U4201-47), a voltage of 13 VDC is being...

2-8THEORY OF OPERATION
The VCOBIC together with Fractional-N synthesizer (U4201) generates the required frequencies in 
both  transmit and receive modes. The TRB line (U4301 pin 19) determines which tank circuits and 
internal buffers are to be enabled. A high level on TRB enables TX tank and TX output (pin 10), and  
a low enables RX tank and RX output (pin 8). A sample of the signal from the enabled output is 
routed from U4301 pin 12 (PRESC_OUT), via a low pass filter, to pin 32 of U4201 (PREIN).
A...

UHF (403-470MHz) Transmitter Power Amplifier (PA) 40W 2-9
5.0 UHF (403-470MHz) Transmitter Power Amplifier (PA) 40W
The radio’s 40 W PA is a four stage amplifier used to amplify the output from the VCOBIC to the radio 
transmit level. It consists of the following four stages in the line-up. The first stage is a LDMOS 
predriver (U4401) that is controlled by pin 4 of PCIC (U4501) via Q4473 (CNTLVLTG). It is followed 
by another LDMOS stage (Q4421), an LDMOS stage (Q4431) and a bipolar final stage (Q4441)....

2-10THEORY OF OPERATION
5.3 Driver Stage
The following stage is an enhancement-mode N-Channel MOSFET device (Q4431) providing a gain 
of 10dB. This device also requires a positive gate bias and a quiescent current flow for proper 
operation. The voltage of the line Bias_2_UHF_PA_1 is set in transmit mode by the ASFIC and fed to 
the gate of Q4431 via the resistive network R4632, R4631, R4485 and R4486. This bias voltage is 
also tuned in the factory. If the transistor is replaced, the bias voltage must be...