Motorola Vhf Manual

Chapter 2
THEORY OF OPERATION
1.0 Introduction
This Chapter provides a detailed theory of operation for the VHF 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 VHF (136-174MHz) Receiver
2.1 Receiver Front-End
The receiver is able to cover the VHF range from 136 to 174 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...

VHF (136-174MHz) Transmitter Power Amplifier (PA) 25 W 2-3
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 quadrature detector using a ceramic phase-shift element (Y3102) to provide 
audio detection....

2-4THEORY OF OPERATION
Op-amp U3402-1 monitors the drain current of U3401 via resistor R3444 and adjusts the bias 
voltage of U3401 so that the current remains constant. The PCIC (U3501) provides a DC output 
voltage at pin 4 (INT) which sets the reference voltage of the current control loop. A raising power 
output causes the DC voltage from the PCIC to fall, and U3402-1 adjusts the bias voltage for a lower 
drain current to lower the gain of the stage.
In receive mode the DC voltage from PCIC pin 23...

VHF (136-174MHz) Transmitter Power Amplifier (PA) 25 W 2-5
3.5 Antenna Switch
The antenna switch consists of two PIN diodes, D3471 and D3472. In the receive mode, both diodes 
are off. Signals applied at the antenna jack J3401 are routed, via the harmonic filter, through 
network L3472, C3474 and C3475, to the receiver input. In the transmit mode, K9V1 turns on Q3471 
which enables current sink Q3472, set to 96 mA by R3473 and VR3471. This completes a DC path 
from PASUPVLTG, through L3437, D3471, L3472,...

2-6THEORY OF OPERATION
4.0 VHF (136-174MHz) Frequency Synthesis
The frequency synthesizer subsystem consists of the reference oscillator (Y3261 or Y3263), the 
Low Voltage Fractional-N synthesizer (LVFRAC-N, U3201), and the voltage-controlled oscillators 
and buffer amplifiers (U3301, Q3301-2 and associated circuitry).
4.1 Reference Oscillator
The reference oscillator (Y3263) contains a temperature compensated crystal oscillator with a 
frequency of 16.8 MHz. An analog to digital (A/D) converter internal...

VHF (136-174MHz) Frequency Synthesis2-7
A voltage of 5V applied to the super filter input (U3201 pin 30) supplies an output voltage of 4.5 VDC 
(VSF) at pin 28. It supplies the VCO, VCO modulation bias circuit (via R3363) and the synthesizer 
charge pump resistor network (R3251, R3252). The synthesizer supply voltage is provided by the 
5V regulator U3211.
In order to generate a high voltage to supply the phase detector (charge pump) output stage at pin 
VCP (U3201-47), a voltage of 13 VDC is being...

2-8THEORY OF OPERATION
The VCOBIC together with the Fractional-N synthesizer (U3201) generates the required frequencies 
in both the transmit and receive modes. The TRB line (U3301 pin 19) determines which tank circuits 
and internal buffers are to be enabled. A high level on TRB enables the TX tank and TX output (pin 
10), and a low enables the RX tank and RX output (pin 8). A sample of the signal from the enabled 
RF output is routed from U3301 pin 12 (PRESC_OUT), via a low pass filter, to pin 32 of...

VHF (136-174MHz) Transmitter Power Amplifier (PA) 45 W 2-9
signal into a digital code that is applied to the loop divider, thereby causing the carrier to deviate. The 
balance attenuator is used to adjust the VCO’s deviation sensitivity to high frequency modulating 
signals. The output of the balance attenuator is present at the MODOUT port (U3201-41) and 
connected to the VCO modulation diode D3362 via R3364.
5.0 VHF (136-174MHz) Transmitter Power Amplifier (PA) 45 W
The radio’s 45 W PA is a four stage...

2-10THEORY OF OPERATION
5.2 Pre-Driver Stage
The next stage is an LDMOS device (Q3421) providing a gain of 13 dB. This device requires a 
positive gate bias and a quiescent current flow for proper operation. The voltage of the line 
PCIC_MOSBIAS_1 is set during transmit mode by the PCIC pin 24, and fed to the gate of Q3421 
via the resistive network R3410, R3415, and R3416. The bias voltage is tuned in the factory.
5.3 Driver Stage
The following stage is an enhancement-mode N-Channel MOSFET device (Q3431)...