Front-End Receiver Assembly; Rf Board Basic; Voltage-Controlled Oscillator; Vhf Radios - Motorola ASTRO Digital Spectra Service Manual

2-6
2.6

Front-End Receiver Assembly

The receiver front-end consists of a preselector, a mixer circuit, and an injection filter. The receiver
injection (1st local oscillator) comes from the VCO assembly through a coax cable. The injection filter
is either fixed-tuned or tuned at the factory depending upon the bandsplit. The output of the filter is
connected to the mixer.
The preselector is a fixed-tuned filter. The receiver signal is fed to the preselector from the antenna
switch in the PA for the 800 MHz and UHF radios, or the preamp output for VHF. The signal is then
sent to the mixer integrated circuit where it is connected to the mixer transistor. The receiver injection
is also fed to this point. The mixer output is at the 1st IF center frequency of 109.65 MHz. This signal
is sent to the 1st IF amplifier stage on the RF board through a coaxial cable.
2.7

RF Board Basic

The RF board contains the common synthesizer circuits, dual IF receiver and demodulation circuits.
A 4-pole crystal filter at 109.65 MHz provides first IF selectivity. (For HRN6014D, HRN6020C,
HRN6019C, HRN4009D, HRN4010C and later RF board kits, two 2-pole crystal filters provide first IF
selectivity at 109.65 MHz.) The output of the filter circuit is fed directly to the custom digital back-end
circuit module. An amplification circuit at 109.65 MHz, the second mixer, the second IF amplifiers (at
450 kHz), the IF digital-to-analog converter, and the baseband down-converter comprise the digital
back-end circuit module.
Synthesizing for the first and second VCO is performed by the prescaler and synthesizer ICs. These
ICs are programmed through a serial data bus from signals generated on the VOCON board. A DC
voltage generated on the command board, sets the synthesizer's reference oscillator frequency of
16.8 MHz. This voltage is controlled by the digital-to-analog converter (D/A), and is the only element
of the RF board requiring alignment.
The second local oscillator runs at 109.2 MHz (low-side injection), or 110.1 MHz (high-side injection)
and consists of a VCO which is frequency-locked to the reference oscillator. Part of the local
oscillator's circuitry is in the prescaler IC.
A clamp and rectifier circuit on the RF board generates a negative DC voltage of -4 V (nominal) for
increasing the total voltage available to the first VCO and second local oscillator's VCO. The circuit
receives a 300 kHz square wave output from the prescaler IC, then clamps, rectifies, and filters the
signal for use as the negative steering line for the two VCOs.
2.8

Voltage-Controlled Oscillator

This section discusses the voltage-controlled oscillator components and basic operation for each
band.
2.8.1

VHF Radios

The voltage-controlled oscillator (VCO) assembly utilizes a common-gate Field Effect Transistor
(FET) in a Colpitts configuration as the gain device. The LC tank circuit's capacitive portion consists
of a varactor bank and a laser-trimmed stub capacitor. The inductive portion consists of microstrip
transmission line resonators. The stub capacitor serves to tune out build variations. Tuning is
performed at the factory and is not field adjustable. The varactor network changes the oscillator
frequency when the DC voltage of the steering line changes. The microstrip transmission lines are
shifted in and out of the tank by PIN diodes for coarse frequency jumps. A third varactor is used in a
modulation circuit to modulate the oscillator during transmit.
July 1, 2002
General Overview: Front-End Receiver Assembly
68P81076C25-C