User's Manual

Theory of Operation GMH Series VHF Radio
Page 4-4 BK RADIO
4.3.2 RX/TX Board
4.3.2.1 Receiver
Buffer - Q101 is an L.O. buffer which provides approximately 15 dB of gain to supply the mixer and
transmitter line-up.
Front End - The preselectors are varactor-tuned direct coupled filters with impedance transformations built
into the configuration. The bipolar preamp provides approximately 18 dB of gain to overcome filter losses
and provides a low noise figure.
Mixer/IF - The active singly-balanced JFET mixer converts the signal to an IF of 16.9 MHz where it is
filtered by the crystal filters and amplified by the IF amplifier.
Demodulator IC - I1 is a multi-function IC which provides a second mixer, second IF oscillator, amplifier,
and quadrature detection.
Antenna Switch- The antenna switch provides RX/TX isolation and switching using a TX series PIN diode
pair (which is located on the High Level PA Board) and RX shunt PIN diode, with the coaxial cable running
from the High Level PA Board to the RX/TX Board being one quarter wave-length at mid-band.
Front End Tuning- I2 is a Hex DAC that provides an independent tuning capability for each of the five
varactor elements in the two pre-selectors.
4.3.2.2 Low Level Power Amplifier
Line-up- The low level power amplifier consists of the following stages and power levels:
Low Level Amp to 0.02 watt
Driver to 0.2 watt
Final to 2.0 watts
Broadband matching networks are used throughout the line-up. The Low Level Amp runs class A, and the
driver and final are operated in class C mode. The Power control voltage is applied to the driver stage
collector.