User Manual

CIRCUIT DESCRIPTION
4-2
Part No. 001-3422-003
4.1.3 RECEIVER
The receiver is a double-conversion type with intermediate frequencies of 21.45 MHz / 450 kHz. Varactor
tuned LC bandpass filters reject the image, half IF, injection, and other unwanted frequencies. A four-pole crystal
filter enhances receiver selectivity.
4.1.4 TRANSMITTER
The transmitter produces a nominal RF power output of 5W at 13.3V DC, adjustable down to 1W. Frequency
modulation of the transmit signal occurs in the synthesizer. Transmit audio processing circuitry is contained in the
Loader board or customer-supplied equipment.
4.1.5 LOADER BOARD
The Loader board, Part No. 023-3240-330, is a plug-in circuit board used to load the synthesizer with a
desired frequency and filters data/audio to and from the user interface connector. The Loader board is programmed
by a personal computer and software.
4.2 SYNTHESIZER
A block diagram of the transceiver is shown in Figure 4-1 and a block diagram of Synthesizer IC U811 is
shown in Figure 4-2. The synthesizer output signal (produced by a VCO) is controlled by a DC voltage produced
by the phase detector in U811. The phase detector senses the phase and frequency of the two input signals and
causes the VCO control voltage to increase or decrease (if they are not the same). The VCO is then "locked" on
frequency.
Synthesizer programming provides the data necessary for the internal prescaler and counters. One input
signal is the reference frequency. This frequency is produced by the 14.85 MHz reference oscillator (TCXO). The
other input signal is the VCO frequency.