User Manual

4-1
June 1997
Part No. 001-3412-002
SECTION 4 CIRCUIT DESCRIPTION
4.1 GENERAL
4.1.1 INTRODUCTION
The main subassemblies of this transceiver are
the RF board, VCO board, TCXO and Loader board.
A block diagram of the transceiver is located in Figure
4-1. The 3412 is also available in transmit only and
receive only models.
The VCO board is enclosed by a metal shield and
soldered directly to the RF board. The VCO is not
serviceable.
The 3412 has a reference oscillator stability of
±1.5 PPM. The 17.5 MHz TCXO (Temperature Com-
pensated Crystal Oscillator) is soldered directly to the
RF board. The TCXO is not serviceable.
4.1.2 SYNTHESIZER
The VCO (voltage-controlled oscillator) output
signal is the receiver first injection frequency in the
Receive mode and the transmit frequency in the
Transmit mode. The first injection frequency is
52.95 MHz above the receive frequency. The fre-
quency of this oscillator is controlled by a DC voltage
produced by the phase detector in synthesizer chip
U811.
Channels are selected by programming counters
in U811 to divide by a certain number. This program-
ming is performed over a serial bus formed by the
Synth Clock, Synth Enable, and Data pins of J201.
This programming is performed by the Loader board
or user supplied hardware/software (see Section 3).
The frequency stability of the synthesizer in both
the receive and transmit modes is established by the
stability of the reference oscillator described in the
preceding section. These oscillators are stable over a
temperature range of -30° to +60° C (-22° to +140° F).
4.1.3 RECEIVER
The receiver is a double-conversion type with
intermediate frequencies of 52.95 MHz / 450 kHz.
Two helical bandpass filters reject the image, half IF,
injection, and other unwanted frequencies. A four-
pole crystal filter and an 8-pole ceramic filter enhance
receiver selectivity
4.1.4 TRANSMITTER
The transmitter produces a nominal RF power
output of 5W at 13.6V 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. See Section 1.2.2.
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. As stated previously,
the synthesizer output signal is produced by a VCO
(voltage controlled oscillator). The VCO frequency 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 fre-
quency.
Programming of the synthesizer provides the data
necessary for the internal prescaler and counters. One
input signal is the reference frequency. This frequency
is produced by the 17.5 MHz reference oscillator
(TCXO). The other input signal is the VCO
frequency.