SOUTHERN AVIONICS COMPANY
Model SA100 Dual 2-1Theory of Operation, Radiobeacon
2 Theory of Operation, Radiobeacon
2.1 Functional Description: SA100
Synthesizer: The RF signal is generated on the KWOSYN PWB by a voltage controlled LC
oscillator operating at twice the RF frequency. This signal is divided to produce a 1 KHz signal
that is fed to a phase locked loop (PLL) and compared with a 1 KHz signal produced by a divided
4.096 MHz crystal oscillator signal. The PLL voltage controls a tuning diode in the LC oscillator.
The LC oscillator output is divided by two to produce a square wave output at the frequency
dialed in with three rotary switches on the board.
RF Driver: The square wave output of the synthesizer board is buffered with an inverter on the
EXCITER MOTHERBOARD PWB before being routed to the Power Amplifier input.
Tone Key: Two separate audio tones, 400 Hz and 1020 Hz, are generated on the TONE KEY
PWB. One of these tones is selected and fed through a gate controlled by the Keyer, a gate
controlled by the AUDIO PWB, and a front panel Modulation Level Control.
Audio (optional): Whenever a voice signal is added to the standard beacon signal, it is
processed on the AUDIO PWB. The processing consists of audio filtering, automatic gain control
(AGC), squelch control, and clipping.
Modulator Driver: The code tone signal, a DC level, and the voice signal are added, buffered,
and level adjusted on the EXCITER MOTHERBOARD PWB before being routed to the modulator
input.
Keyer: The Keyer consists of a KEYER CODE PWB, and one or two CODER SHIFT
REGISTER PWBs depending on the length of the code. An 8 Hz (adjustable) clock on the
KEYER CODE PWB serially shifts code bits stored on the CODER SHIFT REGISTER PWBs to
the tone gate on the TONE KEY PWB. The code is programmed with DIP switches on the
CODER SHIFT REGISTER PWB in a straightforward manner; ie., a dot is one switch closure, a
dash is three switch closures, space between letters is three switch opens, etc., so that a dot is a
125 msec tone, a dash is a 375 msec tone, etc.
Modulator: The composite signal from the modulator driver drives a pulse width modulator that
generates a 100 KHz pulse train with pulse widths proportional to the input voltage. These
pulses drive a step down switching regulator which generates the modulating voltage for the
power amplifier.
Power Amplifier: The half-bridge switching power amplifier amplifies the RF Drive signal and
delivers an amplitude modulated signal with a power level up to 400 watts peak to the filter
module.
Filter: The filter module covers 190-535 KHz in 5 bands. Bands are selected with jumpers.
Monitor: The filter output passes through the HWRF PWB to the 50 Ohm RF output. Forward
and reflected power are sampled on this board and used to drive a meter reading for forward
power, reflected power and percent modulation. The forward and reflected power signals are
processed on the MONITOR CTRL PWB to provide shutdown signals for low power, low tone
level, continuous tone, and high reflected power.
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