SOUTHERN AVIONICS COMPANY
Model SA100 Dual 2-7Theory of Operation, Radiobeacon
2.3 Detailed Circuit Analysis: SA100
2.3.1 Synthesizer PWB (KWOSYN)
Q1 and its associated circuitry form a voltage controlled Colpitts oscillator with frequency variable
from two times 190 KHz to two times 535 KHz. With L2 in the circuit, the frequency can be tuned
from two times 190 KHz to two times 320 KHz, and with L2 switched out, the frequency can be
tuned from two times 320 KHz to two times 535 KHz, depending on the control voltage from pins
9 and 13 of the phase locked loop (PLL) U2 and the adjustment of L1 and L2. The desired fre-
quency is selected with switches S2, S3, and S4. Closing J1 adds 0.5 KHz to the frequency
dialed in with S2, S3, and S4. This programs the divide by N counter U4 to divide by two times
the frequency in KHz so that the output of U4, pin 23, is 1 KHz. U1 divides the 4.096 MHz crystal
oscillator signal by 4096 so that its output at pin 1 is 1 KHz. These signals are fed to the PLL U2
which locks the two signals by varying the voltage at pins 9 and 13 to control the tuning diode
CR1. The oscillator signal is divided by two by U3 to obtain a square wave output at the same
frequency selected by the programming switches. This signal is fed out to the board at pin 7 to
the RF driver. The signal is also available at TP1. TP2 is used to check the control voltage
which should be between 1 and 8 volts. Q3 and Q4 drive LED DS1 which illuminates whenever
the PLL is in lock.
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