3: Audio/Filter Board
PRC1099A-MS
3-5
position (refer to “SQUELCH Position” on page 12-4).
The audio output from product detector U4 is fed into two sections of quad op
amp U1. These stages operate at high gain and the noise or signal is amplitude
limited, producing a square wave output to trigger one-shot U2A and U2B.
The output from the one-shot is a series of pulses that go through the
high-pass filter formed by C2, R5, C3, and R6 and the low-pass filter R7, C4,
R8 and C5. This presents a DC level signal proportional to the input frequency
of the one shot to the inverting input of U1C.
The DC level changes as the frequency shifts, representing the change in
frequency of the human voice at a syllabic rate between 0.5 and 2.5 Hz. This
signal is AC coupled through C6 to low frequency differentiation amplifier
U1C. The output is applied to U1D, which pulls down D4 if any signal
appears at D2/D3 with a positive or negative amplitude of 0.9V or greater.
This unipolar converter enables the squelch to detect the first syllabic rate
change at the output of the product detector.
Diode D4 is connected to U2C and U2D that form a timer with R16 on the
input and C9 on the output to hold the audio on for approximately four
seconds after the last syllabic change is detected. The squelch is turned off
when the timer output is grounded through R18 (MUTE). The microprocessor
can also disable the audio through D23 by setting the output at U9 pin 5 low.
Squelch Gates
Squelch switch PNP bipolar transistor Q6 turns off the receive audio by
switching off the +12 V supply to U5. Q6 is controlled by the input from side
tone switch Q24 or by Q1 and Q2 that are, in turn, driven by the squelch timer
U2C and U2D or the MUTE signal from the processor. The output from Q1
also controls current reduction circuitry in the Mixer board. This circuitry is
also controlled by the T8 line, isolated by diode gates D5 and D6.
Audio Amplifier
Handset/Speaker
The output from product detector U4 is coupled through the front panel audio
gain control potentiometer (VOLUME) R1 to audio amplifier U5, with a gain
of 26 dB. This device has a constant voltage output into a wide range of load
impedances. It also provides direct output to the handset or earphones
(50 OHM RX AUDIO).
Audio Amplifier
600 Ohm
From product detector U4, the receive audio signal is applied to audio
amplifier U10A. U10A is half of a fixed-gain operational amplifier that
provides a separate fixed-level audio output for external devices such as
RTTY modems. The amplifier has low impedance output and provides a level
of 0 dBm into a 600 ohm load (600 OHM RX AUDIO) connected to the front
panel Accessory connector.
