Operating instructions

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RF Technology R800 Page 11
5.6 Noise Filter, Amplifier and Detector 5 CIRCUIT DESCRIPTION
the audio to the line and monitor output circuits. The monitor output can be set for noise
squelch only operation by S1.
The audio from U17a is adjusted by the volume control before connecting to the monitor
output amplifier U5. U5 drives the internal speaker and can also supply 3-5 watts to an
external loudspeaker.
The audio from U17b is adjusted by RV3 before connecting to the line output I.C. (U22a,b).
U22 is a dual amplifier connected in a bridge configuration to drive the 600 Ohm line output
transformer T1.
5.6 Noise Filter, Amplifier and Detector
The unfiltered audio from the discriminator is fed to trimpot RV4 which is used to set the
noise squelch threshold. From RV4 the audio goes to the noise filter (U27a). This is a
10KHz high pass filter and is used to eliminate voice frequency components.
The noise signal is then amplified by U27d and fed to the noise detector. The noise detector
consists of D6, Q17 and U26c. D6 and Q17 are a charge pump detector and pull the input to
U26c low as the noise increases. U26c has positive feedback and acts like a schmidt trigger.
The output of U26c goes high when noise is detected. It connects to the microcontroller and
to analog switch U17d. U17d varies the gain of the noise amplifier to provide approximately
2dB hysteressis.
5.7 Subtone Filter and C.T.C.S.S.
The discriminator audio is fed through cascaded low pass filters U28a and U28b to filter out
the voice frequency components. The filtered sub-tone audio is supplied to the C.T.C.S.S.
hybrid and the rear panel system connector. The filtered output can be used for re-
transmission of C.T.C.S.S. or D.C.S.
The C.T.C.S.S. decoder module is a microcontroller base hybrid module. Under control of
the main microprocessor U15 it can decode all 38 E.I.A. tones and 12 additional commonly
used tones. The decode bandwidth is set to 1% but may be changed to 2% by a jumper on
the printed circuit board.
5.8 External Squelch
The audio output can be muted through pin 8 of the receiver system connector P1. When pin
8 is pulled to less than 1 Volt above ground, the microcontroller U15 will mute the audio
output.
This facility can be used to mute the audio during transmission, as is required in single
frequency systems, by simply connecting pin 8 of the receiver to the transmitter T/R relay
driver output (pin 16 on Eclipse transmitters).