Operating instructions

Page 12 RF Technology R350/R500
5 CIRCUIT DESCRIPTION 5.5 Audio Signal Processing
5.5 Audio Signal Processing
A 4 kHz low pass filter (U27b) is used to remove high frequency noise from the
signal. A 300 Hz high pass filter (U26a,b) then removes the sub-audible tones. A 240
Hz notch filter (U26c,d) is used to improve the rejection of tones above 200 Hz.
The audio can be set for either 750 uS de-emphasis or a flat response by JP2. JP2
selects the feedback network of amplifier U27c.
After de-emphasis and filtering, the audio signal is applied to the inputs of two analog
switches (U17a,b). These switches are controlled by the micro-controller and squelch
or mute the audio to the line and monitor output circuits. The monitor audio can be
unsquelched by S1 which keeps U17a switched on.
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 IC
(U22a,b). U22 is a dual amplifier connected in a bridge configuration to drive the
600Ω 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 10 kHz 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 micro-controller and to analog switch U17d. U17d varies the gain of
the noise amplifier to provide approximately 2dB hysteressis.
5.7 Sub-Tone Filter and CTCSS
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 CTCSS hybrid and the rear panel system connector. The filtered output can be
used for re-transmission of CTCSS or DCS.
The CTCSS decoder module is a micro-controller base hybrid module. Under control
of the main microprocessor U15 it can decode all 38 EIA 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.