Local Microphone Input; Ctcss And Tone Filter; Audio Signal Processing - RF Technology Eclipse Series Operation And Maintenance Manual

5 CIRCUIT DESCRIPTION
5.7 Local Micr ophone Input
The local microphone input is provided for use with a standard low impedance dynamic
microphone. The microphone output is amplified by U9a before connecting to analogue
switch U10a. U10b inverts the local microphone PTT input to switch U10a ON when the
microphone PTT button is pressed. U10a is OFF at all other times.
The local microphone audio has priority over the other inputs.
microphone PTT input switches OFF the audio from the line or direct inputs through D16 and
U10c
5.8

CTCSS and Tone Filter

The CTCSS encoder module H1, under control of the main microprocessor U13, can encode
all 38 EIA tones and (on some models) additional commonly-used tones.
The tone output of H1 connects to jumper JP8 which is used to select either H1 or an external
tone source. The selected source is coupled to U9c which is a balanced input unity gain
amplifier. The buffered tone from U9c is fed to 300 Hz low pass filter U7c.
On Rev 4 or later revisions, the low pass filter can be by passed by inserting a jumper onto
JP17.
RV3, the tone deviation trimmer, is used to adjust the level of the tone from U7c before it is
combined with the voice audio signal in the summing amplifier U7a.
Back to back diodes D4 and D5 limit the maximum tone signal amplitude to prevent
excessive tone deviation when external tone sources are used.
The subtone amplifier, filter and limiter can be bypassed on Rev 4 or later exciters by
removing the link from JP22 and moving the link in JP16 to the alternate position.
5.9

Audio Signal Processing

Jumper JP4 selects either the line or direct input source.
connected to JP6. JP6 can be removed to provide 20 dB attenuation when the input level is
above 10 dBm to expand the useful range of the line level trimmer RV4. The wiper of RV4 is
coupled to the input of the input amplifier U9d. U9d provides a voltage gain of ten before
connecting to the input of analogue switch U10c.
The outputs of U10a and U10c are connected to the frequency response shaping networks
C52, R133 (for 750µs pre-emphasis) and C61, R55 (for flat response). JP7 selects the pre-
emphasized or flat response.
The audio signal is further amplified 100 times by U7d. U7d also provides the symmetrical
clipping required to limit the maximum deviation. The output level from U7d is adjusted by
RV1, the deviation adjustment, before being combined with the tone audio signal in the
summing amplifier U7a.
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RF Technology T800

5.7 Local Microphone Input

Activation of the local
The selected source is then
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