Operating instructions

RF Technology T350/T500 Page 14
5.7 Local Microphone Input 5 CIRCUIT DESCRIPTION
The bridge configuration allows audio signal inversion by interchanging the positive
and negative inputs and minimizes ground loop problems. Both inputs should be
connected, with one lead going to the source output pin and the other connected to the
source audio ground.
5.7 Local Microphone 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. Activation of the local
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.
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.
5.9 Audio Signal Processing
Jumper JP4 selects either the line or direct input source. The selected source is then
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.
The composite audio from U7a is fed through the 3Khz low pass filter U7b. When the
links on JP23 are in their default state, the filtered audio is coupled to the TCXO voltage
tuning input and the modulation balance trimmer RV2. RV2, R99 and R98 attenuate
the modulation signal before applying it to the VCO via varactor D3.