Audio Signal Processing; Ptt And Dc Remote Control - RF Technology T220 Operation And Maintenance Manual

5 CIRCUIT DESCRIPTION
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.
5.8

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
votlage gain of ten before connecting to the input of analog switch U10c.
The outputs of U10a and U10c are connected to the frequency response shaping
networks C52, R133 (for 750 µ s pre-emphasis) and C61, R55 (for flat response). JP7
selects the pre-emphasised or flat response.
The audio signal is further amplified 100 times by U7d. U7d also provides the
symetrical 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. The
filtered audio is coupled to the TCXO voltage tuning input and the modulation
balance trimmer RV2.
RV2 adjusts level of the audio used to modulate the VCO. This primarily effects the
deviation of audio frequencies above 500 Hz. RV2 is used to balance the high and
low frequency deviation to obtain a flat frequency response relative to the desired
characteristic.
5.9

PTT and DC Remote Control

Two main PTT inputs are provided. The first, a direct logic level input, is connected
to pin 3 of the system connector. The transmitter can be keyed by applying a logic
low or ground on pin 3. Pin 3 connects to the PTT logic and microprocessor through
D10.
DC current loop control can be used for remote PTT operation. The current loop can
be configured by JP9, JP10 and JP11 for use with either a remote free switch or a
remote switched source.
Opto-isolator ISO1 is used to isolate the loop current signal from the transmitter PTT
logic. The loop current passes through the input of ISO1 and the output of ISO1
connects to the PTT logic.
Page 12 RF Technology T220