Operation and Maintenance Manual
RF Technology T220 Page 11
5 CIRCUIT DESCRIPTION
The resulting control voltage supplies Q2 through R10, R12 and completes the power
leveling control loop.
5.4 600
Ω
ΩΩ
Ω
Line Input
The 600
Ω
balanced line input connects to line isolation transformer T1. T1 has two
150
Ω
primary windings which are normally connected in series for 600
Ω
lines. The
dual primary windings can be used to privide DC loop PTT signalling or a 2/4 wire
hybrid connection. All four leads are available at the rear panel system connector.
The secondary of T1 can be terminated with an internal 600
Ω
load through JP5 or
left unterminated in high impedance applications.
5.5 Direct Coupled Audio Input
A high impedance (10k
Ω
) direct AC coupled input is available at the system
connector. The direct coupled input connects to U9a which is configured as a unity
gain bridge amplifier.
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.
5.6 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 analog 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.
5.7 CTCSS and Tone Filter
The CTCSS encoder module H1, under control of the main microprocessor U13, can
encode all 38 E.I.A.\ 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