TA2922 Universal Line Interface Service Manual 15
© Tait Electronics Limited March 2010
The balanced output is isolated by a 1:1 600 ohm line transformer, T1. The
transformer secondary has a centre tap for use with DC keying options.
Unbalanced audio is sourced from one half of the power amplifier’s output which
drives the line transformer. The signal is AC coupled and has an output impedance
of 100 ohms, making it suitable for driving loads such as audio headsets.
Line in
The line in audio interface features:
■ balanced 600 ohm line in audio to radio TX audio
■ unbalanced 50k ohm impedance audio input to radio TX audio.
The line in audio is always available to the radio transmitter at the AUD_TAP_IN
line of the radio internal option connector. The transmitter is enabled when the
TX Key or PTT input to the TA2922 board is activated. The audio filtering and
flat/pre-emphasis selection is defined in the programming of the radio audio
options.
Balanced line input enters the board through a 1:1 600 ohm line transformer, T2,
which also provides line isolation. If a 2-wire audio interface is configured,
transformer T2 is not used and instead all transmit and receive line audio is via
transformer T1. In this 2-wire configuration, link LK4/R62 is fitted for audio
routing which makes the unbalanced audio input unavailable.
When link LK4/R62 is not fitted, the unbalanced line input is available for ground
referenced audio. Input impedance is 50k ohms and the signal is AC coupled.
2.2 TX Key Input
Both opto-isolated and un-isolated transmitter keying options are available.
The optocoupler input operates between 10 to 50V and is not polarity sensitive. It
can be connected to two input lines on the external connector, or be connected
to the centre tap of transformer T1 for line DC keying (also known as phantom
keying) with the suitable placement of option links. Both positive and negative
keying voltages are supported.
The un-isolated TX key input PTT_DIRECT is 3.3V logic compatible where a
DC voltage of under 2.5V keys the transmitter. A 10k ohm 3.3V pull up resistor is
included on this line. The maximum voltage on this line should not normally
exceed 5V. This line can be driven directly from any open collector or relay contact
output.
2.3 RX Gate Output
Both relay isolated and un-isolated open collector outputs are available for RX
gate/busy detect output.
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