CTI Products TSAM Transmitter Steering & Audio Matrix - Manual (Page 73)

TSAM Installation and Maintenance Rev. 2.10 Chapter 8
CTI Products, Inc. Maintenance Theory
65
8.2.4 Transmitter Audio Circuitry
Transmit audio from the console's Main and secondary channels connects to the Master TSAM. The TX
audio line receivers match the line impedance, and adjust the input audio to the proper level. The line
receiver circuit is adjusted so peak input audio is at 0 dBm before the keying tones are inserted. This level
is necessary so that keying tones inserted into the transmit audio path have the proper levels.
Line receiver circuits consist of a jumper selectable input attenuator and an adjustable gain input amplifier.
The input amplifier has a selectable 600 Ohm or 10K Ohm input impedances.
The keying tone circuit consists of tone generator IC106, 10 dB and 30 dB resistive pads, and an analog
multiplexer IC108. When sending HLGT or Function Tones, the mux passes audio direct from the tone
generator or through the 10 dB pad, but console transmit audio is muted. When sending LLGT, the mux
passes audio from the 30 dB pad as well as console transmit audio, allowing LLGT mixed with console
audio to reach the transmitter.
Figure 18 TX Board Audio Block Diagram
Crosspoint Audio Switch
Audio and keying tones from analog mux IC108 are routed to the transmit line drivers through the
crosspoint switch IC111. Peak voice and HLGT levels are adjusted to 0 dBm at the switch. This level
optimizes signal to noise and minimizes crosstalk.
Transmit Audio Line Drivers
Referring to the transmit board schematic and examining the TX 1 output in detail, we see IC101A and
IC101B make up a differential driver. This differential drive is applied to the line transformer T101
through a pair of impedance matching resistors. Op amp IC101 is optimized for driving the low
impedance wireline loads. R117 controls the gain of the first inverting amplifier in the differential driver.
The second amplifier inverts the output of the first amp and has a gain of 1. Zener diodes DZ101-102 and
gas tube GT101 provide transient over-voltage protection for the drive circuit.