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TN-1114-AN Page 5 of 10

5 Connecting the TB8100 to an External Modem

Connection Connect the TB8100 to the external modem using the unbalanced line

input (Tx audio) and output (Rx audio) on the system interface connector.

If required, you can also use the Rx Gate and Tx Key lines, as described

below.

Details of the pin allocations for the different types of system interface

connector are provided in the “Connection” chapter in the TB8100

Installation and Operation Manual, and also in the TB8100 Service Kit

(Configure > Base Station > System Interface).

AC Coupled

Unbalanced Audio

Input

The modem selected may have a DC offset present for both the transmitter

and the receiver. On the standard system interface board, the transmit path

is DC coupled (with a bias of 2.5V), and the receive path is AC coupled.

We recommend that you enable the high pass filter in the Signal Path tab of

the channel profile dialogue box in the Service Kit (refer to “Configuring

the Base Station” on page 7). This will ensure that the transmitted signal is

exactly on the configured carrier frequency. If you do not enable the high

pass filter, ensure that you set the transmit carrier frequency exactly to the

required frequency by adjusting the DC bias of the external modem.

Note The CODECs used to sample the unbalanced line input are DC

capable. However, because of component variations, the DC

offset incorrectly reported by the CODEC can lead to a small

offset in the transmit carrier frequency, which is removed by

selecting the high pass filter.

Figure 1 Typical connection

Tx Key

Rx Gate

Rx Audio

Tx Audio

Ground

TB8100

System

Interface

Modem

Tait TB8100 Using

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