Nortel Circuit Card, Installation Manual, Page: 71 / 908

Overview Page 71 of 906
Circuit Card Description and Installation
Variable gain filters
Audio signals received from the analog phone trunk are passed through a
low-pass A/D monolithic filter that limits the frequency spread of the input
signal to a nominal 200–3400 Hz bandwidth. The audio signal is then applied
to the input of the codec. Audio signals coming from the CODEC are passed
through a low-pass A/D monolithic filter that integrates the amplitude
modulated pulses coming from the CODEC, and then filters and amplifies the
result.
On some of the trunk cards, the gain of these filters can be programmed by
the system controller. This allows the system to make up for line losses
according to the loss plan.
Balancing network
Depending on the card type, the balancing network is capable of providing
either a 600 ohm or a 900 ohm (or both) impedance matching network. It also
converts the 2-wire transmission path (tip and ring) to a 4-wire transmission
path (Rx/ground and Tx/ground). The balancing network is a transformer/
analog (hybrid) circuit combination.
Signaling circuits
Signaling circuits are relays that place outgoing call signaling onto the trunk.
Signal detection circuits monitor the incoming call signaling.
Control signals
Control signals and logic are provided when the trunk is going to be
connected to special services such as recorded announcement and paging
equipment.