4RF Aprisa XE User Manual page 111

Circuit Levels
The 8 bit digital word for each analogue sample encoded (A law), has a maximum of 255 quantizing code
steps, a maximum of + 127 for positive signals and a minimum of - 127 for negative signals. No signal is
represented by the code step 0.
A nominal level of 0 dBm generates a peak code of ± 118 which allows up to + 3.14 dBm0 of headroom
before the maximum step of 127 is obtained. Any level greater than + 3.14 dBm0 will be distorted
(clipped) which will cause severe problems with analogue data transmission.
It is therefore important that analogue signals presented from the DFXO / DFXS line interface be
normalized to fit within the ± 127 quantizing steps of the encoder. This is done by adjusting the circuit
levels relative to the 0 dBm (± 118 peak code) for example:
If a nominal input level of +1 dBm is applied to the DFXS line interface, the DFXS Input Level must
be set to +1.0 dBr. This will effectively attenuate the sent signal by 1 dB to produce a digital
signal with a 118 peak code (0 dBm).
If a nominal output level of -6 dBm is required from the DFXS line interface, the DFXS Output Level
must be set to -6.0 dBr. This will effectively attenuate the received signal by 6 dB.
The circuit levels and the transhybrid loss of both ends of the circuit, also determine the stability of the
circuit. If the circuit levels are too high and the transhybrid loss figures achieved are too low, the circuit
can have a positive loop gain and can recirculate (sometimes called singing).
Typically, an end to end 2 wire voice circuit is engineered to have a 2-3 dB loss in both directions of
transmission.
Exchange
Line Card
0.0 dBm
4WR
0 dBr
Transmission
Reference Point
Z
B
0 dBr
4WS
-2.0 dBm
Overall Loss
= 3.0 dB
Derived System Level Plan
DFXO
Interface
-6.0 dBm
-6.0 dBr -4.0 dBr
2WR 2WS
2WS 2WR
+1.0 dBr -1.0 dBr
-1.0 dBm
Note 1: The derived system loss is 2 dB in both directions due to the deliberate 2 dB level mismatch between
Aprisa XE User Manual
Configuring the Traffic Interfaces | 109
Aprisa XE
DFXS
Interface
-2.0 dBm
4WS 4WR
0 dBr
Z
Z
B B
0 dBr
4WR 4WS
0.0 dBm
the exchange line card and the DFXO interface unit
Overall Loss
= 8.0 dB
-8.0 dBm
-6.0 dBr
2WR output
2WS input
+1.0 dBr
+1.0 dBm