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APPENDIX D
Detailed Reference
ECHO RETURN LOSS
(ERL)
INSERTION LOSS
TERMINOLOGY
D-4
The following paragraphs provide details on terms and concepts which
are used in this appendix. You should refer to this material for precise
definitions of terms used throughout the appendix.
Echo return loss is the signal which escapes through the transmit path of
the hybrid when a signal is being received. The return loss of any hybrid
should be at least 18 dB. The CELP is capable of accepting return losses of
as little as 6 dB although this is not recommended.
Insertion loss is the loss from 2-wire side to the 4-wire side of the hybrid
and vice versa. Many hybrids exhibit a 6 dB insertion loss. Generally,
active hybrids are designed to operate at 0 dB insertion loss.
2-WIRE
Z IN
036R480DI
It is important to understand some terms and concepts when aligning
voice circuits. Two important terms that come to mind are 'dBm0' and
'TLP'. 'dBm0' is the signal level relative to the TLP. 'TLP' is the level of
a 0 dB tone. This level is used when aligning circuits. when reading the
following statements pay close attention to the use of dBm, dBm0 and dB.
A Voice channel module operates best when the input is close to 0 dB at the
CODEC chip input. Therefore a Voice channel optioned for 0 dB gain or
loss on the transmit side, would require that the VF signal (analog) input
to the Voice channel be aligned to 0 dBm, for optimum performance.
Therefore the module is said to have a 0 dB TLP or 0 TLP. If 16 dB of
gain were selected, then –16 dB would be the TLP.
A Voice channel optioned for –4 dB (4 dB of attenuation), would require a
+4 dBm VF input for optimum performance. If you were asked to inject a
–10 dBm0 test tone, you would be correct to inject a –6 dBm signal, since
+4 dB VF input TLP would be the 0 dBm level, simply because of the at-
tenuation option setting of –4 dB.
HYBRID
TRANSMIT
4-WIRE
IMPEDANCE
MATCHING
CIRCUIT
RECEIVE
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