ia3222 integration, ia3222 Datasheet - Page 40

• Manufacturer Part Number: ia3222
• Description: Ia3222/ia3223 Ez Daa? Chipset With Analog Interface
• Manufacturer: integration

IA3222/IA3223
RETURN LOSS AND TRANS-HYBRID RETURN LOSS
Telephone devices transmit and receive bi-directionally down a twisted-pair line. All of the transmitted signal would be present on the
receiver were it not for a cancellation circuit called hybrid or two-to-four-wire hybrid. A hybrid works by canceling the actual echo back
from the line with the expected transmitted reflection. The expected reflection is determined by applying the transmitted signal
through an analog of the driver impedance applied across an analog of the line impedance (also called a “line mirror” short for “line-
mirrored impedance” or “balance network”). If the actual line impedance and drive impedance is identical or balanced perfectly by
the line mirror and mirror drive impedance, then the echo cancellation will be complete. In practice, the impedance varies
significantly with the length of the line and with the presence of bridged taps (parallel open-circuit twisted pair stubs), so that echo
cancellation varies with the frequency of the transmitted signal and with the line.
The standard measure of the cancellation of the hybrid balance is return loss. This is a measure of the reflection from the transmit
path back to the receive path in terms of loss (attenuation) normalized to levels on the telephone line and expressed in dB. The
higher the loss, the lower the reflection and the better the hybrid balance. For example, a 20 dB return loss at a given frequency
indicates that the transmitted signal at the receiver will be 20 dB lower than if the same level of signal was received on the
telephone line from an outside source. Return loss for a line and its terminating impedance can also be calculated if both
impedances are known.
Hybrids are normally present both at the SLIC or switch end and at the CPE or loop termination end. Confusion sometimes arises
since return loss is used to measure both the accuracy of the impedance termination on either end of the phone line and also to
measure the efficacy of the hybrid in cancelling the reflection. This latter is called trans-hybrid or four-wire return loss. The return-loss
figure on the termination impedance indicates the reflected signal amplitude due to the impedance mismatch from an ideal
termination.
Many telephone line interfaces, whether SLIC or DAA, may provide several levels of hybrid balancing. Commonly, a first-order analog
compromise line mirror is used, which may provide only about 10 dB of balance over a range of telephone lines. Its purpose is to
reduce the dynamic range of the receiver channel (or codec) by this amount since telephone systems require about 80 dB of
dynamic range. If the application requires better hybrid balancing, then some form of dynamic or line calibration is required.
The degree of total hybrid balancing needed is a function of the telephone application. The main purpose of Central Office or End
Office hybrid balancing is to reduce far-end voice echoes that can be very annoying and degrade the quality of service. Surface
communication paths (optical fiber, coax, etc.) round-trip echoes between the North American West Coast and Europe can approach
200ms. On a new line installation, the hybrid mirror at the Central Office requires tuning at least for loaded versus non-loaded lines.
Telephone-network echo cancellers dynamically adjust hybrid balance, but in order to work best require some minimum compromise
hybrid balance. Central Office hybrid tuning assumes that the telephone line is terminated with a standard impedance. For most of
the world this is 600 Ω. In some countries, the 600 Ω impedance is replaced with an RC network that better approximates the
complex impedance of a long non-loaded telephone line. This reduces hybrid-balancing differences between short and long lines.
In practice, the return-loss matching of the terminating impedance is not very critical for two reasons. First, the bi-directional loss
down a typical telephone line reduces the effects of any 600 Ω or complex impedance mismatch by twice the nominal loss. For
example, since the average telephone line loss is about 4dB, on such a line the effects of a termination mismatch on the Central
Office SLIC hybrid balance will be reduced by 8 dB (the attenuation in each direction). Secondly, the line-length variation of the return
loss is the dominant effect, swamping most production termination impedance variations. For example, a median-length telephone
line of 5,000 ft has an added 400 Ω of series resistance and a distributed capacitance of around 80 nF. Since most speech audio is
below 1 kHz, the telephone-line resistance has the dominant effect on echo that is not mitigated much by the distributed
capacitance. Evidence that termination impedance variation is not critical is suggested by very wide margins on return loss
requirements in mainstream regulatory approval. US FCC Part 68 has no return-loss impedance requirement, that of TBR21 is very
low (only 8 dB) and JATE requires only a few dB.
On the CPE side of the telephone line, there is no regulatory requirement for trans-hybrid return loss, but that parameter has a more
significant functional impact on modem performance or audio quality.
All high-speed full-duplex modems (V.32, V.34, V.90 and V.92) during training build an exact line mirror (of what is left over from the
DAA compromise hybrid) to cancel the echo to better than 70 dB. The IA3222 has a compromise trans-hybrid balance network on
the Line Side that improves the dynamic range performance of the analog channel by minimizing the transmitted noise and
distortion reflected back into the receiver channel. Both of these are very critical for high-speed modems. A hidden critical element
for high-speed modems is hybrid thermal drift. If the hybrid return loss, transmitter or receiver gain drifts by even a small amount
(less than -60 dB) between initial training and several minutes later, the modem performance can be greatly degraded since the un-
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