TR-41.3-01-02-003
STANDARDS PROJECT
PN-4350
TITLE
History of Return Loss Requirement
SOURCE
VTech Innovation
246 Industrial Way West
Eatontown, NJ 07724
CONTACT
Stephen R Whitesell
Phone 732 544 3017
Fax
732 544 3540
Email swhitesell@vtechphones.com
DATE
February 12, 2001
DISTRIBUTION
TR-41.3.5
Abstract
A question was raised about the history of the Return Loss requirement at the November 2000 meeting in
Savannah. It specifically had to do with what drove the rather substantial change in the requirement
between 470-A and 470-B. This contributor indicated he believed it was done to harmonization with the
CSA T510 standard but agreed to check and see what documentation was available in his files. The
documentation clearly points to that harmonization effort as being the driving factor.
Disclaimer:
This contribution has been prepared to assist the TIA standards development process. It is offered as a basis for
discussion and is not a binding proposal on VTech Innovations. The contents are subject to change in form,
numerical value, or both after further study. VTech Innovations specifically reserves the right to add to, revise, or
otherwise amend the information contained herein.
Intellectual Property Statement:
The individual preparing this contribution is unaware of patents, the use of which may be essential to a standard
resulting in whole or in part from this contribution.
Copyright Notice:
The contributor grants a free, irrevocable, non-exclusive license to the Telecommunications Industry Association
(TIA) to incorporate text contained in this contribution and any modification thereof in the creation of a TIA standards
publication; to copyright in TIA’s name any standards publication even though it may include portions of this
contribution; and at TIA’s sole discretion to permit others to reproduce in whole or in part the resulting TIA standards
publication.
2
History
The oldest document related to return loss that I have is the Bell Laboratories Transmission Objectives
Handbook, dated June 1966. It clearly distinguishes between echo return loss and singing return loss in
section 9.4.3:
Return losses are controlled by two objectives, echo return loss and singing return loss. Echo
return loss is a weighted average of the return losses in the echo band (500 to 2500 cps) and is
intended to be a measure of the relative importance of echo to the return losses. At the present
time there are several weightings being used in determining echo return loss, each of them
resulting in slightly different values. The determination of the correct weighting is being currently
studied. Singing return loss is the lowest return loss at any single frequency within the
transmission band. It is intended to measure return losses, particularly at the frequencies where
singing usually occurs (200 to 500 cps and 2500 to 3200 cps).
However, this document is not particularly useful in establishing requirements for terminal equipment.
Instead, it provides an objective for the return loss looking into a loop with an off-hook telephone from the
“end office.” This objective is measured against 900 Ω + 2.16 µF. The objective for echo return loss
(ERL) is stated as a mean value of 11 dB with a standard deviation of 3 dB. The objective for singing
return loss (SRL) is stated as a mean value of 6 dB with a standard deviation of 2 dB. I do note, however,
that 11 dB is the same number as used for the echo return loss limit in TIA/EIA-470-B, even though it is
measured against a different impedance, differs by whether twice the loss of the loop is included, and is
expressed in one case as a minimum limit and in the other as an objective for the mean.
This concept of a singing return loss requirement for the 200 to 3200 Hz band and an additional echo
return loss requirement for the 500 to 2500 Hz band appears in several internal documents that I have.
The first documentation I can find of return loss requirements for telephones is in some internal
documents dating from 1978. This information was contributed to EIA when EIA RS-470 was being
developed as SP-1286 in the 1977 to 1980 time frame. When RS-470 was published in January 1981, it
contained the return loss requirement reproduced in Appendix A. Virtually the same requirement was
also published in January 1980 in AT&T PUB 48005.
The requirement is framed as having two criteria that must be met: (a) 3.5 dB of return loss over the 200
to 3200 Hz band and (b) 7.0 dB of return loss over the 500 to 2500 Hz band. Neither EIA RS-470 nor
AT&T PUB 48005 distinguish the 7.0 dB requirement to be a weighted average across the 500 to 2500
Hz frequency band. The use of an oscillator (as opposed to a broad band noise generator) for the
measurement as depicted in Figure 18 of RS-470 (and its equivalent in PUB 48005), may be taken as an
indication that swept frequency measurements were to be made. From a conservative point of view, if
the EUT met a 7 dB return loss requirement at every point in the 500 to 2500 Hz band, it would certainly
meet a 7 dB echo return loss requirement averaged across that band.
The 470-A version of the document made some minor changes to the return loss requirement, but did not
make any changes to the two fundamental criteria noted above. The text for this document is also
reproduced in Appendix A, with the changes from the 470 version noted using change tracking.
However, the 470-B version is a completely rewritten requirement as also shown in Appendix A. In it, the
requirement for the 500 to 2500 Hz frequency range is clearly identified as applying to echo return loss in
the talk state only, but the minimum acceptable level is increased to 11 dB. The 200 to 3200 Hz return
loss requirement remains, but it is identified as applying in all off-hook states including hold and mute.
The allow limit is also dropped slightly to 3 dB minimum. All return loss requirements are now against 600
Ω + 2.16 µF, whereas they had been against 600 Ω in the earlier versions.
These changes can be traced to the harmonization effort that was made to align 470-B with CSA T5101995. Appendix A also contains two scanned pages from Northern Telecom (now Nortel Networks)
Contribution TR-41.3/92-09-018 made by Dennis Rittenhouse. It shows a side-by-side comparison of the
requirements wording in 470-A and T510-M92. Note the wording of T510-M92 is essentially the same as
that adopted for 470-B and T510-1995.
Appendix A
3
EIA RS-470, January 1981
4.5.2
4.5.2.1
Off-Hook Tip-to-Ring Impedance
Definition
Singing and barrel effect result when the telephone impedance is significantly different from the
line impedance at the interface. A measure of this discontinuity can be determined by return loss,
defined as follows:
Return Loss (dB) = 20 log |(Z1 + Z2)/(Z1-Z2)|
where Z1 and Z2 are the two impedances at the interface in the frequency band 200 to 3200 Hz.
4.5.2.2
Method of Measurement
Connect the telephone and a suitable dc supply circuit as in Figure 18 and measure the return
loss against 600 ohms over the loop conditions specified in Figure 3.
4.5.2.3
Standard
The return loss of a telephone at the output terminals measured against a 600-ohm resistor shall
be greater at each frequency than the following, over the range of loop current of the loop
simulator of Figure 3.
Frequency (Hz)
Minimum Return Loss (dB)
200-3200
3.5
500-2500
7.0
It is desirable that the off-hook impedance be 600 ohms, across the 200-to-3200 Hz band.
+
–
125 µF
DC Portion
of Loop
Simulator
Figure 3
600 Ω
External Balance Network
Tip
Return Loss
Measuring Set
Ring
(Wiltron 9031,
or Equivalent)
Telephone
OSC
External Oscillator
600Ω Impedance
0 dBm Level
200-3200 Hz
Figure 18
Return Loss Test Configuration
[SRW Comment: The value and polarity markings for the capacitor are shown in Figure 18 of the
published document, but the actual capacitor symbol is missing. It was present in the SP-1286
and SP-1286-A copies of the document that were circulated for ballot.]
4
EIA-470-A, July 1987
4.5.2
4.5.2.1
Off-Hook Tip-to-Ring Impedance
Definition
Singing and barrel effect result when the telephone impedance is significantly different from the
line impedance at the interface. A measure of this discontinuity can be determined by return loss,
defined as follows:
Return Loss (dB) = 20 log |(Z1 + Z2)/(Z1-Z2)|
where Z1 and Z2 are the two impedances at the interface in the frequency band 200 to 3200 Hz.
4.5.2.2
Method of Measurement
Connect the telephone and a suitable dc supply circuit as in Figure 4-18 and measure the return
loss against 600 ohms over the loop conditions specified in Figure 4-3. The transmitter cap and
ear cap of the telephone set under test should be provided with sound-attenuating covers to
exclude ambient sound pressure and provide proper acoustic termination for the receiver.
4.5.2.3
Standard
The return loss of a telephone at the output terminals measured against a 600-ohm resistor shall
be greater at each frequency than the following, over the range of loop current of the loop
simulator of Figure 4-3.
Frequency (Hz)
Minimum Return Loss (dB)
200-3200
3.5
500-2500
7.0
It is desirable that the off-hook impedance be 600 ohms, across the 200-to-3200 Hz band.
125 µF
DC Portion
of Loop
Simulator
Figure 3
600 Ω
External Balance Network
Tip
Return Loss
Measuring Set
Ring
(Wiltron 9031,
or Equivalent)
Telephone
OSC
External Oscillator
600Ω Impedance
0 dBm Level
200-3200 Hz
Figure 4-18
Return Loss Test Configuration
[SRW Comment: The capacitor in Figure 4-18 has been changed to non-polarized.]
5
TIA/EIA-470-B, November 1997
4.4.4 Off-Hook Metallic Impedance
4.4.4.1
Requirements
The off-hook metallic impedance of the telephone set shall meet the following requirements,
when measured as specified in Clause 4.4.4.2:
(a) The off-hook impedance shall meet an echo return loss (ERL) of 11 dB over a frequency
band of 500 to 2500 Hz when measured against 600 Ω in series with 2.16 µF. This
requirement shall apply during the talk state.
(b) The return loss in the frequency band 200 to 3500 Hz shall be greater than 3 dB when
measured against 600 Ω in series with 2.16 µF. This requirement applies during all operating
states, including, for example, hold and mute.
(c) The return loss during generation of DTMF signals shall be greater than 3.5 dB over the
frequency range of 200 to 3200 Hz and over the range of loop currents given in Figure 40.
4.4.4.2
Method of Measurement
The following steps shall be taken:
(a) Connect the telephone set to the test circuit as shown in Figure 28.
(b) Condition the telephone set not to transmit any signals in the off-hook mode.
(c) Put the telephone set in the off-hook mode with the loop current set at 70 mA or maximum
current, whichever is less.
(d) Arrange the return loss set calibration to measure ERL against 600 Ω in series with 2.16 µF.
Record the meter reading.
(e) Set the return loss set “test type” switch to ”ext. osc.” And calibrate the set to measure return
loss vs frequency against 600 Ω in series with 2.16 µF.
(f) Vary the oscillator frequency from 200 to 3500 Hz and record the minimum value of return
loss and the frequency at which it occurs. Adjust the current from the dc feed circuit for the
minimum value obtainable but not less than 20 mA. Vary the oscillator frequency as before,
and record the minimum value of return loss and the frequency at which it occurs.
(g) Repeat steps (c) to (f) for all remaining off-hook operating states.