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W orking Cover Page
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TIA-TSB-31C
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Rationale and Measurement Guidelines for TIA-968
Draft 1
19-August, 2002
W arning: This document is a “work in progress” by TIA TR41.9 and as such it’s contents
may change.
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Appendix F, Alternate Transverse Balance, Digital EUT
A.1 Background
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This annex offers a current based measurement technique, where a ratio of the
stimulating metallic current to the resulting longitudinal current represents the
effective transverse balance. This ''ratio of currents'' technique uses completely
passive toroidal current probes as the detection elements in conjunction with a
well-balanced balun transformer as shown in Figure F-1.
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The test fixture exhibits substantial immunity to parasitic effects of capacitance,
inductance, and laboratory RF noise, while maintaining an extremely accurate
response in the non-traditional telephony frequency bands of ISDN and DSL.
The magnetic devices are available as ''off the shelf devices'', and they are
directly compatible with typical 50 ohm BNC connections to measurement
equipment such as a network or spectrum analyzer. Additionally, the passive
devices completely isolate the analyzer from the Equipment Under Test (EUT),
leaving absolutely no paths through ground or isolation resistors. The transverse
balance of the EUT easily plots as a logarithmic ratio of currents on the screen of
an analyzer for immediate analysis.
Transverse Balance = 20log
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i 1+ i 2
i 1-i 2
- 6dB
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In effect, the transverse balance is a ratio in dB of an applied metallic current
and the resulting longitudinal current. The presence of longitudinal current is
undesirable, and it is well known that such current may disturb adjacent circuits.
Therefore it is highly desirable to identify equipment that emits longitudinal
current.
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A ratio of currents measurement easily and accurately achieves the primary goal
of identifying current absent from a desired equal current flow from tip to ring
during metallic stimulation, across the frequency band of interest. The absent or
lost current, referred to as longitudinal current iL, is quantified by the toroid that
measures i1-i2. The stimulating metallic current iM is quantified by the toroid that
measures i1+i2. The term i1+i2 is nearly twice the metallic current, and could
more accurately be expressed as (i1+i2)-iL, but the iL term is several orders of
magnitude less than iM, and therefore iL is neglected in the numerator.
Neglecting iL in the numerator of the transverse balance equation compensates
for the analyzer’s measuring twice the metallic current, in effect the user
subtracts 6dB from the analyzer's results to get the true transverse balance.
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The mathematical error introduced by this test configuration (where iL is
neglected in the numerator) is insignificant, and can be shown mathematically to
be less than 0.136dB for an analyzer plot that shows 36dB for a given frequency
(transverse balance of 30dB). For the quantified range for pass fail criteria (35dB
minimum), a worst case error of 0.136dB at 30dB is completely acceptable. Of
course, as the quality of the EUT improves, the error reduces proportionately, so
the error at 35dB is truly negligible.
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A.2 Purpose
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To determine transverse balance of digital EUT, by using a ratio of currents.
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A.3 Equipment
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(1)
A precision 50 ohm to 100 ohm balun with greater than 60dB of longitudinal
balance with respect to the center tap on the 100 ohm side (similar to North
Hills 0311LB) SEL# XX.
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(2)
Two of the same model precision wound toroidal current monitors (similar to
Pearson 4100) SEL# XX.
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(3)
A typical network or spectrum analyzer with a frequency range
encompassing the desired frequency of test (See A.5(1) below.). SEL# XX.
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A.4 Equipment States Subject To Test
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Active state with appropriate grounding applied.
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NOTE: Terminal equipment may require special attention to ensure it is properly
configured for this test. For example, if the equipment would normally be
connected to ac-power ground, cold-water-pipe ground, or if it has a
metallic or partially metallic exposed surface, then these points shall be
connected to the test ground plane. Similarly, if the EUT provides
connections to other equipment through which ground may be introduced
to the equipment, then these points shall be connected to the test ground
plane. Equipment which does not contain any of these potential
connections to ground shall be placed on a conductive plate which is
connected to the test ground plane (see comment (1)); this applies to
both non-powered and ac-powered equipment.
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A.5 Procedure
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(1)
Assemble the circuit shown in Figure F-1 and connect the equipment to the
circuit as shown. The frequency range for the analyzer should be at least
200Hz < f < 2MHz.
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(2)
Set the analyzer's 50-ohm tracking generator output to +3dB (equivalent to
0dB into 100 ohms). With switch S2 set in position B, terminate the tip
conductor to ground while the ring conductor is open. This represents the
worst case transverse balance condition, and there is no metallic current
flow. Switch S1 is then toggled and the ground termination is attached to the
opposite conductor, again a worst case transverse balance condition. For
both positions of S1 the analyzer should read about 0dB for any frequency in
the specified transverse balance frequency band. Essentially this process
verifies conductivity and wiring for the test circuit.
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(3)
With switch S2 in position A, the analyzer should read within 1dB of 36dB for
the termination shown (Za=100, Zb=10K, and Zc=1158), which is a
transverse balance of 30dB. Toggle switch S1 and the analyzer should read
within 1dB of the previous measurement. Should this not be the case, it is
then necessary to add the variable capacitors and adjust them to achieve at
most a 1dB difference for the two positions of S1, for the termination shown.
These caps, when properly adjusted, compensate for the imperfections in the
transformer's windings and possible parasitic effects such as interwinding
capacitance.
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(4)
After the test fixture is calibrated as described above, set switch S2 to
position C and measure the transverse balance of the EUT. The pass/fail
limits in dB for transverse balance versus frequency using the ratio of
currents method should be the same as the limits specified for the voltage
method described in the primary text of Part 68. However, as is true of any
alternative method of test the burden of proof of correlation between the
standard and alternative method lies upon the user of the alternative
methodology.* Recall that the display on the analyzer must be reduced by
6dB to get the true transverse balance for the EUT.
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*Test methodologies and illustrative circuits specified in TIA-968 have been
determined by historically understood use and recommended practice to
provide ease of test and lab-to-lab repeatability, while they may not always
be the most expedient or technically appropriate way to perform a test in a
specific situation. Correlating test results with this alternative current method
might include lab data for both voltage and current methods under conditions
which can be expected to yield clean data with each (i.e., at the low-
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frequency end of the test range and/or away from frequencies and/or levels
which might suggest the voltage method is questionable).
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FIGURE F-1: TEST FIXTURE TO MEASURE TRANSVERSE BALANCE USING A
RATIO OF CURRENTS
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