Basic transmission impairments II

Most communications field technicians will have some involvement with multiplex (mux) or telephone company (TELCO) leased-line circuits. When circuit-related problems arise, the technician equipped with an understanding of basic transmission impairments will have one foot forward in the evaluation and resolution process.

As mentioned in the first part of this series, circuit impairments are usually placed in one of two categories: steady-state or transient.

Steady-state measurements redux

This article picks up with the fourth item in the list of steady-state measurements: Phase Jitter.

4. Phase Jitter

Phase Jitter is the unwanted phase modulation of a primary signal by one or more other signals through the transmission process. It can also be defined as any unwanted variations in the zero crossings of the recovered signal. Phase Jitter is expressed in degrees relating to peak-to-peak deviation over specified frequency ranges (20Hz-300Hz, 4Hz-300Hz, 4Hz-20Hz). When excessive, jitter causes errors in data recovery. Jitter measurements are often made in conjunction with noise measurements, since noise and jitter can have a measurable impact on one another.

Phase jitter may be transient or periodic in nature. Periodic jitter is often caused by coupling from low-frequency sources such as 20Hz ringing generators, 60Hz AC power, and their second through fifth harmonics (20Hz-300Hz). Jitter modulation components in the 4Hz to 20Hz range are less common, but not unusual. The transient type can be the result of noise in the transmission path.

5. Amplitude Jitter

Amplitude Jitter is the measure of incidental amplitude modulation of the transmitted holding tone. It can be caused by many of the same sources as phase jitter, and indirectly by attenuation distortion and envelope delay. These latter two can create amplitude jitter from phase jitter, and vise versa. Noise also can affect amplitude jitter measurements. Because there are several possible contributors, it is often necessary to accomplish measurements for these parameters as well to determine the true cause of the impairment. Amplitude jitter is evaluated over the same frequency ranges as phase jitter, and is expressed in percent.

6. Envelope Delay Distortion (EDD)

Envelope delay describes a condition where various frequency components of a transmitted signal exhibit different propagation delays. The greatest delay occurs near the upper and lower limits of the circuit passband due to inductive and capacitive reactance.

An EDD test is essentially a measurement of circuit phase linearity. A curve can be developed by plotting changes in phase delay as the test frequency is varied. Delay distortion is the difference in delay between the frequency under test and a reference frequency near the center of the circuit passband (the point of minimum delay). When delays between various frequency components of the data transmission become great enough, intersymbol interference can result. Envelope delay is expressed in microseconds.

7. Return loss, related measurements

Return loss is expressed in dB, and is the ratio of power levels between a signal transmitted into a circuit and its power reflected back (with the far-end properly terminated). As the return loss increases, the reflected power decreases and the quality of the circuit improves.

Return loss is most often an issue with circuits that convert between two-wire (2W) and four-wire (4W) operation. Impedance mismatches or discontinuities (often occurring in or at the 2W/4W hybrid) cause the incident signal to be reflected back to the source. Over significant distances, this can result in line echoes. Low return loss values can be the result of poor send/receive isolation within network equipment or wiring and can cause a hollow “rain barrel” effect, singing, and producing ripples in the envelope delay response. Return loss measurements should specify the frequency or frequency band under test as well as whether the circuit is 2W or 4W.

Echo Return Loss (ERL) is an averaging return loss measurement for circuit mid-band frequencies, with -3dB points of 560Hz and 1965Hz. Singing Return Loss Low (SRL Low) and Singing Return Loss High (SRL High) provide an indicator of the margin against circuit singing. These are also averaging measurements that evaluate the lower and upper-frequency portions of the circuit. The frequency ranges for SRL Low and SRL High measurements are 260Hz to 500Hz, and 2200Hz to 3400Hz respectively (-3dB points). Since the ERL, SRL Low and SRL High measurements each evaluate a band of frequencies, tests can be accomplished using a sinusoidal sweep or a band-limited noise source.

Impedance discontinuities may be present at multiple points along the length of the circuit, resulting in more than one echo, each with its own path traveled, level and delay. Echo Path Loss (EPL) is a measurement of the difference in levels between the incident signal and its reflection, or echo, from one or more paths. Echo Path Delay (EPD) measures the difference in time between the incident signal and its reflections.

8. Peak-to-average ratio

The Peak-to-average ratio (P/AR) measurement assigns a number value, or figure of merit to the circuit under test. It is an indicator of circuit quality with regard to intersymbol interference for voice-band data applications. Values range from one to 100, with 100 being the best.

This test is accomplished through the simultaneous transmission of 16 phase-related tones of known peak-to-average ratio. This signal creates an envelope similar to voice-band data. Spreading, or dispersion of the signal is measured at the far-end of the circuit. Low P/AR values most often result from problems with envelope delay, and return loss (creating gain and phase ripples). Circuit bandwidth reduction, attenuation distortion, non-linear distortion and noise also can lower the values. Since the P/AR measurement can be affected by several parameters, when values are low, additional tests must be accomplished to isolate the cause. P/AR measurements are fairly immune to transient impairments.

9. Intermodulation distortion

This impairment test offers indication of non-linearities within a circuit by measurement of second and third order IMD products through one of several methods. The most common is the four-tone method, accomplished through transmission of four, equal-level tones: two near 860Hz, and two near 1380Hz. IMD products result in three narrow bands centered at 520Hz (second order), 1.9kHz (third order) and 2240Hz (second order). The levels of the second and third order IMD products are expressed separately, in dB, relative to that of the received test signal.

Transient Measurements

Summary

The first step toward resolving multiplex circuit or TELCO leased-line problems is an understanding of transmission impairments. This, in conjunction with the proper test equipment, and knowledge of test procedures and service provider specifications will help the technician identify and/or resolve circuit performance issues.

Editors note: This is the second and final article in a series.