AN3547| Application Note
Maxim/Dallas > App Notes > TELECOM Keywords: return loss, LIU, line interface unit, SCT, single chip transceivers, DS3150, DS3151, DS3152, DS3153, DS3154, DS3251, DS3252, DS3253, DS3254, DS3170, DS3171, DS3172, DS3173, DS3174, DS3181, DS3182, DS3183, DS3184
May 26, 2005
APPLICATION NOTE 3547
Measuring Return Loss on T3/E3/STS-1 LIUs
Abstract: This application note describes how to measure and improve the return loss (RL) on the Dallas Semiconductor DS3150 line interface unit (LIU). In this application note, the definition, requirements, measurement, and improvement of return loss are discussed.
Return-Loss Definition
When a high-speed signal reaches the end of a transmission line, if the transmission line is imperfectly terminated, a portion of the signal energy is reflected back toward the transmitter. This reflected signal mixes with the original signal, distorting that original signal and making it more difficult for the LIU receiver to correctly recover clock and data. Return loss is the ratio (expressed in dB) of the power of the original signal vs. the power of the reflected signal. Thus, return loss indicates the relative size of the reflected signal and, therefore, how perfectly or imperfectly the transmission line is terminated. If the measured return loss of an LIU card is 20dB at a given frequency then the reflected signal has 20dB less power than the original signal at that same frequency.
Return-Loss Requirements
For E3, ITU G.703, and ETS 300-686, the input return-loss requirements are shown in Table 1, and the output return-loss requirements in Table 2. Table 1. Input Port Minimum Return Loss Frequency Range 860kHz to 1720kHz 1720kHz to 34368kHz Return Loss 12dB 18dB
34368kHz to 51550kHz 14dB Table 2. Output Port Minimum Return Loss Frequency Range Return Loss 860kHz to 1720kHz 6dB 1720kHz to 51550kHz 8dB
Measuring Return Loss on Dallas Semiconductor LIUs
The test setups and procedures for measuring E3 return loss are described in subclauses A.2.5 and A.2.6 of the ETS 300-686 specification. The test configuration in Figure 1 is designed to measure the input return loss and verify compliance with the requirements shown in Table 1. The output return-loss configuration is similar, with
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