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GS1578A

HD-LINX® II Multi-Rate Dual Slew-Rate Cable Driver

Gennum -> Semtech 

GS1578A Data Sheet

4.3 Output Return Loss Measurement

To perform a practical return loss measurement, it is necessary to force the

GS1578A output to a DC high or low condition. The actual measured return loss

will be based on the outputs being static at VCC or VCC-1.6V. Under normal

operating conditions the outputs of the device swing between VCC-0.4V and

VCC-1.2V, so the measured value of return loss will not represent the actual

operating return loss.

A simple method of calculating the values of actual operating return loss is to

interpolate the two return loss measurements. In this way, the values of return loss

are estimated at VCC-0.4V and VCC-1.2V based on the measurements at VCC and

VCC-1.6V.

The two values of return loss (high and low) will typically differ by several decibels.

If the measured return loss is RH for logic high and RL for logic low, then the two

values can be interpolated as follows:

RIH = RH- (RH-RL)/4 and

RIL = RL+(RH-RL)/4

where RIH is the interpolated logic high value and RIL is the interpolated logic low

value.

For example, if RH = -18dB and RL = -14dB, then the interpolated values are

RIH = -17dB and RIL = -15dB.

4.4 Output Amplitude (RSET)

The output amplitude of the GS1578A is set to 800mVp-p with a tolerance of ±7%

using an RSET resistor of 750Ω. A ±1% SMT resistor should be used.

The RSET resistor is part of the high speed output circuit of the GS1578A. The

resistor should be placed as close as possible to the RSET pin. In addition, the PCB

capacitance should be minimized at this node by removing the PCB groundplane

beneath the RSET resistor and the RSET pin.

NOTE: Only an RSET value of 750Ω ±1% should be used. Using other values for

RSET is not recommended.

33657 - 4 March 2006

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