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GS1508

HD-LINX™ HDTV Cable Driver

Gennum -> Semtech 

GS1508

SDO

SDO

Figure 3-3: Static Protection Diodes

3.3 Output Return Loss Measurement

To perform a practical return loss measurement, it is necessary to force the GS1508

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 GS1508 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. With this method, 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; the interpolated values are RIH = -17dB and

RIL = -15dB.

3.4 Output Amplitude Adjustment

The output amplitude of the GS1508 can be adjusted by changing the value of the RSET

resistor as shown in Figure 3-4 and Table 3-1. For an 800mVp-p output with a nominal

±7% tolerance, a value of 53.6% is required. A ±1% SMT resistor should be used.

The RSET resistor is part of the high-speed output circuit of the GS1508. The resistor

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

be minimized at this node by removing the PCB groundplane beneath the RSET resistor

and the RSET pin.

GS1508 HD-LINX™ HDTV Cable Driver

Data Sheet

9500 - 6

August 2010

7 of 14

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