GS9078A Data Sheet
34165 - 4 March 2006 10 of 14
4.3 Output Return Loss Measurement
To perform a practical return loss measurement, it is necessary to force the
GS9078A output to a DC high or low condition. The actual measured return loss
will be based on the outputs being static at V
CC
or V
CC
-1.6V. Under normal
operating conditions the outputs of the device swing between V
CC
-0.4V and
V
CC
-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 V
CC
-0.4V and V
CC
-1.2V based on the measurements at V
CC
and
V
CC
-1.6V.
The two values of return loss (high and low) will typically differ by several decibels.
If the measured return loss is R
H
for logic high and R
L
for logic low, then the two
values can be interpolated as follows:
R
IH
= R
H
- (R
H
-R
L
)/4 and
R
IL
= R
L
+(R
H
-R
L
)/4
where R
IH
is the interpolated logic high value and R
IL
is the interpolated logic low
value.
For example, if R
H
= -18dB and R
L
= -14dB, then the interpolated values are
R
IH
= -17dB and R
IL
= -15dB.
4.4 Output Amplitude (RSET)
The output amplitude of the GS9078A is set to 800mV
p-p
with a tolerance of ±7%
using an RSET resistor of 750Ω. A ±1% SMT resistor should be used.
The R
SET
resistor is part of the high speed output circuit of the GS9078A. The
resistor should be placed as close as possible to the R
SET
pin. In addition, the PCB
capacitance should be minimized at this node by removing the PCB groundplane
beneath the R
SET
resistor and the R
SET
pin.
NOTE: Only an R
SET
value of 750Ω ±1% should be used. Using other values for
R
SET
is not recommended.
