Lambda SR1, Operation Manual, Page: 187 / 317

SR1 Operation 187
© 2014 Stanford Research Systems
Voltage Offset Leads to Different Positive and Negative Pulse Widths
The same effect can by caused by a transmitter with different time constants for rising and falling
edges. On an eye diagram an offset can be detected by observing that the points at which the positive
and negative going pulses cross is above the "0 Volts" line. The eye diagram for an actual digital audio
signal with a small offset is shown below.
Eye Diagram of Offset Digital Audio Carrier
Another way of observing the effect of asymmetry is to examine the histogram of pulse widths. For an
ideal square wave signal there would be a single peak in the histogram of pulse widths corresponding to
1/2 the square wave period. For the digital audio signal above we can see that the 2 UI peak has been
split into two peaks— the higher one corresponding to the slightly longer positive going pulses and the
peak on the left corresponding to the slightly shorter negative going pulses.
Splitting of 2 UI Peak Caused by Pulse Asym m etry