Network element delay measurements
Triggering on 8b/10b serial patterns can be used for measuring the time delay of an active network element. One might think this an easy task to solve even without special triggering on 8b/10b. But it can be challenging when it’s necessary to measure the time delay under real conditions. The setup for this measurement is shown in figure 7
. The input signal of the network element is connected to Channel 1 and the output data stream is connected to channel 2 of the oscilloscope.
A data generator will provide the required data stream to the input of the network element (DUT). A unique pattern inside the data stream will perform as a timing reference. This timing reference needs to be a very rare pattern to prevent confusion with another timing position. If the pattern has been defined you can search for that pattern in the acquired signal of channel 1 and channel 2 and then measure the time between the two locations.
A software decoding function could help to find the sequence in the data stream. Because of its infrequent occurrence the search for the pattern can be very difficult. Given oscilloscope acquisition memory limitations, the chance of finding the pattern for the timing reference is low. Unlike searching, 8b/10b triggering makes it much to find the sequence. This is because triggering ensures that the pattern will always be inside of the acquisition window.
Without 8b/10b triggering the delay measurement would require a trigger signal from the data generator to the oscilloscope. That would be the only way to synchronize the starting point of the 8b/10b timing reference at the output of the data generator with the acquisition window of the oscilloscope. However, this method fails to replicate real world conditions.
In the set up shown in figure 7
, a bidirectional link connects the two network elements. Network element A acts as a data source to ensure that network element B (DUT) works in a desired operating mode. The communication link has to be established by proprietary commands and the data flow of this commands needs to be maintained during the measurements to keep the DUT in the desired operation mode. Therefore a static data pattern provided by any data generator will not work.
Figure 7: Time delay measurement setup
Similar to the previous example a unique pattern is required for a timing reference (marker) in the data stream. Since the pattern is very infrequent and there is no trigger signal available from the source network element A, an 8b/10b trigger is required to find the pattern in the input and the output signal of the DUT.
The optimum way to see and measure the delay between the input and output of the network element is to use two zoom windows. The first zoom window is placed at the beginning of the pattern sequence at channel 1 and the second zoom window at the beginning of the pattern sequence at channel 2. For the delay measurement the acquisition time window of the oscilloscope should be equal to or greater than the delay time.
About the author:
Dean Miles is a Senior Technical Marketing Manager at Tektronix. Visit Tektronix here www.tek.com
Courtesy of EETimes Europe
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