Tektronix 071-0590-00 user manual Pattern Lock, Pattern Loading Software, Error Insertion

BERT Primer

Pattern Lock

Before a BER measurement can be made, the BERT Transmitter and Receiver must be using the same test pattern. This may be guaranteed if they use the same PRBS or fixed word pattern and are running at the same clock rate, but there is still the issue of having the bit comparison begin at the same point in the pattern. All of our general purpose BERTs have the ability to do an “auto search” for pattern lock. There are two techniques commonly used for this: the “bit slip” or “clock slip” method, and the “feed forward” method.

In the “bit slip” method, each error detected causes a one-bit slip in the reference pattern, until the beginning of the pattern is reached. This is normally the only method that can be used with custom data structures or word patterns. This is the method the GB1400 uses to synchronize any data pattern. This is also the method used for WORD patterns with the GB700 and GB1400 instruments. With the “bit slip” method, synchronization times will vary by the length of the pattern. The longer the pattern, the longer the sync time.

In the “feed forward” method, which works for PRBS patterns only, a certain number of arriving bits are latched into the Receivers PRBS shift register and used as the “seed” for the start of the PRBS generation. This is the method the GB700 and GB1400 use for PRBS synchronization.

Both methods rely on realizing some minimum acceptable BER before the Receiver considers it has obtained “pattern lock”. It will then begin accumulating “bit errors” in the error counter register.

Pattern Loading Software

Almost all BERTs have the ability to accept loading of reference patterns into internal memories in both transmitter and receiver. In many cases this is via computer interface, either RS-232 or GPIB. The GB1400 has an front panel floppy disk drive to load patterns. The GB700 and GB1400 use either RS-232 or GPIB.

Pattern creation software is a valuable aid in developing custom patterns. A simple spread-sheet like tool is available for our BERT products to help with this need.

Error Insertion

Most BERTs have the ability to internally generate bit errors, either singly or at specific bit error rates. This calls for altering bits in the output pattern which are opposite from the bits the receiver will use at that location, thus causing the Receiver to tally a bit error. This feature may be important in measuring ability of device under test to withstand or recover from such errors. Some BERTs, including our GB700, will allow an external “trigger” signal to cause an immediate bit error to be output.

Error insertion is also a useful tool to test the “integrity” of your equipment setup. You should always insert a few errors in your setup to conform correct operation before you start your data collection.

GB1400 User Manual B-11