8 Channel Differential Scanner with Absolute Reference

A prime contractor came to PRL with a requirement to translate and distribute a set of GPS signals (1 PPS, IRIG-B, and 10 MHz) throughout a test station with very low skew, measured both from channel to channel and also from unit to unit.

PRL designed 3 new products:

  • PRL-4505, 1:8 Differential Fanout Buffer System, TTL or Differential NECL Input, RS422 Output
  • PRL-4506, 1:8 Differential Fanout Buffer System, TTL or Differential NECL Input, Differential TTL Output
  • PRL-4508, 1:8 Differential Fanout Buffer System, TTL or Differential NECL Input, Differential NECL Output

Fig. 1: The UUT

Testing a whole fleet of these units to the required skew tolerance, over a delivery schedule of several years, required a test mux with extremely low skew.

The task:

Route 8 pairs of differential NECL, TTL, or RS-422 signals to an oscilloscope to measure AC and DC characteristics. The test mux required an absolute reference path so that we could measure total propagation delay through each channels. This allowed us to  test unit-to-unit skew down to tens of picoseconds.

Finally, we wanted to build this test mux using all off-the-shelf products from PRL, so that we would not have to do any engineering.

The Solution:

Routing the differential outputs signals to the oscilloscope was simple. We selected the PRL-8216N-SMA, 16:2 Scanner with NECL Terminations, controlled by the PRL-USBIO-1 module.

For the drive signal we selected the PRL-175N-10, NECL Crystal Clock Source, 10 MHz. This module provided us with two:

 

photo:

The resulting test set performed very well. When the UUT is replaced by an SMA bullet the measured skew is < 20 ps, despite the signals passing through 5 cables and 3 switching stages between the original clock source and the scope.

We were able to test the fanout buffer systems to < 100 ps typical (NECL output) and <200 ps typical (TTL and RS422 outputs). 

Other Versions

Similar test muxes were created for the TTL and RS422 versions of the fanout buffer. We have delivered many units over a span of 8 years, and we can trace unit-to-unit skew back to the first units ever delivered.