Figure 23: Example of LVDS differential signal
The complementary single-ended signals labeled A and B in the previous figure must have well matched signal timing (for
example, low skew) to function as a useful differential signal. If the signal skew becomes comparable in size to the rise time
of the complementary signals, then the rise time of the resulting differential signal will be slower than expected. Skew is a
measure of the time delay mismatch between two supposedly matched signals.
Although very accurate low skew measurements usually require TDR or VNA measurement equipment, it is possible to
make relatively accurate skew measurements of a differential signal pair using a TDP7700 Series probe and a TekFlex
probe tip. A flex circuit based solder tip should first be soldered to the DUT differential signal, taking care to match the A and
B input wire lengths, because the wiring length mismatch will contribute to the skew measurement error. A DUT ground
reference connection should also be made to one of the TekFlex tip ground vias using a wire length as short as possible.
At first it might seem reasonable to try to make a skew measurement by directly comparing the A and B signal timing.
Although it is possible to display both the single-ended A and B signals that make up the differential signal using the
TriMode measurement selection function, both signals cannot be displayed with a single signal acquisition of the
oscilloscope. Even though it is possible to capture the A signal and store it as a reference waveform for comparison with a
follow-on B signal acquisition, the trigger timing uncertainty between acquisitions might make this a less than accurate, if not
impossible approach.
A more indirect, but more accurate, skew measurement is possible by switching the probe input mode to the common mode
setting. Because the common mode setting for a TekFlex probe tip features full probe bandwidth, the response should be
fast enough to display the relative skew performance. The common mode response for a perfectly matched, zero skew,
complementary signal pair should be a flat trace at the common DC bias voltage level. As the skew of the signal pair
increases, the common mode waveform begins to show narrow pulses at the signal logic transitions. The pulse width of
these narrow transition pulses is a relative measure of the skew.
Theory of operation
TDP7700 Series TriMode™ Probes Technical Reference
19
