System and method for testing differential signal crossover using undersampling

A test system comprising:

• a test circuit operable to be coupled to a device under test (DUT), the test circuit configured to test a signal crossover for a differential mode signal provided by the DUT, wherein the test circuit comprises,
• a pair of window comparators, each window comparator coupled to a signal from a pair of signals making up the differential mode signal, each window comparator configured to compare a value of the signal with a voltage threshold boundary and produce an output value based on the comparison;
• an enable signal line coupled to the pair of window comparators, the enable signal line to provide an enable signal to initiate the comparison in the window comparators, wherein the enable signal is periodic with a period that is substantially equal to an integer multiple of a period of the signal crossover plus a small time value;
• and
• an automatic test equipment (ATE) coupled to the DUT and the test circuit, the ATE configured to provide clock signals to the DUT and an enable signal to the test circuit and process data produced by the test circuit.

A method for testing a circuit for meeting signal crossover timing constraints, the method comprising:

• setting an operating frequency of the circuit and a sampling frequency of an undersampling comparator, wherein the sampling frequency of the undersampling comparator is substantially equal to an integer multiple of the operating frequency of the circuit plus a small frequency value;
• setting a voltage threshold based upon a timing constraint;
• initiating the testing of the circuit;
• and
• counting a number of times an output of the undersampling comparator is equal to a specified value.

A method for measuring a crossover voltage of a differential mode signal produced by a circuit, the method comprising:

• setting an operating frequency of the circuit and a sampling frequency of an undersampling comparator, wherein the sampling frequency of the undersampling comparator is substantially equal to an integer multiple of the operating frequency of the circuit plus a small frequency value;
• setting a voltage threshold for the undersampling comparator;
• counting a number of times an output of the undersampling comparator is equal to a specified value;
• repeating the second setting and the counting for any remaining voltage thresholds;
• and
• calculating the crossover voltage from a voltage threshold with a largest count of the number of times the output of the undersampling comparator is equal to the specified value.