When M1™ detects a differential pair on two channels from the scope, digitizer, or EDA file, it links those two channels into one virtual measurement. In the graphic shown, it calls the differential pair found on channel 2 and channel 3 a virtual channel 2/3. In addition, M1 automatically chooses the differential crossover voltage for the mid-transition point for all timing measurements.

Note: For mid-transition timing measurements, M1 defaults standard single-ended signals to the standard 50% point on the waveform. Normally, the user has the ability to change that default. The user can change the high, low and middle thresholds and can specify them in terms of percentage or volts.

Notice in the dialog shown above that the mid-transition point is not user selectable. This is because M1 uses true differential crossing points for the mid-transition threshold when differential pairs are identified. M1 also calculates the differential crossing point for each cycle in the waveform acquisition so that any variations due to data dependencies may be identified.

Differential Crossover Voltage

When M1™ sees a differential pair,  it enables its Differential Crossover Voltage measurement in the measurement wizard. A differential crossover voltage measurement looks at each differential crossover point in the waveform acquisition. Then for each crossover it calculates the voltage where the crossover occurs. It then plots the results into a histogram or into a time series view for further statistical analysis. The user has three options: Differential XV+, Differential XV- or Differential XV Both. The Differential XV+ measurement only looks at the differential crossover voltage when it sees a rising edge on the lowest numbered channel in the differential pair. The Differential XV- measurement only looks at the differential crossover voltage when it sees a falling edge on the lowest numbered channel in the differential pair. The Differential XV Both measurement only looks at the differential crossover voltage when it sees either a rising or falling edge on the lowest numbered channel in the differential pair.

Differential Transition Time Measurements

When M1™ sees a differential pair, M1 also enables its differential transition time measurements in the NEW measurement wizard. In general, these measurements look at pairs of edges (one rising and the other falling; one from the Positive rail of the differential pair and the other from the Negative rail of the differential pair) that define a particular differential crossover event and subtract the length of one edge from the length of the other in a prescribed manner. See the M1 Help file for complete definitions for each of these measurements. Many designers using differential signals find the measurement flexibility of M1’s differential transition time measurements useful.