It is easy to verify that this meter performs as advertised. The sensor produces a 1.000000 megahertz 2v p-p square wave with an accuracy of +/-20ppm, when it is connected to the display unit where it is terminated with 75 ohms to match the cable. If you remove the BNC connector from the sensor and connect the display unit to a good quality function generator, you can verify that the correct reading is displayed for the simulated difference in time rate. Set the function generator to 1.000000mhz, turn on the DT-Meter and wait for it to auto-calibrate (6 seconds). Change the function generator to 1.001000mhz and the DT-Meter will read a time rate difference of 0.001 seconds per second. The DT-Meter should also beep continuously if you have not changed the alarm setting. Change the function generator to 999000.00 hertz and the DT-Meter will read a time rate difference of -0.001 seconds per second.
The formula that the DT-Meter uses is 1-(f1 / f2) where f1 is the larger value of the sensor frequency as seen at the display unit, or 1mhz, and f2 is the smaller of the sensor frequency as seen at the display unit, or 1mhz. The result is adjusted by a calibration that is set on power-up (or by user command). The reason for using this formula is because a display of 1.00000 seconds per second would change to 0.999999 whenever the sign changed, when what is desired is to know the magnitude of the difference and the sign. It is much better to watch the display change from -0.000001 to 0.000001 rather than from 1.00001 to 0.999999.