Hello All,
I've recently purchased an MSOX2024A oscilloscope and I am enjoying it to no end. I've recently taken on the task of monitoring the atomic time signal broadcast from station WWVB in Fort Collins, CO. This time signal is based on a 60 kHz carrier and can be received by a very cheap receiver / antenna setup.
In measuring both the analog and digital signals from the station I have noticed that there is often a discrepancy between the pulse width as measured on the analog side versus the digital side. Variances can be as much as .07ms between the two at times. Sometimes the pulse widths are dead on exact but it is very infrequent.
My question is why there would be a discrepancy between the two measurements, given that the probes are both reading the same signal and ground pins of the receiver. The difference between the two gives me an uncertainty about which measurement is actually correct (or has less error?). I'd really enjoy hearing from anyone but try to keep the response on a somewhat layman's term basis.
Thanks to all for reading.
Regards,
Chris (New oscilloscope owner)
There are a number of things that can affect this measurement:
- The analog and digital samples are not synchronized. They are on the same timebase, but the actual sample rate for both the analog and digital channels change as you zoom in and zoom out. On both signals, the scope has adjusted sample rate and depth to give you a fast update rate, while keeping signal fidelity. The sample rate for the digital channels could he higher or lower than the analog sampling rate, and is not shown on the display.
- The measurement threshold for the analog measurement is not the same as the digital threshold. The signal has a finite rise time. If the thresholds are not the same then the actual time that an actual rising edge is found by the HW (on the digital channels) and by the SW (on the analog channels).
- There are always inherent uncertainties in any measurement. Some of these are listed in the datasheet and/or manual for the scope, or for any scope.
I can pretty much guarantee that the actual value of the pulse width will ALWAYS be more accurate than the scope you are using. This sounds a little like a makework science project that is designed to help you learn the difference between theory and the real world. Always remember two things:
- The real world is a special case
- In theory, theory matches reality. In reality, they never match.
Al