# AC to constant with mso protocol?

Forum for discussing PicoScope version 6 (non-automotive version)

### AC to constant with mso protocol?

Having used multimeters I took it for granted that there would be a way to plot the RMS value of a signal. I change my meter to AC and the cycling data becomes constant. I always figured that's what coupling would do on a scope. I finally took the time to learn and realize that is not what coupling does.

I can't flatten my ac wave to a point with math because math is always point based and not period based.

Measurements can't be plotted.

My last hope is maybe using a protocol? Can I plot the result of a protocol while hiding the original waveform?
dnoyeb
Active User

Posts: 12
Joined: Tue Mar 28, 2017 1:16 pm

### Re: AC to constant with mso protocol?

Hi dnoyeb,

If you're trying to get the RMS value from a Math Channel, then you just have to use the right calculation (integral w.r.t. Time, divided by Time = Average) and capture enough cycles, to allow the complete buffer functions to converge on the value, as in the example below:

Code: Select all
`sqrt(integral((A)^2)/T)`

Regards,

Gerry
Gerry
Technical Specialist
Gerry
PICO STAFF

Posts: 226
Joined: Mon Aug 11, 2014 11:14 am

### Re: AC to constant with mso protocol?

When using the above Math channel for RMS, you need to be aware of the following caveats (updated 26/4/17):

1/ The pre-trigger should be set to 0%, (with the yellow trigger diamond positioned on the far left of the screen), and be greater than zero, so that Time is always positive, and the waveform is meaningful.

2/ There needs to be enough cycles of the signal waveform, to allow the Math Channel waveform to converge on a single value. If using measurements, only rulers placed on the converged DC portion of the Math Channel will give correct measurement values.

3/ The RMS Math Channel will not converge quickly enough when zooming in, unless you're zooming in near the start (the only way to change the function so that it does converge quickly would be a not very practical, manual correction to the function itself every time zoom is used).

4/ Also, the y-axis scaling of a Math Channel is not automatically proportional to the input channel/channels being used, so you may need to adjust it to match the Input Channel scaling.

Regards,

Gerry
Gerry
Technical Specialist
Gerry
PICO STAFF

Posts: 226
Joined: Mon Aug 11, 2014 11:14 am