Test Plugin Scale Units

I just received my QA401 this week, and so far I am very pleased with the unit and software.

My questions is, is there any way to change the scale from dBV to dBm for the test plugins?

I know you can change it in the side control panel, but that doesn’t seem to change the it for the test plugins - just the active display and general user interface. I work more in dBm than dBV, and would like the test plugin graph scales to be in dBm. I plan on mostly using the AMP Frequency Response test plugin.


Hi @M2Circuits, there’s no way currently to change the units to dBm. But you could fiddle with offsets and do it. Assume we’re talking about a 600 ohm system (recompute for whatever characteristic impedance you wish):

0 dBV =  1Vrms = 1.66 mW = 2.21 dBm
6 dBV =  2Vrms = 6.66 mW = 8.21 dBm
12 dBV = 4Vrms = 26.6 mW = 14.21 dBm

Note to go from 0 dBV into 600 ohms to dBm requires you to add 2.21 dB. And then, 6 dB more voltage yields 6 dB more power. And 12 dB more voltage yields 12 dB more power. So, if I add an external input gain of -2.21 dB in the plugin setup:

Then the resulting graph is as follows. Note that we output 0 dBV, but it’s measured as 2.21 dBV because of the offset we specified. So, that gives us the correct value.

And then I can change the Y axis title in the graph tool and I get:

So, with a little cheating you can get there.

Hi Matt,

Thank you for your response!

Hopefully you will be able to implement a units selection in either the test plugin dialog box or graph editor in a future update.

But for now, this will work ok.

Thank you!