6dB gain with Differential Cables

QA40x
1

HI, forgive me if this is obvious but should I be seeing a 6dB gain with differential cables vs single ended? I built a set of xlr to bnc cables using stereo mogami 2528.

2

Hi @rsrecords, this is correct and you are right, this can be very confusing. It’s a lot like using a DVM. If you measure the L+ output with a DVM where one leg is connected to ground, you might see 1Vrms. But if you do the same measurement with the DVM connected to L+ and L-, then the voltage reported is twice as much.

In the case of a DVM and the QA40x outputs, the actual voltage delivered to your DUT is a function of how you connect things.

But perhaps what is needed is a checkbox in all the generators that says “Output is balanced” and that will automatically reduce the generated voltage by 6 dB so that the measured is correct. And then an annunciator that says “Output Balanced” or something similar.

Alternately, the QA401 software had a setting in the DBV context menu, adjacent the Input Gain setting. It’s there in the QA40x software, but it’s disabled. But it’d be similar in that you’d enter a 6 dB output gain when using balanced connection. This seems a bit too generic.

Finally, as a third option, there could be another set of measurements added. For example, there’s the Sys:Gen1 measurement. But there could also be a Sys:Gen1 BAL measurement. And that would simply take the Gen1 setting and add 6 dB to it.

So, the 3 options:

  1. A checkbox in all the generators specifying the output is balanced
  2. A generic output gain adjustment
  3. A Sys:Gen1 measurement that is balanced. So, depending on your connection, you’d either select the Sys:Gen1 or Sys:Gen1 BAL to see the level delivered to the DUT

Or, if another option seems better, please feel free to outline it. But I agree it can be very confusing.

3

Ok thanks for clearing that up.
An option in the software would be fantastic.