Input Transformer Measurement

Hi Guys,
I bought QA-402 quite some time back but didnt really have enough time tinkering with this great device.

Would like to measure bandwidth of my DIY input transformer. It’s 600:600R, 1:1, around 40R resistance in Primary/Secondary.

Do I need to add R in series when injecting the signal to its primary? I assume 2Vrms will be sufficient?

Any suggestion welcome.


Hi @Jimmy168, driving 600 ohms shouldn’t be an issue, but remember there’s 100 ohms of output Rin the QA402 outputs. And if you drive balanced that will be 200 ohms total. So, all up, you are probably looking at 800 ohms of load which isn’t an issue for the output opamps (OPA1612).

If your transformer wants to see specific input/output impedances, you will need to add R to get those numbers right. On the output side, it’d be series R. On the input side, it could be a parallel R across the inputs of the analyzer (or outputs of the transformer, depending on how you want to look at it).

And the, you can do a quick chirp at low levels (-10 dBV or so) to learn the frequency response, and then start some automated tests where you step up in voltage to learn more about the transformer saturation etc.

Feel free to post what you are measuring. I think a lot of folks would enjoy learning what you are up to.

Hi Matt, thanks for your reply.

I dont think there will be an issue with the 100R R output as I’m not really measuring on specific impedance.

What my worry was the primary/secondary DCR resistance is around 40R each. Not sure if it will be too low or not?

Noted on the R on input and R load at output.

My first time measuring Input Transformer - as it’s small and low primary/secondary resistance (40R), I really dont warn to damage either the transformer or my QA402… :slight_smile:

Thank you very much.

Done the test… with 1K5 load on output of the transformer. Looks good!

@matt , may I know the input impedance of QA402? Example if I remove 1K5 load from my Input Transformer output and direct to QA402 input, what load that my Input Transformer output side will see?


Hi Jimmy, the input impedance appears as follows: The BNC input goes to a 4.7uF cap in series with a 470 ohm, followed by a shunt of 100k ohms. Overall, we consider it a 100k series input.

Thanks @matt .
Any idea if QA404 will do 384k (so we can measure 200 kHz)?

Hi @Jimmy168, no, the QA403 won’t do 384K.