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DIY Transformer for use with Bode Plots.
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tautech:
Nice thread Mike.
FYI here's some good info on FRA and finding its limits albeit with the little X-E scopes not the 2kX Plus models.
https://www.eevblog.com/forum/testgear/siglent-sds1x04x-e-bodeplot-ii-(sfra)-features-and-testing-(coming)/
mawyatt:
That's a must read thread tautech! rf-loop did a great job with the Bode Function. I need to spend some time reading thru all the posts in the thread, lots of good quality information :-+

Best,
joeqsmith:
My last attempt

25Turn Bifilar, single core 31mm od, 16.5mm id, 12.9mm thick.
Ccoupling @ 10kHz 43pF
Lp @ 10kHz 8.97mH
Ll @ 10kHz 0.863uH
R 0.116 ohms

Calcuated -3dB Lf should be 442 Hz.   I measure 421.   The 0.2dB Lf measures 2.7 kHz. 
joeqsmith:
While that last core was much worse than the initial attempt,  my goal was to get something a little closer to OPs setup.  Again, I am only questioning that 0.2dB cutoff.  It's why I kept asking about the setup.   Not how the transformer is used.

I had made another attempt to make something that would run a bit lower than the first core.  0.2db was 60Hz to a MHz.
joeqsmith:
I marked up Jay_Diddy_B's drawing I linked above.  Obviously in the first two circuits changing the location of CH1 is going to have a major effect what the low frequency response looks like when we measure our transformer.   Consider as we approach DC the primary will start to load the output.  Because we are looking at a ratio of ch1 to 2, it can make the response seem flat.   

I'm not a fan of the added resistor he shows but at DC, hey..   Anyway,  the third circuit, I change the scopes inputs to 50 ohms on both channels and include a splitter.   

I suspect the initial setup is wrong as Jay pointed out.  Calculating the rolloff we won't be anywhere near 10Hz as expected.  Maybe it adds to the confusion.
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