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.
The original setup is not wrong, it's exactly as stated multiple times and illustrated in the Bode Plot video shown above. The Bode measurement with the Siglent DSO Ch1 is
Across the Transformer Primary and Ch2 across the secondary with input impedance set to 50 ohms. Since the intended use and thread title of the transformer as stated is for Bode use, and mostly as an Injection Transformer for Closed Loop measurements, this method as stated and used partially removes the transformer (and source) characteristics from the measurement and revels the effective use of the transformer for Bode use.
This is NOT directly intended (although can be used) for a controlled impedance type use like 50 ohm RF, but generic Bode use where the effects of the input source are partially removed by the input sampling location. The intended case is for Closed Loop measurements the input sampling is actually moved to the transformer secondary side, the transformer is for isolation with the DUT, and one secondary winding wire goes to Ch1 as Input and the other secondary winding wire as Output goes to Ch2, the secondary creates a "floating injection source" inserted in series with the negative feedback (with restrictions) for the DUT. Since the Bode plots are of complex type (Magnitude and phase) the result loop response of the DUT is simply Vout/Vin in magnitude and phase. Note the effects of the signal source, transformer, cables are completely removed (ideally) from the result and why this technique is so powerful when applied properly!!
Here's some previous quotes in this thread answering your questions regarding the transformer intended use, and what the plots show.
Our intent was to show the CM Filter cores in a DIY configuration utilizing the wire from the CM Filter can be useful for Bode type Closed Loop Measurements within a 1MHz frequency range,Here's what I think you are asking for, these are with 50 ohm source drive, transformer input measured with DSO Hi Z (1M) and output terminated with 50 ohms (DSO)The low frequency impedance to the transformer is low, however the input sampling is across the transformer primary so this compensates some for the transformer low frequency characteristics.Of course the intrinsic transformer doesn't have a basic 10Hz 3dB corner, just look at the primary inductance measurements of ~8mH which implies an impedance of 1/2 ohm at 10Hz!!! This is not to say the core transformer low end isn't important for Close Loop Injection use, the transformer must inject a signal into the DUT and this injection level falls off at the high and low frequency end due to transformer and source characteristics, as well as other effects. In the Closed Loop Bode measurement the loop gain of the DUT comes into play, and usually the loop gain has a general low pass type charteristic. So a rising DUT loop gain as frequency decreases causes the DSO Ch1 signal to be small as the negative feedback attempts to "null out" the injected signal which appears to the Closed Loop System as an error signal. The DSO will increase the input sensitivity to help compensate for such, but eventually reaches a sensitivity which can no longer "pull out" the signal from the noise, and the Bode response degrades. The same happens at the high end where the transformer rolls off, thus the injected signal rolls off. However, the DUT loop gain is also falling off which partially helps with DSO detecting the signals since less of the injected signal is "nulled out" by the negative feedback loop gain.
We realize this is a rather complex subject and likely difficult to get ones "arms around", especially with all the nuances involved in proper setup, use, and measurement understanding. Please spend some time studying the mentioned video (and other related papers on Bode Plots and Close Loop Measurement Techniques), this is an excellent resource for getting an understanding of Closed Loop Bode measurements, and even illustrates how this method can be utilized to measure the very complex non-linear nature of SMPS even tho Bode is a linear type function.
Awhile back, well before this thread, some folks downplayed the Bode capability of these DSOs. Likely from a lack of knowledge and/or understanding just how useful this technique is when properly applied with the DSO. PicoScope, Siglent have this capability (we know and use both), Keysight and others also.
Anyway, hope this helps clarify what this thread was intended to illustrate, and how useful these DSOs are for Closed Loop Bode measurements with the simple addition of a repurposed inexpensive Common Mode Filter core for Bode Injection Transformer usage.
Best,
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