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DIY Transformer for use with Bode Plots.
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_Wim_:

--- Quote from: jonpaul on May 19, 2022, 07:31:27 am ---The professional units have much lower LF cutoff than 100 Hz, 1..10 Hz.

--- End quote ---

That quite achievable DIY also. Quite a while ago I made these following these excellent instructions: (http://www.simprojects.nl/images/DIY_signal_injection_transformer.pdf)

My results were posted here:
https://www.eevblog.com/forum/testgear/looking-for-a-low-cost-way-of-measuring-dc-dc-converter-control-loop-response/msg1738013/#msg1738013

joeqsmith:
Looking at the Jay link, I can't understand why he was showing 10dB/div.   Interesting is he shows a // resistor with the secondary.   I wonder if he was driving a 1M source.


--- Quote from: mawyatt on May 18, 2022, 06:23:11 pm ---Here's a plot of the Transformer using a 50 ohm source @ 0.2Vpp (SDG2042) and Siglent SDS2104X Plus under Bode mode.

--- End quote ---

Your source is 50ohms and you are driving your transformer that has a 50ohm in // with the primary?   Then the secondary is also in // with a 50ohm which drives a 50ohm input? 

When you built the second transformer (series parallel) you did not use the // resistors and you also changed how you measured it.  Could you go back and measure it the same way as the first?   


--- Quote from: TopQuark on May 22, 2022, 07:45:04 am ---I have been down this path of building wide BW transformers before as well.

--- End quote ---

Could you change the vertical scale to 0.2dB/div and repost?
mawyatt:

--- Quote from: joeqsmith on May 23, 2022, 12:55:41 pm ---Looking at the Jay link, I can't understand why he was showing 10dB/div.   Interesting is he shows a // resistor with the secondary.   I wonder if he was driving a 1M source.


--- Quote from: mawyatt on May 18, 2022, 06:23:11 pm ---Here's a plot of the Transformer using a 50 ohm source @ 0.2Vpp (SDG2042) and Siglent SDS2104X Plus under Bode mode.

--- End quote ---

Your source is 50ohms and you are driving your transformer that has a 50ohm in // with the primary?   Then the secondary is also in // with a 50ohm which drives a 50ohm input? 

When you built the second transformer (series parallel) you did not use the // resistors and you also changed how you measured it.  Could you go back and measure it the same way as the first?   


--- End quote ---

We actually removed the all termination resistors on all the transformers, these were originally included to save from having an external termination for some other tests. The series/parallel version would require a termination of 12.5 ohms from proper termination with a 50 ohm source drive.

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). First plot is larger core mentioned, second is smaller core and third is with series/parallel transformer externally terminated with 50 ohms (DSO) and a pair of 33 ohm shunt resistors, for a combo of 12.41 ohms (close to ideal 12.5 ohms for series/parallel transformer).

I'll post the primary and leakage inductance of the transform later.

Edit: Here's the transformer measured details measured @ 10KHz.

Large Core in Grey Box Lp 8.2mH, Ll 4.5uH, Cc 122pF
Small Core in Blue Box Lp 8.0mH, Ll 8uH, Cc 71pF
Series/Parallel with Small Core in small Grey Box Lp 5.4mH, Ll 3.6uH, Cc 48pF

Anyway, hope this helps and what you are looking for.

Best,
joeqsmith:
Thank you.  This was very helpful and makes much more sense.   Could you display them with 0.2dB/div rather than 1 like you show with your first plots.    Maybe 2deg/div.   I'm not sure what the Siglent can show.

It sounds like the Siglent can't route these signals internally so you have some output  that is 50 ohms that you connect to a T.  One leg of the T goes to one channel of the scope set to 1M.  The other leg of the T goes to the transformer primary.  The secondary goes to a second channel of the scope set to 50 ohms.    Is that correct?

When you run these tests, how do you calibrate the system?   Do you just use a section of coax for the thru and normalize to that?   

***
Sorry, but I had one other question.   Assuming the circuit you are testing applies some DC bias, are you testing this effect?   What sort of breakdown voltage do you need between the two windings?   Things go wrong, seems like it could go really bad.
mawyatt:
There's no way to calibrate the system we know of other that taking cal measurements and then creating plots from collected data with cal corrections. These Siglent DSOs are relatively new to our lab, so maybe someone with more experience with these can chime in.

I would not place much emphasis in the plots supplied beyond 1MHz, since this is the area of our interest resides. We made no special attempts to minimize or equalize cable lengths, or even include cables effects, or low loss cables....just some stuff we had laying around, even had scope inputs set on 20MHz BW! Likely much of the "artifacts" past 1MHz are due to the scope BW, crude setup, cabling and such. For a more thorough test one would use full scope BW, controlled setup, quality short cables, connectors & such as you would expect for proper RF measurements...which this thread wasn't intended for!!

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, thus frequency range note on case covers. I would not use the CM Filter wire if the use entailed a large voltage across the transformer, this would require a proper insulated twisted pair with thick insulation, again our usable is for Closed Loop measurements for Op Amps and such, not high voltage circuits like SMPS.

DC bias is a concern and why we got the larger core, thinking a larger core should have a higher DC saturation capability vs. the smaller core, even though we don't know what the core details are. Likely we'll get a proper core and windings later if need be, thus questions to others that have utilized proper cores.

This effort was just a quick DIYer to "see" if these cores would work and get familiar with the Closed Loop capability using the SDS2104X Plus and especially to "see" if this capability could be applied to high loop gain circuits such as Op Amp based circuits.

The setup has been torn down, but if we get some time will try and rerun with the tighter dB and phase scales.

Anyway, hope this helps explain the DIY effort behind these CM Filters, reconfigured as Isolation Transformers for Bode Injection use.

Edit: Added finer resolution plots for a Thru, Large Core, Small Core and Series/Parallel with Small Core and scope BW is 100MHz.

Best,
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