Products > Test Equipment
DIY Transformer for use with Bode Plots.
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jonpaul:
Rebonjour...wideband transformer design is a compromise and tradeoff, LF ..HF BW, ratio, # turns Lp, Llkg, Cp-s, Cshunt.

See many fine refs on WB trafo design, one example is /Magnetic Components   from our old friend Steve SMITH,

https://www.amazon.com/Magnetic-Components-Applications-Steve-Smith/dp/0442203977

RE stability/bode polts/injection trafos, i suggest   original papers and hardware from Deane VENABLE in 1980s

https://www.venableinstruments.com/loop-stability

 the excellent modern systems and transformers from Omicron Labs

https://www.omicron-lab.com/fileadmin/assets/Training_and_Events/Webinar/2014-11_Webinar_LoopGain.pdf

https://www.omicron-lab.com

Bon chance!

Jon


mawyatt:
We decided to make another transformer out of the last CM Filter we have, and removed the windings then twisted the pair together as before. This time we cut the twisted pair in the middle and created two twisted pair windings on the core. The two windings were arranged as primary in series and secondary in parallel. This should yield a Vo/Vi transfer of 1/2 or -6dBv, and we measured -6.85dBv.

For sanity checks on all 3 transformers used as Injection Bode types, we setup a simple circuit for Closed Loop Bode evaluation. The circuit is a simple Op-Amp (LM358) non-inverting gain of 21 with the feedback resistor of 20K and the shunt to ground resistor of 1K. The + Op-Amp input was grounded thru a 1K resistor and the transformers were connected from the 20K and 1K junction to the Op-Amp - input. The Op-Amp was power by +-10V.

A Siglent SDG2042X AWG was used as the input (we could have used the scope built-in AWG). This was on a LAN with the Siglent SDS2104X Plus scope. Amplitude was set to 0.05Vpp so as not to saturate the small trasnfomers, and confirmed by monitoring the transformer output waveforms.

Thanks to tautech for helping us in the past to get familiar with LAN use (and earlier the SDS2104X Plus), as we didn't know "Diddley Squat" about LANs. BTW Dave used this old redneck engineering term "Diddley Squat" meaning "not much", in his latest How a LCR Meter Works video! Guess this term found it's way to the Land Down-Under  ;D


Here's what the latest transformer rendition looks like with the series/parallel windings.

The Bode plots are shown for the mentioned Op-Amp circuit above, measuring the Open-Loop Gain and Phase of a Close-Loop-System. The plots are for the 1st & 2nd transformers with the different cores, and the last is with the split winding transformer. Please note the indicated Gain and Phase Margin indicated by P1 and P2, you see the frequencies at Markers X1 and X2.

Not sure what's going on around 100~200Hz on the left, likely some interaction with the transformer, this should be a smooth plot coming away from 100Hz.

Note the use of the 10 bit mode in the scope, big help with the large dynamic range involved, and with the dynamic channel input scaling the scope performs in Bode Mode, extending the DR even further.

Anyway, this simple, cheap Isolation Transformer seems to work quite well with the SDS2104X Plus and the built in Bode Plot capability. Very powerful feature being able to do Open-Loop measurements on Closed Loop systems without any additional equipment (you can use the scope built-in AWG) other than a DIY low cost Isolation Transformer  :-+



Best,
T3sl4co1l:
Yeh specifically they're strontium ferrite, a magnetically hard material (wide BH loop); you need MnZn (LF/broad) or NiZn (high freq), soft materials with small loop.

Also amorphous/nanocrystalline, which show up from time to time in pulse transformers, CMCs, etc.

Tim
mawyatt:
Have no idea what these CM Filter cores are made of, actually surprised they worked as well as they did. Noted on the other thread linked in first post that some folks had used Vacuumschmelze Nanocrystal Core type toroidal cores, only cores I recall using were from Micrometals and TDK, and that was eons ago, never heard of these!! Digikey has a few in stock that run from $10~20 per core.

Has anyone had success with these Vacuumschmeize cores, or other types?

One thing that Jay_Diddy touched on in some of the other threads is that in very high open-loop gain circuits (like op-amps) the negative feedback return signal creates an almost null voltage at one of the transformer terminals. This gives the scope input a difficult time with accurate amplitude and phase measurements, which contributes to the uncertainty at lower frequencies, and may be more of an issue than the transformer core material.

Open Loop response shown below with a OP-07 (Closed-Loop non-inverting gain of  ~17.7dBv) and the region between 100 and 1KHz is chaotic, thus showing from 1KHz to 1MHz which looks good considering it's just plug-in on a board with jumpers. Just for fun swapped the transformer leads and you should see the inverted response (negative in dB & phase response) which is shown in 2nd image.


BTW whether you like the Siglent or not, that's pretty impressive on screen dynamic range for a mid-level DSO that's based upon a core 8 bit ADC, can't wait to see how well the 12 bit version behaves!!


Best,
T3sl4co1l:
Yes, they live up to the hype -- check datasheets, they're pretty solid.  The best example is the appnote comparing CMCs with equivalent size ferrite parts, the Am./NC are a fair cut above at low frequencies, and remain above even into the skin effect and capacitive cutoff regions.  (As laminated metal, they do exhibit skin effect starting around 20kHz or so.)  Likewise, keep in mind that the A_L figure varies with frequency -- for the same reason, it goes quite lossy at that point (R = X) and so L is decreasing with frequency in that range.  So keep that in mind.  You're just designing for impedance, and they're great for pulse transformers and CMCs because that impedance is just so high, even as it drops off through the capacitive region.

An important but now-archaic application was ISDN transformers, something like DSL but shifted lower I think?  Very wide.  Not sure what's commercially available now, or if it's basically going to be homemade, but anyway for stuff like this, homemade is fine. :)

Tim
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