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| Injection transformer for bode plot - my DIY project |
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| Weston:
One subtle thing to keep into account when testing: When wound with twisted pair these are transmission line transformers. It looks like you are using a pair from CAT-5 cable, which is 100 ohms impedance, and you are terminating in to 50 ohms, but even so, there should be some transmission line effects. Depending on which wire of the output is grounded you can get transmission line behavior. If the transformer is inverting and you have a common ground in your measurement setup it will act as a transmission line. If it is non-inverting its a normal transformer that likely has lower bandwidth. A normal transformer is limited by the stray L and C, but in a transmission line transformer that gets folded into the transmission line and the bandwidth is really only limited by loss in the transmission line and impedance mismatch causing reflections. Discussion to be had on if this actually impacts anything in application, but for loop injection one side should be low impedance, which is basically a ground. So you can use it like a transmission line transformer in some situations. Also, for your measurements you should twist the input and output wires together to lower inductance, it might impact your measurements. |
| TopQuark:
So I have had a bit of a breakthrough in the injection transformer design. Usually in injection transformer builds, there's a resonance valley in the transmission path (S21) around 10MHz. I am thinking it has to do with a LC resonance between the interwinding capacitance and the leakage inductance of the transformer. The measured LC values match up with the measured resonance frequency. Somehow the fix I found is by wrapping either the input pair or output pair around an EMC suppression ferrite. Or better yet, route the input pair against the output pair through the ferrite. See the attached photos on what I mean. The ferrites I used are TDK ZCAT clamp on type. The way I am currently thinking about it is imagining the transformer as a perfect 1:1 transformer, existing as an invisible blip in an otherwise perfect coax path. Then the ferrite is just acting as a common mode choke, with the coax wrapped around it. Anyways, now I am able to supress the S21 resonant valley around 10MHz, and have a smooth roll-off past 10MHz. The -3dB is now 20MHz, and very usable as an injection transformer to 50MHz IMO. |
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