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Author Topic: Common mode supression  (Read 1080 times)

Offline Quinta

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Common mode supression
« on: March 18, 2023, 0935 UTC »
My english still very bad, but i should tell here about my experiments.
Common mode noise i measure between PC case and grounding in 9 floor building via 10:1 voltage divider - almost 13 volts.
http://hoshinokoe.ru/files/0ARTICLES/common_mode/interf_pc_case.jpg
My previos schmatic for CM supression. Miniature galvanic isolated 9:1 transformer instead of balun, second miniature 1:1 transformer near receiver
http://hoshinokoe.ru/files/0ARTICLES/common_mode/common_mode_protection.jpg
But in the big building grounding very noisy, almost like a mains power. So intead of classic long wire and grounding much better use a two random wires attached to balun.
Measuring of some variants.
http://hoshinokoe.ru/files/0ARTICLES/common_mode/LW_and_noise.jpg
Then i add a long, 30 cm common mode choke, made like a anode choke, but winded with two paralel wires, to reach maximum reactance.
http://hoshinokoe.ru/files/0ARTICLES/balun_for_SWL/SWL_balun1.jpg
I still not measure this long balun, but it seems t be its better then only miniature transformer.
Today i made a Austin tranformer for HF to measure its reactance for common mode.
http://hoshinokoe.ru/files/0ARTICLES/balun_for_SWL/Austin_trans_isolation.jpg
Certainly reactance depends on design, bit this particular design has 4-5 db better common mode suppression in 50 ohm schematic compared with compact balun from aliexpress.
http://hoshinokoe.ru/files/0ARTICLES/balun_for_SWL/wideband_trans_isolation.jpg
The only problem with austin balun - this is a magnetic transformer, so you can't use random ferrites ripped from vga cable chokes. And it has high leakage inductance, so losses in poorly designed austin transformer can ruin advantages of low winding capacitance.
Now i use "long" balun with my personal sdrs and parallel with web-sdr on my personal webpage.
For example - coastal station Skagen on 1700KHz, distance approx 1200km.
http://hoshinokoe.ru/files/0ARTICLES/balun_for_SWL/skagen_coastam_mf.mp4
Yes, still bad but with classic amateur contruction in my case noise floor would be much higher







« Last Edit: March 18, 2023, 0941 UTC by Quinta »

Offline Charlie_Dont_Surf

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Re: Common mode supression
« Reply #1 on: April 17, 2023, 2035 UTC »
My english still very bad, but i should tell here about my experiments.
Your English is good enough to write this, which is reasonably high. I can see that you paid attention in English class.  :)


Common mode noise i measure between PC case and grounding in 9 floor building via 10:1 voltage divider - almost 13 volts.
http://hoshinokoe.ru/files/0ARTICLES/common_mode/interf_pc_case.jpg

OK, understood, however, I don't think that this is completely unusual and don't take this as positive proof that you have "a lot of noise". Your photos of the SDR screen do indicate that there is quite a bit of 1-30 MHz noise but I would not say that the oscilloscope image is indicative of the SDR waterfall images. That is, one does not necessarily lead to the other.

A few things to note:
1) Oscilloscope probes that are not grounded will often pick up (receive) noise like this.
2) I believe that you have connected the oscilloscope probe to a AC to DC power inverter output (but without a ground connection?). Understand that this output should be a high impedance node for anything above ~10 KHz (because the parasitic L increases the output impedance at higher frequencies), making the DC cable a very nice antenna, so the oscilloscope (which also has a high impedance input) will pick up quite a bit of noise from everything in the general area.
3) I think that the oscilloscope may not be connected to the same AC mains outlet (220V outlet) as some of the other items on your desk. This can create a ground loop that can create enough noise to show up on an oscilloscope by virtue of the different ground voltages at frequencies greater than DC.
4)  The oscilloscope probe is ~30 cm from the computer screen and there are likely many power inverters within ~1 meter of the probe.

So, again, it looks ugly but I wouldn't make a lot of decisions about what to do about it based upon this measurement that you show.


I still not measure this long balun, but it seems t be its better then only miniature transformer.
Today i made a Austin tranformer for HF to measure its reactance for common mode.
http://hoshinokoe.ru/files/0ARTICLES/balun_for_SWL/Austin_trans_isolation.jpg
...
Now i use "long" balun with my personal sdrs and parallel with web-sdr on my personal webpage.

You may want to consider using two antennas (e.g., the 8m wire and the 30 m wire) and a variable phasing network to be able to rotate (change) the phase of the two wires relative to each other to attempt to partially cancel some of the noise by making it out of phase.
« Last Edit: April 18, 2023, 0236 UTC by Charlie_Dont_Surf »
I don't STRETCH the truth.

"Every minute I spend in this room, my signal gets weaker.
Every minute Charlie squats in the bush, his signal gets stronger."

Offline Quinta

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Re: Common mode supression
« Reply #2 on: July 20, 2023, 2228 UTC »
My english still very bad, but i should tell here about my experiments.
Your English is good enough to write this, which is reasonably high. I can see that you paid attention in English class.  :)

I simply love to watch AVGN :)

So, after some measuring i make the best antenna transformer for receiving: magnetic transformer on N87 ring, relatively thin and big diameter, so capacitance between windings minimal.