[time-nuts] Noise and non-linear behaviour of ferrite transformers

Attila Kinali attila at kinali.ch
Sun Jul 20 05:10:23 EDT 2014

On Sat, 19 Jul 2014 22:16:33 -0700
"John Miles" <john at miles.io> wrote:

> That may be due to tempco, as Tom mentioned.  The ferrite cores don't seem
> to be noisy in and of themselves, 

My current research tells me, that transformers have an inherent
noise (Barkhausen noise), but so far i could not find any numbers
on how large the effect actually is.

> but the slightest air currents on your PCB
> (including just convection currents from nearby hot components with no
> external airflow at all) will look like close-in phase noise that a
> 3048A/E5500-class instrument will be able to see.  
> Let's see if these plots come through.  This is the residual performance of
> a two-channel digitizer built with an AD9268 demo board, where the blue
> trace has the input transformers covered in RTV162: 

Interesting data! Thanks a lot!

> All that said, the real hazard with transformers is that people tend to use
> them to drive unbalanced coax cables with balanced signals.  This turns the
> coax shield into an antenna, at which point you may end up with with more
> noise and spurs than you had before. 

Could you explain this a little bit more? Because this would be exactly
what i would like to do.

			Attila Kinali

I pity people who can't find laughter or at least some bit of amusement in
the little doings of the day. I believe I could find something ridiculous
even in the saddest moment, if necessary. It has nothing to do with being
superficial. It's a matter of joy in life.
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