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

John Miles john at miles.io
Sun Jul 20 01:16:33 EDT 2014


> -----Original Message-----

> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On

> Behalf Of Richard (Rick) Karlquist


> I did some tests of residual phase noise using an

> Agilent E5505A and found that air coil inductors

> did not add noise (at least down to my noise threshold)

> but that ferrite core inductors had easily seen noise.

> It was on the order of ADEV = 1E-10 close to the

> carrier.  I would describe this as noise rather

> than non-linearity....



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



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. 


-- john, KE5FX

Miles Design LLC


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