[time-nuts] Minicircuits 10% discount in December
Richard (Rick) Karlquist
richard at karlquist.com
Fri Nov 28 15:27:33 EST 2014
On 11/28/2014 10:08 AM, Dave M wrote:
> Thanks for the brief review of MiniCircuits stuff (I'm not connected
> with them in any way except as a customer).
> Since you've characterized some of their parts, perhaps you could help
> answer a question that someone else posted, and one that I would like to
> have answered as well.
> Have you measured the effects of DC current in the windings of an RF
> transformer, such as is seen if the transformer is in the collector
> circuit of an amplifier? If so, could you provide a generalization of
> the effects, such as changes in frequency response, losses, etc.?
> Many Thanks!,
> Dave M
The very tiny cores on MiniCircuits transformers will start to saturate
at hundreds of mA. The effect is that the magnetizing inductance drops,
which matters more at low frequencies than high frequencies. I try
to avoid feeding DC to an amplifier through a transformer winding.
Instead I use a separate RF choke for that. However, it would probably
work OK for, say, up to 25 mADC for a small signal transistor, but
why take a chance.
If you are using a DC feed through a transformer winding, be careful
not to accidently short circuit it, causing the full available current
from the power supply to flow through the transformer. This can
actually magnetize the core and permanently damage it. Saturation
via DC is much more deleterious than saturation via AC.
It is easy to calculate the flux density using Ampere's law, which
is one of the four Maxwell's equations. H = I/(2piR). Since R
(radius) is in the denominator, cores saturate from the inside
first before the whole core is saturated. Multiply H by mu,
(as any time nut knows) to get B. If R is 1 mm, and I is 628 mA,
then H = 10 ampere turns per meter. If mu-relative is 1000, then
B = 4piX10^-7 X 1000 X 10 = 125 mT. That is a hefty 1250 Gauss.
Some materials may be affected at 1/10 this flux density.
Now a days, a lot of RF is differential, in which case you are
free to feed DC through the output transformer without worrying
about this issue.
I worked for several companies where those 6 hole cylindrical chokes
were ubiquitous. I specifically characterized those and established
a maximum current rating of only 100 mA. Of course, many production
designs exceeded this limit and "worked" anyway. I actually observed
someone try to put 20A through one of these. The tantalum capacitors
on the "cold" side of the bead actually exploded due to RF current.
Rick Karlquist N6RK
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