[HomeBrew] winding toroids

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[I am wondering where I can read up on winding trifilar toroids for double
balanced mixers. I understand the phasing, toroid types and winding, but 
how
does one calculate just how many turns are needed through the core? Most
cookbooks say 15 turns on a type 43 core, but never explain how that figure
is arrived at. Is it size dependant? Frequency dependant? Broadband 
dependant?]

As Jim has already pointed out, the inductive reactance of the winding ( 
Xl ), should be about 10X the the lowest impedance port at the lowest 
frequency of operation.  Note: never less than 5X.

This insures proper transformer action at this frequency. Note: In a 
broadband transformer, the  low end loss is heavily influenced  by the 
primary inductance while the hign end loss is attributted to 
interwinding capacitance and seriies resistance.

The formula for Xl is: 

(1)   Xl = 2 pie x f x L   Where f is in Hz and L is in H

Note: Formula also works if f is in MHz and L in uH.

The inductance of a winding on a toriod core is:

(2)  L = (Al x N^2) / 1,000  Where L is in uH and Al is the inductance 
index (mH/1,000 turns).

Al for a FT-37-43 ferrite core is 420

Note: Al changes for cores of difffernt size and material.

An example:

 From (2), the inductance for 15 turns on a FT-37-43 ferrite core is:

L(uH) = (420 x 15^) / 1,000 = (420 x 225) / 1,000 = 94.5uH

At what frequency does the Xl equal 500 ohms (10 x 50ohms) at an 
inductance of 94.5uH?

Solving for f in euation (1)

f = Xl / (2 pie L) = 500 / (6.28 x 94.5) = .843Mhz

Therefore a 15 turns on a FT-37-43 core will provide transformer action 
over the HF frequency range.

Note: At higher frequencies, the effect of the core is reduced, however, 
there is ample magnetic field and mutual coupling (until interwinding 
capacitance and series resistance losses become to high).

Ed, W1AAZ

Lots of books, parts and kits for sale at:

http://mysite.verizon.net/topossibilities/