LC circuits and ferrite cores

Ferrite cores

Cores suitable for amateur radio up to 30MHz in order of AL:

Type		Colour		AL		Size (mm)
FT37-67				20		9.5
T37-7		White		32		9.5
T37-2		Red		40		9.5
T50-2		Red		49		13

FT37-43				350		9.5
FT50-43				440		13
FT82-43				470		21
FT240-43			1075		61

BN43-1502	Shiny Black	1050		13
BN43-2402	Shiny Black	1440		7
BN43-202	Shiny Black	2200		13

The inductance of a coil on a toroid is

L(uH) = (AL * n^2)/1000

Baluns

Balanced to unbalanced transformers. Configuration:



Example of use in a PA to drive two MOSFETs, inductance of FT50-43/10t is 44uH, which is suitable for a broadband transformer where the inductance wants to be at least x4-x6 of the impedance:

Transformers

A common arrangement is the auto transformer, like this



tin / tout = vin/vout = sqrt(Zin/Zout),

tout is the sum of the turns of the two windings.

Example: if tout = 2*tin then the transformer will have a ratio of 1:4, so transforming a 50R input to a 200R output, or a 12R input (MOSFET load) to a LPF/Antenna of 50R.

In the full circuit above the turns ratio of the output transformer is 2:3 or 3/2. So the impedance is translated by (3/2)^2 or x2.2. This means the transistors will drive an impedance of 50/2.2 = 23R. The core chosen for this circuit is the BN43-3312 which has a AL = 7000. This gives impedances of 28uH:63uH.

Another example, a balanced mixer for a direct conversion receiver:



Each coil is 10t on an FT37-43 core, or 35uH impedance. The impedance is high enough to provide a wide band transformer at an input and output of 50R.

Here's a nice amplifier:



The 2N 5109 is driven directly from the 50R input, there is drop down transformer at its output to drive the gate of the IRF510. The output of the IRF510 has a step up transformer to match the 50R load.

Matching circuits

These come in two varieties CL or LC. CL match hi Z to Low Z, and LC Lo Z to Hi Z. Here's the configurations:





Common usage for these circuits is in ATUs, to match 50R to hi her or lower aerial impedances.