# Hello, i need formula for if IF transformer 42IF123-RC to design in ltspice

#### michael1978

##### Member
Hello

can somebody help me
for IF transformer
42IF123-RC i need to resonate at 10.7Mhz, i want to design in Ltspice

i read one Post here, but i dont understand
i dont know the formula, for inductor in ltspice

here my circuit in ltspice

#### ronsimpson

##### Well-Known Member

The capacitor is 4.7pF + 5pF = 5.2pF
From 1-3 is 4.5uH (3.2 to 5.7uH)
There is no 4k resistor inside.
1-2 = 5 turns
2-3 = 9 turns
1-3 = 14 turns 4.5uH
4=6 = 2 turns
The formula for inductance and turns ratio is turns^2 (turns squared)
We know that 14 turns = 4.5uH.
You need to work out the inductance for 1-2, 2-3, and 4-6. Then plug them into spice.
You have the phasing of 1-2 & 2-3 so the inductance will add not substract so that is good.

I hope this helps. RonS.

#### michael1978

##### Member
Hello Rons
thanks for help
i realy dont understand

PLEASE can you show me one example?
i dont want to bother nobody here

so 1-3 is 4.5uH, come 14 turns = 4.5uH
i get 4.5uH ^ 2 we get 2025uh

I rely dont know, how come 14 TURNS = 4.5uH

oh man can you show me example please...……..

thanks

#### alec_t

##### Well-Known Member
There are 14 turns between pins 1 and 3. Per the datasheet there is 4.5uH between pins 1 and 3.
There are 5 turns between pins 1 and 2, so the inductance between those pins will be 4.5uH x (5/14)^2 = 4.5uH x 0.128 = 0.57uH.

#### michael1978

##### Member
ooh thanks Alec_t
perfect MAN, i understand now

and wat about 4-6(2 turns) how i have to calculate this one ? please if you know let me know and i go to build...……..

is like this correct 4.5u / (14/2)^2 = 0.9uH

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#### ronsimpson

##### Well-Known Member
is like this correct 4.5u / (14/2)^2 = 0.9uH
Yes
I think you got it. In SPICE you do not have 14T = 4.5uH BUT you have three different values.

#### audioguru

##### Well-Known Member
This is the first time I have ever seen somebody try to simulate an FM radio. Why????

#### ronsimpson

##### Well-Known Member
This is the first time I have ever seen somebody try to simulate an FM radio. Why????
Because turning the "on" knob is hard. lol

I have often SPICE-ed the IF filters to get the bandwidth right. Probably I have done every block of a FM radio many times but never the entire thing.
One thing, I do not have enough computer to do the entire radio. I started with SPICE on a 8088 PC and "if my computer was a little faster". Now with 1000x faster it is too slow.

#### audioguru

##### Well-Known Member
The crystal filter defines the bandwidth. The simulation program does not know about crystal filters.

#### michael1978

##### Member

thanks ronsimpson…..

thank to everyvone