# I am trying to work out the BW of my transformer

1. Nov 18, 2008

### KennyB

I am trying to work out the BW of my transformer.

My source resistance, Rs, is 50R. My load resistance, RL=50R. The windings ratio ,N, is 1:1. The magnetising current, Lmag=7.96uH. The data i have suggests that to find the lower -3dB point, you refer the secondary load to the primary. This gives RL'=50R. In order to find the cut off point, the resistance of the primary is equated to the reactance of Lmag. The combined resistance is defined as Rs//RL'= 50R//50R = 25R. The lower cut-off is then evaluated as 500KHz. A little confused by this because although the leakage inductances are essentially shorts at this frequency, Lmag appears in // with the primary referred load, RL'...and these two components combined are in // with Rs. Unsure.

Now for the upper -3dBpoint...the data now says that the total resistance is Rs + RL'. Why is Thevenin's not applicable here?! The primary winding leakage inductance equals the secondary winding leakage inductance = (1-K)*Lmag where K = 0.998333. The upper -3dB point was evaluated as 600MHz.

Can somebody please clear this up for me?

PS: The reason why i'm interested in the maths of this is that i want to input this data into SIMetrix for analysis with additional analogue front-end circuitry.

Last edited: Nov 18, 2008
2. Nov 18, 2008

### Pumblechook

Re: Tranformers

There will be stray capacitance involved at 600MHz which may produces resonances. I wouldn't expect the attenuation/frequency curve to be smooth. There might be frequencies where the attenuation becames very high..deep nulls. Any core material will behave differently at UHF compared to LF. There will be so many unknowns that it will be impossible to calculate. The only way is measurement of a real transformer. Even then terminations and even short lengths of coax will distort the results.

If you consider it as a purely (ideal) mathematical exercise that is a different thing.

3. Nov 21, 2008

### chedor

Re: Tranformers

helo pro Engs.

I am new here i will like to know more of this transformer calculation .i intend to be faithful explain to ,i am expecting thing of fabulous ...

Thanks
rasheed

4. Nov 21, 2008

### Staff: Mentor

Re: Tranformers

Welcome to the PF, chedor. What is your background so far with transformers? Have you read the introduction to transformers at wikipedia.org yet? If you have specific questions, you could start a new thread here (or in the Homework Help forums if your questions are for schoolwork), and list your specific questions.

5. Nov 23, 2008

### chedor

Re: Tranformers

thanks for early reply...i am new in the field but i have faith that with time i will be a profeesional ,the you have email.. that we can dicus better that this..i have little idea of transformer ..

thanks
rasheed