3256	2009-06-28 09:33:26	chuck adams	Measurements of k and M for transformers
Message	Date	From	Subject
I had posted earlier a note, that I don't think made the group. Changing email addresses messes up a lot of things in our lives..... If you still have the transformers that you experimented with, try the following to measure the coefficient of coupling and the mutual inductance. http://www.conradhoffman.com/handyform1.doc I am starting an series of experiments myself for transformers wound on 35mm plastic canisters that you can pick up for free at just about any one-hour photo place. Get 'em now while they still exist, because when the 35mm film disappears due to digital technology, then we'll have to find another substitute. I'm doing this in studying Paul Harden's (NA5N) regen receiver called the Desert Ratt 3 on the web. FYI chuck, k7qo
3257	2009-06-28 10:09:01	Chris Trask	Re: Measurements of k and M for transformers
> >I had posted earlier a note, that I don't think >made the group. Changing email addresses messes >up a lot of things in our lives..... > >If you still have the transformers that you >experimented with, try the following to measure >the coefficient of coupling and the mutual >inductance. > >http://www.conradhoffman.com/handyform1.doc > Late last year I overhauled my method for measuring wideband transformer reactances and calculating the model parameters: http://www.home.earthlink.net/~christrask/Paper014.html Measurements can be made with a signal generator and a 'scope, though an inductance and capacitance meter is helpful and can save a lot of time. Chris ,----------------------. High Performance Mixers and / What's all this \ Amplifiers for RF Communications / extinct stuff, anyhow? / \ _______,--------------' Chris Trask / N7ZWY _
3258	2009-06-28 11:28:28	victorkoren	Re: Measurements of k and M for transformers
Many years ago I made some measurements on Pot-Core wideband transformers. Pot-core transformers are built from a plastic bobbin that hold the windings and two ferrite cores that are mounted on the bobbin. One conclusion I got is that the Leakage inductance does not change much if you mount the ferrite cores or not. It seems strange but on second thought it is not impossible. Mounting the cores increases the inductance of the primary and secondary, so the K and M (coupling factors) improve markedly, lowering the low -3dB cutoff frequency. On the other side if we use the transformer on a constant reference impedance of 50 Ohm, the upper -3 dB frequency cutoff frequency will stay the same if we mount the cores or not. The conclusi
3259	2009-06-28 11:49:01	Chris Trask	Re: Measurements of k and M for transformers
Victor, > > Many years ago I made some measurements on Pot-Core wideband transformers. Pot-core > transformers are built from a plastic bobbin that hold the windings and two ferrite > cores that are mounted on the bobbin. > One conclusion I got is that the Leakage inductance does not change much if you > mount the ferrite cores or not. It seems strange but on second thought it is not > impossible. Mounting the cores increases the inductance of the primary and > secondary, so the K and M (coupling factors) improve markedly, lowering the low > -3dB cutoff frequency. > On the other side if we use the transformer on a constant reference impedance of > 50 Ohm, the upper -3 dB frequency cutoff frequency will stay the same if we mount > the cores or not. > The conclusion is: make the primary and secondary windings well coupled (bifilar or > trifilar are excellent) without regard to the "coupling effect" of the magnetic > core. > There are additional benefits to using bifilar and trifilar windings. Along with the better coupling coefficient, I found that I can improve the IMD performance of lossless feedback amplifiers by arranging the feedback transformer windings such that they are wound with bifilar or trifilar bindles, provided that the net signal current in the bundle is zero. I came across this while trying to devise a simpler method for constructing the 1:1:2 feedback transfoemr of a 6dB Norton amplifier. The IIP3 performance of the amplifier was improved by more than 10dB by using this methodology in the transformer construction. The reason for this improvement is that the signal current in the wires did not induce any signal flux in the core, thereby reducing or even eliminating the distortion effects that result from the nonlinear behaviour of the hysteresis curve of the core material. Not all gain configurations of the Norton amplifier can benefit from this method, and I need to do a detailed study of it before I publish the full details, although I did discuss this subject briefly in my 2-part series on the Norton amplifier in Electronics World earlier this year. Chris ,----------------------. High Performance Mixers and / What's all this \ Amplifiers for RF Communications / extinct stuff, anyhow? / \ _______,--------------' Chris Trask / N7ZWY _