I have looked at forum history and although there's lots of transformer posts, I cannot find this answer.

I am a semi-retired tech guy and getting back into discrete circuits as a retirement hobby and I do not remember if I ran in to this in my past (I'm nearly 60 and have not played around with circuits since 1991).

My function generator putting out a sine wave at 30 khz on a very small (physically small) step-up transformer that shows a step-up on the secondary of 160:1 at the 30khz frequency.

The transformer is not in a circuit, I'm testing it standalone, at this stage I'm trying to re-learn all the stuff I haven't played with for nearly 30 years.

So at 1 volt peak to peak out of my function generator, sine wave at 30 khz, I see 160 volts p-p on my oscilloscope (I am using a x10 probe by the way so the scope actually reads 16 volts p-p).

I don't remember what would cause the secondary's output voltage to drop from 160 volts p-p to only 500 millivolts p-p as I increase the frequency to 1 Mhz.

The output voltage also drops off as I *decrease* the voltage from 30khz. Again there is nothing attached to this transformer except the function generator on the primary, and the oscilloscope on the output of the secondary, so I don't know why it would have a resonance at any particular frequency.

I remember the notion of hysteresis, ie. loss in the iron core due to heat -- is that why my output voltage keeps dropping as I increase the frequency from 30khz to 1 Mhz?

 

basically Yes.
There are losses due to eddy currents in the core of the transformer.
There are also losses due to capacitance of the windings.
And increase due to higher effective resistance caused by skin effect.

 

It really has to do with the core properties. Try selecting a Ferrite for a toroidal transfomer.

 

Okay, thank you both for the info. Question then, this will help me figure it out more, should I expect that it's possible to have a single transformer be able to output the same voltage when the frequency varies from 30khz to 1 Mhz, or are transformers designed so that any given transformer is optimized for a narrow band of frequencies only? That would strike me as odd, because if you think about an audio amplifier, or an RF amplifier, both of those operate over a broad range of frequencies, and although I'm not certain, I'm thinking there is just one transformer needed for each type of amplifier.

 

Chances are the majority of the problem is core loss, as others have mentioned. Another thing that comes into play is imperfect magnetic coupling due to winding design. Even if you have a very good high-frequency core material, the magnetic coupling between primary and secondary is never perfect. You can model this imperfection as being another inductor in series with the primary winding (not the only way, but adequate for illustration). This is referred to as "leakage inductance" which is a rather odd term, unless you think of it as inductance due to leakage flux - magnetic flux that "leaks out" of where it should be. The leakage inductance is of course reactive, and since it doesn't couple to the secondary, it just acts to reduce the signal to the part of the winding that is coupled.

Unfortunately, winding designs that produce excellent magnetic coupling also tend to introduce fairly high capacitance between primary and secondary.

I suggest a look at the Magnetics, Inc. website. I think you'll find ap notes and other useful info for design of transformers of the sort you want. You might also find something useful at Amidon Associates, which is a reseller of magnetic cores. Micrometals may also be worth a look. I've used lots of Micrometals cores, but just for power inductors.