equipment terminal impedance

What kind of transformer, what frequency?

Most transformers will have impedances substantially less than 50 ohms if used below their design frequency range. This will cause lotsa distortion if driven from a typical piece of bench equipment with a 50 ohm output impedance.

Some small power transformers will saturate in normal usage with no or little load, this can cause extremely funky current waveforms. I think this is a "design feature".

Tim.

You are correct about 50 Ohms being the source resistance. This is typical of signal generators that are designed to drive a transmission line with a 50 Ohm characteristic impedance. The distortion is probably due to 1 of 2 things: . The load impedance reflected back to the signal generator through the square of the turns ratio of the transformer is less than the signal generator is capable of handling. . The leakage inductance of the transformer is so high that its inductive reactance at the frequency of interest is so high that it forms a voltage divider with the reflected load resistnce. Think of a voltage divider consiting of an inductance (the leakage inductance) as the series element and the reflected load as the shunt element. The leakage inductance can be measured as follows: .. Short the secondary .. Measure the indactance across the primary terminals.

You don't say anything about the impedance ratio of the transformer, or what the resistive load is that is connected to the secondary of the transformer. What is the frequency? If you have no load on the generator, other than the transformer, the generator might not like to see a reactive load. Also, the transformer may not be suitable for the frequency you are trying to pass. What happens when you drive a 50 Ohm resistive load?

Low power RF signal generators that I have seen do actually have near 50 Ohm output impedance.

Tam

Yes.

You can usually load signal generators pretty heavily. What does the signal look like when you connect the signal generator up to a 50 Ohm load instead of to your transformer? Also, how much current is actually going into the resistor? (i.e., what is the peak Voltage across the resistor and what is the resistor value?)

We could probably save a lot of time if you would refer us to a schematic of your test setup. Some very important details are: Where is the current sense resistor, and how are you measuring the voltage across it? What load do you have on the secondary of the transformer? What frequency (roughly) are you operating at? We also need to know what the turns ratio of the transformer is, and whether it is rated for the frequencies you are using it at. For example, a line power transformer cannot be used at RF.

Another random thought: when probing current-sensing resistors, it is important to remember that the oscilloscope probe ground is connected to Earth ground, so you have to be careful where you attach it. This is why I would like you to include that information in the schematic of your test setup.

--Mac