Clifton Labs

Non-Linear Xformer Behavior

(Editor’s Note: The following article is from the archives of experimenter, inventor, friend of the Ham Radio community, and founder of Clifton Laboratories, Jack Smith, K8ZOA (SK).)

I’ve written about linear transformer models at Audio Transformer Data and Modeling, and at Softrock Lite 6.2 and about ferrite core based RF transformers at Ferrite Transformers. And, I’ve explored in some detail the problems stemming from Elecraft’s choice of a Tamura TTC-108 audio transformer to provide LINE OUT isolation in its K3 transceiver at Elecraft K3 Receive Audio.

My Elecraft K3 Receive Audio page has a great deal of additional non-linear performance measurements for the Tamura TTC-108 transformer and should be read as a supplement to this page for a full understanding of non-linear transformer behavior. My Elecraft K3 studies, unlike the other pages, looked at both linear and non-linear problems with the TTC-108. Linear transformer concerns are most commonly related to frequency response and, less commonly, phase shift. Non-linear behavior results in waveform distortion, evidenced by harmonic generation and intermodulation distortion.

This page focuses on non-linear transformer behavior. Although started as an extension to my Elecraft K3 audio explorations, I’ve expanded the scope of these studies to other transformers and hence decided the topic justifies its own web page.

Albert Einstein one said–although in more formal language–�everything should be made as simple as possible, but no simpler.� To understand why transformers produce nonlinear distortion requires a detailed look at magnetic material behavior. I’ll make the explanations and mathematics as simple as possible, erring on the side of over-simplification.

I’ll also add that I’ve written many magazine articles, an 800 page book on computer programming as well as this web page, not to mention thousands of documents I’ve worked on professionally whilst practicing law for 30 years. This web page turns out to be among the most difficult I’ve written, and fails to meet my standards of clarity. Generally this means I don’t understand the subject as well as I should, and I apologize in advance. When growing up, I recall an expression used in my family, �he doesn’t understand all he knows about it.� This expression applies here�I have learned quite a bit about transformers in my measurements and research, but I fear that I don’t quite understand all that I’ve learned.

See the Jack Smith’s entire entry on Non-Linear Xformer Behavior in the file below:

Sidebars

Editor’s Note: The following sidebars were written by Ward Silver, N0AX

Hysteresis

As the article states, hysteresis causes some behavior of a material or circuit to depend on what happened in the past— or, the current behavior depends on the previous behavior. Hysteresis in a circuit’s electronic component usually results in non-linear operation. Magnetic materials, such as those used in transformers and inductors, usually exhibit some amount of hysteresis, so circuit designers need to take steps to minimize its effects.

In a linear circuit, a change in the input causes the output to change in the same way every time.  If the circuit’s behavior includes hysteresis, the input’s change doesn’t always result in the same output change. This is non-linear operation. Non-linear behavior might be useful, such as in a comparator circuit called a Schmitt trigger, which is a type of switch.  The hysteresis keeps the switch from rapidly turning on and off when the input is noisy or changing slowly. In an audio amplifier or an impedance matching transformer, hysteresis causes distortion of the output waveform compared to the input. 

Effect of Non-Linear Transformers

To create distortion of an output signal, other signals besides the desired replica of the input are required.  These other signals might be harmonics or if more than one input signal is present, intermodulation distortion (IMD) is the result as the input signal components are combined. In an audio circuit, distortion of a received signal creates a harsh or buzzy tone due to the higher harmonics and other distortion products. In an RF circuit, such as an impedance transformer, distortion creates spurious signals that can create interference in a receiver or to other stations if transmitted.

Distortion can be minimized through careful component selection or through the use of negative feedback so that the circuit can cancel the effect of the non-linear component. The article shows several types of audio transformers tested by the author.  Each transformer has a power rating either in mW or dBm at a specified impedance level. It is important that the transformer not be overloaded to avoid distortion.

The DX Engineering RXShare audio isolation and switching unit uses Triad TY-146P transformers (see figure) rated at 1 watt, although most headphones operate at lower power levels.  These larger transformers are more expensive but avoid being overloaded at high audio levels.  The higher power rating avoids distortion caused by the low headphone impedance (typically 32 to 300 ohms) and IMD from the input signals mixing together. Avoiding distortion in transmitting applications is just as important. When purchasing a transformer like the 4:1 Maxi-Core Unun, make sure it is adequately rated and don’t operate at its maximum power capability. Not only might excess power damage the transformer core, non-linearity can result in harmonics or other spurious signals. The bands are crowded enough without an accessory adding more signals!

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