George Harker

Building a DIY signal injection transformer

To test switched mode power supply designs it is sometimes necessary to inject signal disturbances into the feedback loop to classify behavior. To do this you need a signal generator and oscilloscope or a network analyzer (expensive). You also need a way to decouple the signal generator from the circuit under test. One technique is to use a signal injection transformer. These too are unfortunately pretty expensive.

Following the guide here and eevblog posts here it is possible to make an injection transformer by hand.

I used the following parts:

The cores were taped together to make handling easier.

A long pair of wires (about 60x the height of the core stack were cut). These were twisted together and wrapped onto the coil (about 25 windings round the core which is about as much as can fit. Others note a different core which allows for better spacing- though more windings is unlikely helpful).

Care was taken to note the matching ends of the winding before pairing each loop back to the coax connector (so that the phase / connection to the signal pin is same orientation).

Testing was done using a siglent SSA1104x-e as the oscilloscope, and a siglent SDG2042x as the waveform generator. Connections to the oscilloscope were BNC terminated with 50ohm through terminators.

A bode plot of 10Hz to 10MHz was taken.

The results are -3dB at 28.6Hz rising to flat at approx 110kHz and remaining flat to just under 1MHz before rolling off smoothly to -3dB at 5.6MHz.

It would be nice if the HF end was a little higher gain further out. But this will serve my purposes pretty well. And it’s a lot cheaper than $560 for a picotest.

Note ideally at least device under test end should be fused. I didn’t. I won’t be using this on particularly high voltage / current. But… I should have.