Why do I need an injection isolator?

Figure 2 shows how the loop of a power supply is measured with the AP300 frequency response analyzer. I learned this measurement technique first-hand from Dr. Middlebrook.  It requires a transformer to inject differentially into the feedback loop of the power supply. Middlebrook suggested the use of a current probe, driven backwards, to inject into the loop.

Figure 2: Standard loop injection technique uses a wide-band transformer for signal injection.

While a current probe can inject a signal over a wide frequency range, the injected signal is quite small and inadequate for noisy switching power supplies. Many engineers designed their own transformers to work over the frequency range that they needed. With good transformer design, it was reasonable to expect several decades of operation of the injection transformer. Passive transformers are the technology of choice for this application since they provide a fully isolated signal with no added electronic noise.

How many injection isolators do I need?

The frequency response analyzer from AP Instruments covers the very wide range from 0.01 Hz to 30 MHz.  It is necessary to go as low as 0.1 Hz, or less, in order to measure the loop gains of power factor correction circuits, which typically cross over at about 1 Hz. The 30 MHz upper frequency range is critical for characterizing power components, filters, and impedances. An injection isolator is necessary for many of these measurements.

Over the years, students have asked me how many injection isolators are necessary to cover this range.  I was always puzzled by this question since I have always only had a single injection isolator for all of my applications. I saw after teaching for many years, that the industry’s range of application was much wider than I expected.  It was from this perspective that I created a new product to meet these needs.

At Ridley Engineering, we are now on our fourth generation injection isolator. The first model we created covered the range from about 100 Hz to 15 MHz. Our second product, released about 7 years ago, extended this range from 5 Hz to 15 MHz. The third iteration increased the range, and the present product lowered the output impedance of the device as we will see later.

The latest injection isolator, designed and manufactured by Ridley Engineering, is a truly amazing device. The RIDLEY UNIVERSAL  INJECTOR covers the range from 0.1 Hz to 30 MHz. When used with the AP300 analyzer from AP Instruments, it is the only injection isolator you will need.  In applications where a substantial current source is needed, it provides unmatched performance.