The demo concludes for the ISL55210 Active Balun Evaluation Platform and discusses input impedance match.


Hello, this is Michael Steffes, Senior Applications Manager for high-speed signal path products at Renesas. And we're gonna conclude our series on the active balun approach of using the ISL55210 to get a very broadband single to differential conversion. We've already seen that it gives an extremely flat frequency response through 500MHz. But does it really give a good input impedance match, as the equations would predict?

Turning to a very venerable piece of equipment, the HP or Agilent 4195, we've configured this box with one of our active balun boards to measure the input impedance here. So we've done a calibration from 1MHz to 500MHz, in an S1 one calibration, to measure the input impedance. If we look over that span, the board is powered, and we are taking a measurement, we can trigger it, and we can see that it will update the curves just slightly.

At 10MHz, we're seeing 51ohm, and a minus two degrees, almost perfect match at 10MHz, but continuing and moving out markers up to 500MHz, we'll see that we only deviate up to 53ohm approximately, at 500MHz, and 3.3 degrees. Looking at the magnitudes here, that's better than 30dB return loss over the range of probably about 2MHz to 500MHz. That's a range of input match, just really not supported by any balance. But particularly if you consider we're giving about a 10dB gain to the match load here, which is equivalent to having a turns ratio of about 3.3.

So let's turn back to some of the other characteristics of this circuit, we're obviously are getting a very good input match. So we're trying to emulate a doubly terminated 50ohm environment for output intercept measurement purposes, and that's the purpose of this circuit.

The next step in our intercept measurement, was to come up with an extremely low distortion input signal which we're getting with this pair of power amplifiers, which isolate our signal sources during the measurement process. The test signal coming out here would have the two test tones on them, and we're presenting probably better than a 120dB dynamic range of the source signals, using this approach.

Let's look at our measured data. This is measured using the active balun circuits, set up for a gain of 10dB net to the match load, and a 50ohm input match, that we've discussed previously. Here we’ve got some measured data, and then a fit line, since what we're seeing here is the open loop gain roll off of the ISL55210, up through frequency.

At 200MHz, we're measuring approximately 40dBm to the match load, in a standard output intercept kind of definition. And at 300MHz, we're still above 30dBm. So for a 115mW type device, this is actually exceptional intercept through very high frequencies. The approach certainly works as an IF Amplifier, what else would we use that for?

We could potentially use this in a number of areas, wherever you need a good input match, and of course, it's extremely flexible. If for instance, you need to terminate a cable, and the cable itself is not exactly 50ohm, you can go just go into the equations and put in what you're trying to terminate to, and adjust the resister value to get exact match. So for instance, if you have a cable that needs to be matched, and the cable is not exactly 50ohm, it's 53ohm perhaps, or 55ohm, you can adjust the input resistors to get perfect match over a huge range here.

75ohm systems can be easily be implemented, obviously for cable modem kind of applications. It’s just a very flexible circuit, and wide range of applications.

So go to the Renesas website and order the active balun implementation of the ISL55210, see how you can use it and let us know.