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I am planning to build an amplifier with the HEMT CGH40010. In its datasheet (http://www.wolfspeed.com/downloads/dl/file/id/317/product/117/cgh40010.pdf), there is a picture of a evaluation kit on page 8, and an even better picture is on page 9. There, one can see several small 'copper islands' near the microstrip lines from the connectors to the FET, and near C1 for example is a tuning stub (that one which is bent towards the left hand side) and at the end of this stub, there are also a few of these strange copper islands.

I wonder what the purpose of these small copper pieces is. I guess it is either some kind of resonator, or it is for tuning. But in the 2nd case, how does it work? is the idea to 'scratch off' some of the copper with a scalpel to tune the stub, or what exactly is it?

Photo of that board (Source: wolfspeed.com)

PCB schematic (Source: wolfspeed.com)

Enric Blanco
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T. Pluess
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  • These islands are called tuning dots or "chicken dots". A similar question can be found here: http://electronics.stackexchange.com/questions/175593/what-are-tuning-dots-and-how-to-effectively-use-them – DakotaD Mar 31 '17 at 12:57

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Those small islands are there for tuning the input and output impedances seen by the power amplifier. They're not to be removed! Another example (it's common practice, in fact):

Tuning sections

You can solder several of them together in order to build a stub of the length you need, placed at the distance you need for impedance and/or noise matching.

There are also sections of different width at the output. You can tune the length of each section by soldering some of those islands together.

Enric Blanco
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  • oh, nice. How is the distance and the size of the islands calculated? do you just put some solder on it to connect them? – T. Pluess Mar 31 '17 at 11:17
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    The cleanest method is to use a silver (or other metal) ribbon and solder it to bond the desired islands and the line together. This is far more reliable and controllable than just using solder bridges (blobs). I've seen people tuning amplifiers by just moving a piece of silver ribbon around the islands and pushing it with a stick (to ensure electrical contact) while looking at how I/O matching changes in the VNA. When good matching was achieved then they soldered the piece of ribbon in place. People who say RF/microwaves are dark arts are absolutely right sometimes. :D – Enric Blanco Mar 31 '17 at 18:54
  • Regarding distance and size: this is up entirely to the kind and range of adjustments you wish to make. Apart from that, just make sure (by simulating and making some preliminary measurements) that the effect when they are unconnected is negligible, and that the dimensions of the island are compatible with the ribbon widths you'll be using to tune. – Enric Blanco Mar 31 '17 at 18:58
  • For gold-finished ceramic substrates, wire bonding is used to connect the islands instead. – Enric Blanco Mar 31 '17 at 19:40