Archive for May 2010


Front-End Blocking

If you’re an RF engineer and you haven’t heard of front-end blocking then you’re lucky. I suspect you will hear about it soon, during the last few years I’ve done a lot of work on this problem. The diagram below shows the basic problem. A radio receiver only needs to be sensitive to the band of frequencies that it must receive, there is no need for the radio to have any sensitivity to other frequencies. However, that doesn’t mean that a receiver will reject incoming signals that are out-of-band. Most designs of receiver front-end are sensitive to a much broader range of frequencies than the band in question.

Suppose that a signal is being transmitted in the frequency range marked in green on the picture above. That signal will be captured by the receiver front-end along with the wanted signal. Then the receiver may cause the two signals to interact garbling the information from the wanted signal. There are several different ways that this can happen in different parts of the receiver circuit, I may write about those another day.

In this scenario the interfering signal isn’t in-band, it’s out-of-band, but it still causes a problem. For the sensitive receivers used in modern digital communications systems the power of the interfering signal doesn’t have to be high to cause reduced performance or complete link failure.

There are six general approaches to mitigating this problem:

  • Move the receiving antenna away from sources of interference
  • Break the path between the source of interference and the receiving antenna
  • Make the receiving antenna more narrowband
  • Add in a band-limiting filter between the antenna and the receiver
  • Make the receiver less sensitive to out-of-band signals
  • Change the receiver design so that the wanted signal and out-of-band signals don’t interact or interact less

I’ve used most of these strategies, in the future I’ll write about them in more detail.

Antennas for Portable Handsets

.Cellphone Image

My first job in Antenna design was designing internal antennas for cellphones. The cellphone market is enormous and every handset needs at least one antenna, and often two. It’s important for many of the people involved to understand the problems, so this is a very basic overview.

Due to the differences between handsets nearly every new model requires a new bespoke antenna design. The volume allotted for the antenna is different in each new handset, and it’s a different shape. The circuit board and the metal parts of the handset provide a groundplane for the antenna. This groundplane generally changes in size and shape with every new model. The groundplane characteristics affect the antenna significantly. An antenna built for a particular groundplane will not work well on a groundplane with a different size and shape. The plastics surrounding the antenna, and other nearby components also affect the antenna’s parameters.

Because of these issues it is virtually impossible to reuse designs directly. It can sometimes be done with external antennas such as whip antennas and stubby antennas, but not with the internal antennas commonly used today. It can sometimes be done with Bluetooth and Wi-Fi antennas, but not often with the main antenna for the cellular service.

So, Antenna Designers use a different sort of reuse. They gather knowledge about certain types of antennas, such as PIFAs and sub-types of them, they then learn how those sub-types behave and what factors affect their performance. An Antenna Designer learns how to “tune up” a few types of antenna. This allows him or her to quickly respond when given a particular product to work on.

To make this process as efficient as possible the major antenna companies have invested in rapid prototyping equipment, rapid antenna measurement equipment and other infrastructure.