Coming to a Mobile Phone Near You: AM Radio

By: Shahram J. Tadayon, Silicon Laboratories
( 1 Sep 2008 )

FM radios in handsets and portable media players are becoming commonplace. In fact, most portable media players have an integrated FM player or a sleek FM accessory. The FM adoption success in handsets is an even larger success story with more than 320 million handsets shipping each year, increasing to close to 500 million handsets per year by 2010.

Having an FM radio has been a great, lowcost way to add multimedia capability to portable phones, but what happens when the consumer wants to listen to the ballgame, news, political events, or a favorite talk show? Some might come to the conclusion that AM radio is being phased out, but upon further investigation AM radio is alive and well. In the United States, five out of the 10 most listened to radio stations are AM stations, which is understandable when you consider that there are over 6,000 licensed AM radio stations in the US alone.

Okay, so Americans really like sports and talk radio but what about the rest of the world? The AM listening rate worldwide is even greater. Eight out of the 10 most populous countries have more AM radio stations than FM stations. Two important explanations for this wide adoption of AM radio are that AM radio stations have a larger coverage area than FM, and they tend to provide live content.

AM is a required feature in every FM device on the market except cell phones and MP3 players. The reason for this may not be immediately obvious. Building a small, portable AM radio is very challenging, even given the fact that Walkmans with a tape deck and AM/FM radio were around almost 20 years ago. The problem comes down to trying to fit that walkman into a cell phone.

There are several technical challenges that exist in AM but not in FM, including PCB space, antenna size and manual alignment. The first challenge is board space. Open up the latest AM/FM radio and you will find one or two integrated chips and more than 45 discrete components that are required to make a full AM/FM solution. These solutions require board space in excess of 10cm² or the size of a bite-sized candy bar, which is far too large of a footprint for cell phones. A radio in a cell phone has to be much smaller.

The next challenge is antenna size. A rule of thumb for the size of a good antenna is one quarter wavelength of the broadcast frequency. For FM this is not a problem because that is about two feet and therefore a headphone antenna can easily be used in a portable device. However, with AM frequencies this antenna would need to be more than 100 times larger. Consumer AM radios get around this problem by capturing the AM radio signal using a loop antenna or ferrite loopstick and amplifying the desired signal. Unfortunately, in order to get satisfactory AM reception, most radios require a ferrite loopstick that is 1.5in to 2in long by 0.5in wide, which is still too large for a mobile phone.

In addition to board space and antenna size, AM radios require manual-aligned components such as inductors and variable capacitors. Looking at a home stereo, clock radio or boom box, you will find one or more inductors that have been physically spread apart at the manufacturing line to get the right inductance value and then wax is applied to this inductor to maintain its value. Portable media player and handset manufacturers are not able to support manually-aligned components because of throughput problems, reliability issues, and cost.

Are mobile phones may be the end of AM radio? Not necessarily. There is good news for sports fans and talk show enthusiasts. AM radio is already being adopted by portable media player or handset manufacturers. By using CMOS technology and a digital low-IF architecture the three technical issues are solvable.

For example, a product such as the Silicon Labs' Si4731 is a highly integrated AM/FM receiver that only requires two addition external capacitors. The footprint for this solution is 0.15cm² or about the size of a thumb tack. Combining DSP techniques with high performance mixed-signal circuitry, the Si4731 enables the use of a miniaturized ferrite antenna that can pick up local AM stations. This antenna is 25mm long and 3mm in diameter as opposed to more standard ferrites that easily occupy 10 times that space. The Si4731 has a built-in varactor and offers digital tuning without manuallyadjusted components, and the device can fit into the same footprint as existing Silicon Labs FM radio and FM transceiver ICs, making it is easy for manufacturers to upgrade new models to include both AM and FM.

Beyond just solving the AM technical problems, designers should look for devices that add more features in AM/FM radios such as RDS for radio text in FM, rapid seek, multiple channel bandwidth settings for higher fidelity on strong stations, and better signal reception from weak stations.

By leveraging advanced RF techniques, common process technology and proven mixed-signal expertise, it is possible to take a very challenging problem and resolve it without making compromises that would increase the cost or negatively impact the performance. The end result is the ability to enable mobile handsets to offer both AM and FM radio in a very small footprint, consistent with the handset maker's desire to consistently reduce the size of the handset.

As the next generation phones are designed, a simple, popular AM radio could be an important feature. Users will soon be able to tune their cell phones to AM and listen to history being made whether that's the Beijing Olympics or worldwide election results.


Click here for the illustration:

Figure 1