How to design a digital FM radio
We are witnessing an evolution in the personal computing space. Mobile devices, such as phones and tablets, are on the verge of taking over conventional computers. FM radio is one of the popular entertainment channels in developing countries. This forgotten fact, in addition to the advent of internet radio, can potentially kill the existence of FM radios. Often, OEMs consider not having FM radio on a tablet or a mobile device. However, most of the metropolitan cities still have operational FM radio with no plans to outmode it in the near future.
There is a tiny plug-in device which, when attached to a phone or tablet, can bring FM radio service to mobile devices. The plug-and-play FM receiver can be built using an FM receiver chip with a microcontroller or SOC (system on chip). The microcontroller acts as a USB device when communicating with the USB host on the tablet/mobile and receives commands for operations such as channel scan, channel change, or to set output power level. This bus-powered plug-and-play FM receiver accessory can tune in to the local FM channel while consuming significantly less power than would be required by a mobile broadband radio (Internet radio).
FM radio receivers
FM radio receiver chips provide worldwide coverage of 70-108 MHz catering to the US/EU (87.5-108 MHz), Japan (76-90 MHz) and China (76- 108 MHz). Generally, FM radio receivers have the capability to tune frequencies in steps of 50, 100, or 200 kHz. FM radio can also support radio data system (RDS)/radio broadcast data system (RBDS) functionality, which is fully programmable by the host.
In addition to transmitted audio, RDS is used to receive text information. This could include the title of the song, name of the program being broadcast, or flash news for display. In case of emergencies, RDS can be used to transmit critical information.
Band scan is a method through which the FM radio chip scans through the complete FM band for available radio channels. The radio then stores the strongest channel frequencies in its internal memory that can be read by the host microcontroller or SOC.
Once the channels are stored, there are three ways of tuning to a specific channel:
Preset tuning: In this method, the tuning frequency of the FM receiver is set to a certain channel as defined by the host.
Search tuning: In this method, the receiver automatically searches for the next available valid channel in increasing (search up) or decreasing (search down) frequency direction.
Stepped tuning: In this method, the receiver simply steps by one channel in increasing order (step up) or decreasing order (step down) of frequency.
Most radio receiver chips available today communicate with the host using standard protocols such as I2C and SPI. The radio receiver chip also demands the host’s attention by generating interrupts on critical events such as:
- Low signal quality when the RSSI (received signal strength indicator) value drops below a threshold level
- Mono to stereo transition (and vice-versa)
- RDS synchronization is acquired
- RDS synchronization is lost
- RDS buffer is full
Since this embedded system runs on devices that are battery powered, efficient power management is of paramount importance. The radio receiver chip supports various power modes that are controlled by the SOC to improve battery life. For this reason, a receiver chip supports the following power modes:
Power Off: In this mode, the power supply is turned off and all internal regulators are disabled.
Power Down: The power supply is on but the internal regulators are still disabled.
Standby: The regulators are functional and the mode of the radio is maintained.
Power Up: This is the normal operational mode in which all the regulators are enabled and the radio is fully functional.
The first stage is an analog signal processing stage that converts the RF antenna signal to a low IF (intermediate frequency) digital signal. The AGC (automatic gain control) unit maintains the LNA (low-noise amplifier) in its linear operating range. A mixer is used to downconvert the received RF signal to a low IF signal. The ADC converts the signal to a digital format. The FM demodulation is done in a digital domain. The digital signal processor also handles the RDS data.