Wireless functionality is increasingly common and desired these days, and RF design is no longer something that the average electrical engineer can ignore. Perhaps many of us would like to ignore it—the specialized techniques and unfamiliar components can be intimidating, not to mention ominous tales of RF circuits that can’t pass EMC testing, or that require endless adjusting and tuning, or that work properly only when there’s a full moon.
The challenges presented by RF systems are real, and there is no doubt that developing a high-frequency wireless transceiver is significantly more difficult than, say, designing an embedded system composed of a microcontroller and a few sensors. However, the obstacles are by no means insurmountable, especially when we consider the capabilities of highly integrated RF transceiver ICs.
Custom RF Design
When we think about incorporating wireless functionality into a design, the first things that come to mind might be Bluetooth, or Wi-Fi, or ZigBee. In many cases, it makes sense to use standardized protocols, especially if you want your device to interact with a PC, but it’s important to realize that customized RF interfaces can be preferable when all you want to do is transfer data wirelessly between two circuit boards.
A customized interface allows you to optimize various parameters according to your system’s requirements and constraints. These include low-level parameters such as modulation scheme, frequency band, and output power, as well as higher-level details related to how devices will recognize one another, how data will be formatted, and how packet transmissions will be scheduled and organized.
An example of a packet structure. See this article for more information.
I think it’s fair to say that most of us should not attempt to design a wireless data link using basic RF and digital components (e.g., a microcontroller in conjunction with discrete mixers, PLLs, low-noise amplifiers, etc.). I worked on a project like that once, years ago, and even a well-funded team of professional engineers could not coerce the system to communicate as robustly as we wanted it to.
Fortunately, it is rarely necessary to burden ourselves with all those complicated details: RF transceiver ICs provide flexibility and customization while also freeing us from difficult and potentially maddening design tasks.
A pleasantly simplified block diagram of the ZL70103 from Microsemi. This part is specifically intended for wireless implantable medical devices, but the diagram gives you a good idea of the general functionality of an integrated RF transceiver. The RF block implements wireless transmission and reception, transmitter modulation, and digitization of received signals; the MAC (media access controller) handles digital functionality such as error detection and serial communication.
How to Choose an RF Transceiver IC
The first step is to thoroughly understand the characteristics of your system and to familiarize yourself with relevant RF terminology and concepts. If you don’t have much experience with wireless communication, AAC’s RF textbook is a great place to start. We also have a free space path loss calculator and a link budget calculator.
The following sections discuss prominent features that must be considered when you’re trying to match a transceiver IC to the functional requirements and operating conditions of your system.
In some cases, you need to make sure that the device can transfer bits fast enough to keep up with your data stream. In other cases, you’re looking for an IC that is designed for low data rates. You don’t want a transceiver that is optimized for 2 Mbps (megabits per second) when all you need to do is transfer one temperature reading every ten minutes.
As an example, the ADF7021-V from Analog Devices can go as low as 50 bps. It is also highly integrated (as you can see in the block diagram).
Diagram taken from the ADF7021-V datasheet.
Obviously you don’t want a 2.4 GHz device if your system must stay within the 915 MHz ISM band. If you’re not sure which band is right for your application, look for a transceiver that is compatible with a wide variety of frequencies. The SPIRIT1 (this is the most interesting part number I’ve ever seen) from STMicro can utilize the following bands: 150–174 MHz, 300–348 MHz, 387–470 MHz, and 779–956 MHz.
Suggested application circuit from the SPIRIT1 datasheet.
If you study the pros and cons of the various modulation schemes, you might find that some are better than others given the expected operating conditions of your particular application. For example, if your system will be exposed to high levels of RF noise, frequency shift keying (FSK) is a better choice than amplitude shift keying (ASK) because FSK is less sensitive to variations in the amplitude of the received signal.
Another approach is to choose a transceiver that supports multiple modulation schemes. This allows you to experiment with different options and maybe even adjust your modulation parameters according to changes in the RF environment or the device’s operating mode.
The level of integration exhibited by RF transceiver ICs is, in my opinion, quite impressive. It turns out, though, that even more functionality can be squeezed into a single package: some chips have both a transceiver and a microcontroller. This approach doesn’t appeal to me because I like to transfer microcontroller designs from one project to the next, but if you think that you can save board space or development effort by using one of these devices, there are many options to choose from. One example is the Si106x family from Silicon Labs:
Diagram taken from the Si106x/108x datasheet.
A highly integrated RF transceiver IC is a cost-effective and relatively painless way to introduce wireless communication into electronic systems. Numerous devices are available; if you’re feeling a bit overwhelmed by the multitude of options, a distributor website with good search functionality is a convenient way to find parts that are appropriate for a given application.