Small, Low-Cost, Low-Power Disposable Radios—Part 2: FM, FSK, and PSK

In Part 1 of this series on small, low-cost, disposable radio designs we looked at AM technologies, from single-transistor solutions up to integrated SoCs that are well-positioned to provide reliable, one-way communication. Here in Part 2 we examine some low-cost FM solutions as well as Phase Shift Keying (PSK), Frequency Shift Keying (FSK), and simple carrier On/Off Keying (OOK). In the next installment, Part 3, we will examine single-chip radio transceivers that embed a microcontroller alongside the radio to achieve higher levels of performance. All parts, data sheets, tutorials, development kits, and training modules referenced here can be found on the DigiKey website.

On, off with FM

To save power and keep the airways clear, FM radio chips can be used as carrier on, carrier off to transmit data. Like a framed UART packet, the absence of signal is an inactive state and the presence of signal begins a start bit. Thereafter, each bit position is represented by a duration of active carrier or no carrier. This asynchronous technique can work well and can save some energy since a transmitted signal is not needed for off times and logic-zero bits.

Another benefit of FM transmission schemes is that like AM, they can be very small in size and very low cost, especially in single-transistor implementations (Figures 1a and 1b). Discrete components can be in virtually zero-power states fully turned off by a FET until enabled to oscillate. The enable signal can actually be the data line in pure digital On/Off Keying (OOK) mode.

Figures 1a (left) and 1b (right): Many simple single-transistor, low-cost narrow-band FM transmitter designs are readily available in the public domain using standard off-the-shelf parts. Fixed frequencies or tunable versions are both small, low power, and very low cost.

Also note that linear modes can let analog signals control the frequency. This is advantageous in two ways. First, sensor outputs can directly drive the modulating signal like a Voltage Controlled Oscillator (VCO) to provide analog functionality without a microcontroller, A/D converters, and analog components. This is perfect for ultra-small solutions without a micro. Timed transmission intervals can also be controlled, in this case by a very-low frequency oscillator that can turn the unit on and off for a short duration (Figure 2).

Figure 2: Sensors driving VCOs can modulate carriers directly once attenuated or amplified to take advantage of the entire dynamic range. Using a low-power timed FET power switch extends battery life.

The second benefit is that when a microcontroller is present, either it can drive the modulator with an analog signal from a D/A, or simply use two bits with resistive voltage dividers to provide Frequency Shift Keyed (FSK) communications. A single PWM or Phase Shift Keying can also be used and driven by the internals of the micro. Either a wired OR scenario can be used to start up the oscillator, or a third I/O line from the micro can enable the oscillator.

A benefit here is that the transmit frequencies can be very stable and narrow band. This means many units can be in fairly close proximity without interfering with each other and the receiving station merely “tunes in” the channel it wants to monitor. Conversely, several receivers; each tuned to a narrow and specific band could operate concurrently. This allows the receiving stations to use sharp-notch-type filtering to discriminate heavily against noise and interference, improving overall reliability.

More functionality

While single transistor and discrete designs are seemingly the smallest solution, some chips can rival these small sizes while providing added features and functions. Take, for example, the pure FM SoCs from Silicon Labs; the SI4713-B30-GM or the SI4710-B30-GM are good for 76 MHz to 108 MHz bands. While packaged in a 20-pin QFN package, the footprint is only 3 mm x 3 mm and only two external components are needed to implement the entire radio transmitter. Both use digital audio inputs and serial control so a small micro is needed to feed it with data (see the TechZone article “At Your Disposal: Throwaway Micros”).

Other benefits come with a more advanced transmitter solution. First, data can be transmitted in stereo mode. This can either provide two separate channels, or, allows the flexibility to implement a clocked-transmission scheme. Note, these operate in the standard table radio 76 MHz to 108 MHz mode and are limited to 10 microWatts of power, but will work very well for short-range, line-of-sight designs, and interoperable designs.

To save time and test feasibility, modular solutions can be an engineer’s best choice. Instead of spinning your own design and then having to characterize it and certify it, you can simply obtain an OEM module that is fully tested, characterized, and certified (often for international standards).

This is particularly useful for ISM-band FM transmitters since manufacturers are up to date with regard to regional needs and versions for specific countries. Consider, for instance, the RF Solutions QFM-TX1-433 general-purpose FM transmitter module in a very handy 6-pin surface-mountable form-factor (Figure 3).