The use of wireless to connect devices such as thermostats and light switches can save on installation costs when compared to wired solutions. However, the cost of the node itself can be a significant proportion of the total and hence determine the choice of system implemented. To address the cost issue, Jennic has developed a reference design that enables manufacturers to implement the microcontroller and radio related functions of a system at under $5. The reference design provides excellent RF performance, achieving a range in excess of 1km. Its availability also ensures that product time to market is kept to a minimum by providing a ready to use and proven RF board layout.
The sub $5 price point is achieved as a result of several factors. In the RF circuit, Jennic has eliminated the antenna and RF balun (the other key component in the RF path) components by using a balanced antenna printed on the circuit board, requiring no additional components to match perfectly to the chip’s 200 ohm resistive differential RF interface. The benefits are many – by designing the antenna to interface directly to the chip, the balun losses are removed, giving an instant 1.5dB improvement in receive sensitivity and transmit power. Secondly, the design of the antenna ensures that it has significantly higher gain (around 4dBi) than the ceramic antenna (approximately 1.5dBi) it replaces – due to the design’s larger size, giving higher radiation efficiency, and the lower resistivity of the pcb metallization when compared with the sintered metal commonly used in ceramic antenna designs. The design has been tested for spurious emissions and meets the regulatory requirements of both the US and Europe.
For the crystal, Jennic has used a HC49U surface mounted package measuring approximately 12.5x3.7x4.2mm high, which costs about half the price of equivalent but miniature devices. The increased physical size of the resonating element gives higher performance - the equivalent series resistance is smaller, resulting in lower system phase noise and faster oscillator start-up times, saving on overall system power consumption.
For the PCB, by providing a two-layer reference design measuring just 49x25mm, including the antenna area, the cost of the entire application board can be kept to a minimum. This is important in many low cost products with large application boards such as lighting or remote control, where the design cannot sustain the cost of a four layer board, as used in many conventional RF reference designs. The reference design is freely available from Jennic’s support website – www.jennic.com/support.