Description

MRF49XA by Microchip is a highly integrated RF transceiverthat can be used in the 433, 868 and 915 MHz frequency bands. FSK modulation is internally used by this module. It is mainly used to interface to a PIC microcontroller. A transceiver is a device that can both transmit and receive. Thus, the word ‘transceiver’.

A system that can send and receive data at the same time is called a full duplex system. On the other hand, a system that can only send or receive at a time is called a half-duplex system. Thus, half-duplex systems use only one frequency carrier and the two ends share the same frequency. Full-duplex systems use two carrier frequencies, known as uplink frequency and downlink frequency. This document discusses what is required to successfully develop a half-duplex radio application using the Microchip Technology MRF49XA transceiver. For more information on this transceiver, please refer to the MRF49XA data sheet (DS70590).

Theory of FSK

The most common radio modulation used in Remote Keyless Entry (RKE) systems is the Amplitude Shift Keying (ASK). Data is transmitted by varying the amplitude of a fixed-frequency carrier. When data is encoded as maximum amplitude for a ‘1’ or mark, and zero amplitude – the power amplifier (PA) is switched off – for a ‘0’ or space, this type of modulation is also named On-Off Keying, or OOK. This modulation format allows very simple and low-cost transmitter designs. Another type of modulation is Frequency Shift Keying (FSK). This is done by shifting the carrier’s frequency on either side of an average (or carrier) frequency.

The amount by which the carrier shifts on either side of the carrier’s frequency is known as deviation. The FSK modulation has several advantages over the ASK modulation. While the AM modulation is very sensitive to variations of amplitude and noise, the FSK encoded transmissions are more immune to signal attenuation or other amplitude-based disturbance.

Although the apparent bandwidth is from f0 – Δf to f0 + Δf, in reality, the bandwidth spreads larger than the span between f0 – Δf to f0 + Δf, because the speed of transition between the two frequencies generates additional spectral content.
In short, think of FSK modulation as a more reliable transmission medium having much less noise. In order to achieve a successful design, you will need a deeper understanding of the requirements of an FSK modulated radio link.

Conclusion

MRF49XA is a highly integrated RF transceiver. It requires only a few external components and can be controlled via an SPI interface.

Applications

The MRF49XA is ideal for low-power, short-range radio communications, where the host system is a microcontroller, such as Microchip’s PIC® microcontrollers.

MRF49XA Transceiver