Using Approved Radio Modules for remote data acquisition and control

A search on the internet will quickly bring up a load of AM and FM radio modules which can be used for remote control and data acquisition. The 433 MHz AM modules retail at around £1.50 for a Tx and Rx pair while the FM modules deliver a better range and noise immunity and seem to cost around £7 for the pair. This blog post is about getting the best range from these devices.

The majority of these devices have three pins. Power, Gnd and Data in (for Tx) or Data out (for Rx). A common assumption is that the Tx and Rx behave like a pair of wires. ie feed a string of data into the Data i/p of a Transmitter module and the same data can then be received out of the Data o/p of the Receiver module. This is not the case and the reasons for this are explained below (in brief).

Radio modules transmit analogue data and the receiver has to reconstruct this back into digital data. The receiver reconstructs an 'average' threshold and uses this to decide whether a 1 or a 0 has been received. If it receives a long series of 1s and 0s in the data then this threshold is biased high or low and will reconstruct the wrong data. For the receiver to work correctly one needs to ensure that the transmitter has an equal number of 1s and 0s. That is there is no DC bias to the data. The way this is done is to use coding such as Manchester Coding or Pulse Width Modulated Coding.

In Manchester Coding each data bit is followed by  its inverse. For example the data set 11001110 is transmitted as 1010010110101001, This ensures that the trasnmitter always sends out equal numbers of 1s and 0s, allowing the Receiver to establish a good threshold to work with.The use of this coding technique effectively halves the data rate.

In addition to ensuring a data stream with no DC bias, there are other issues that need to be taken into account. In the absence of a signal the Receiver data threshold will be indeterminate. The transmitter has to send a series of alternating 1's and 0's to condition the receiver. Following this a pre-defined synchronisation pattern so that the receiver can work out the optimim sampling point and 'lock into' the actual data.

An example of scheme that could be implemented is as follows.

A preamble of at least 16 successive pairs of 1's and 0's

An 8 bit synchronisation character

The data 'payload' encoded in Manchester format

This effectively requires 80 bits of data to be transmitted to send a 'payload' of 20 bits. But this reduction in data throuhput is more than made up by the greatly enhanced range that will result.

Another factor that can improve the performance of these modules is the use of a 1/4 wave whip antennae. For a system operating at 433 MHz, the length of this antennae is calculated by the equation 300/(4*Freq in MHz). So for 433Mhz one gets a 1/4 wave length of 17.3 cm. As well as a length of wire of this length soldered to the Tx module one has to make sure that there is a ground plane for this antennae to work against.

In summary, this is a brief insight into using pre-approved RF modules in remote control systems. The book "An Introduction to Low Power Radio" by Peter Birnie and John Fairall is an excellent introduction into this subject for someone new to this field.