Radiometrix Ltd, BIM2/3G Data Sheet

page 7

Application Information

Modulation formats and range extension

The module will produce the specified FM deviation with a 2-level digital input to TXD which toggles between

0V and 5V. Reducing the amplitude of the data input from this value reduces the transmitted FM deviation,

typically to ±20-22kHz minimum at 0 - 2.8V. The receiver will cope with this quite happily and no significant

degradation of link performance should be observed.

TXD is normally driven directly by logic levels but will also accept analogue drive, e.g. 2-tone signalling. In this

case it is recommended that TXD (pin 14) should be DC-biased to 1.5V with the modulation AC-coupled and

limited to a maximum of 3V peak-to-peak. The instantaneous modulation voltage must not swing below 0V or

above 3V at any time if waveform distortion and excessive FM deviation is to be avoided – use a resistive

potential divider and/or level shifter to accomplish this if necessary. The varactor modulator in the transmitter

introduces some 2nd harmonic distortion which may be reduced if necessary by predistortion of the analogue

waveform.

At the other end of the link the AF output (pin 13) can be used to drive an external decoder directly.

Although the module baseband response extends down to DC, data formats containing a DC component are

unsuitable and should not be used. This is because frequency errors and drifts between the transmitter and

receiver occur in normal operation resulting in DC offset errors on the AF output.

The time constant of the adaptive data slicer in the BiM2/3G is set at a reasonable compromise to allow the use

of low code speeds where necessary whilst keeping settling times acceptably fast for battery-economised

operation. RXD output on pin 12 is “true” sense, i.e. as originally fed to the transmitter.

Antenna considerations and options

The choice and positioning of transmitter and receiver antennas is of the utmost importance and is the single

most significant factor in determining system range. The following notes are intended to assist the user in

choosing the most effective arrangement for a given application.

Nearby conducting objects such as a PCB or battery can cause detuning or screening of the antenna which

severely reduces efficiency. Ideally the antenna should stick out from the top of the product and be entirely in

the clear, however this is often not desirable for practical or ergonomic reasons and a compromise may need to

be reached. If an internal antenna must be used, try to keep it away from other metal components and pay

particular attention to the “hot” end (i.e. the far end), as this is generally the most susceptible to detuning. The

space around the antenna is as important as the antenna itself.

Microprocessors and microcontrollers tend to radiate significant amounts of radio frequency hash, which can

cause desensitisation of the receiver if its antenna is in close proximity. 433MHz is generally less prone to this

effect than lower frequencies, but problems can still arise. Things become worse as logic speeds increase,

because fast logic edges are capable of generating harmonics across the UHF range which are then radiated

effectively by the PCB tracking. In extreme cases system range can be reduced by a factor of 3 or more. To

minimise any adverse effects, situate the antenna and module as far as possible from any such circuitry and

keep PCB track lengths to the minimum possible. A ground plane can be highly effective in cutting radiated

interference and its use is strongly recommended.

A simple test for interference is to monitor the receiver RSSI output voltage, which should be the same

regardless of whether the microcontroller or other logic circuitry is running or in reset.