Radiometrix Ltd, BiM2A Data Sheet

page 6

The time constant of the adaptive data slicer in the BiM2A 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.

In applications such as longer range fixed links where data speed is not of primary importance, a

significant increase in range can be obtained by using the slowest possible data rate together with

filtering to reduce the receiver bandwidth to the minimum necessary. In these circumstances the AF

output can be used to drive an external filter and data slicer. The AF output waveform on pin 13 is in

the same sense as that originally fed to the transmitter, i.e. no inversion takes place.

Expected range

Predicting the range obtainable in any given situation is notoriously difficult since there are many

factors involved. The main ones to consider are as follows:

!

Type and location of antennas in use (see below)

!

Type of terrain and degree of obstruction of the link path

!

Sources of interference affecting the receiver

!

“Dead” spots caused by signal reflections from nearby conductive objects

!

Data rate and degree of filtering employed

Assuming the maximum 64kb/s data rate and ¼-wave whip antennas on both transmitter (@10mW) and

receiver, the following ranges may be used as a rough guide only:

Cluttered/obstructed environment, e.g. inside a building

:

50-75m

Open, relatively unobstructed environment

:

200-300m

It must be stressed, however, that range could be much greater or much less than these figures. Range

tests should always be performed before assuming that a particular range can be achieved in any given

application.

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.

Two types of antenna are recommended for use with the BiM2A:

Whip (¼-wave). This consists simply of a piece of wire or rod connected to the module at one end. The

lengths given below are from module pin to antenna tip including any interconnecting wire or tracking

(but not including any 50

Ω coax or microstrip connection). This antenna is simple, cheap, easy to set up

and performs well. It is especially effective when used with a ground plane, which in practice is often

provided by the main PCB or by a metal case.