/td>
Radiometrix

Radiometrix Ltd.
Application note 009

Page 1




By Myk Dormer - Senior RF design engineer, Radiometrix

To the non-specialist, even the simplest of radio modules can seem a strange and demanding device. It
requires ripple free power, proper mounting on a good groundplane, and the provision of a carefully
selected and located aerial. And even then, exacting interface requirements must be met, often the
datastream must be specifically coded, and bandwidth and timing restrictions worked within.

And then everything is guaranteed to work ?

Unfortunately, no. Even when good practice has been followed, and the module is being used well
within its parameters, there will occur occasions when the implemented link achieves nothing like its
calculated, or promised, range. (In rare cases it may fail to operate at all).

Sometimes, this can be due to simple bad practice. Like using an inappropriate aerial, a noisy power
supply, a badly coded datastream, or occasionally even a defective radio. These cases are relatively
easy to diagnose. But sometimes even when every guideline and rule has been followed, and the radio
modules have been re-tested by the manufacturer and found good, the link does not perform.

The problem is most likely not in the module itself at all,
but rather it is the result of an interfering signal. An
interferer can originate from outside the customers
equipment (from nearby electronic equipment, or from
other radio systems) or from within (from switch mode
power supply oscillators, and from digital clock signals).
It can be a single frequency spur, it can be the modulated
carrier of another radio, or it can be a forest of unstable,
digital noise.

A typical ISM band radio receiver can have an input sensitivity better than -115dBm (about 400nV).
An unwanted signal on channel of this order of magnitude will be undetectable on a scope, or a simple
spectrum analyser, but will still compromise the link range significantly.

So how do you go about identifying, and solving, such interference?

There are a few simple diagnostic methods that can save a great deal of heart ache and lost time:

Once youve got the suspicion that things are not all they ought to be you need to be able to reliably
reproduce your findings. A repeatable test set-up will be needed in your lab if you are to make any
meaningful conclusions.

Conducted tests (where a signal generator connected to the aerial port replaces the transmit end of the
link) are useful, but only if interference is entering the radio by radiation through the casing, or
conduction through the interface.

Listen to your radio module

Radiometrix

Hartcran House, 231 Kenton Lane, Harrow, HA3 8RP, England
29 January 2008







Application note 009
Tel: +44 (0) 20 8909 9595, Fax: +44 (0) 20 8909 2233
A 433MHz ISM band receiver
Radiometrix Ltd.
Application note 009

Page 2
Otherwise, a radiated test will be needed. Placing a large value attenuator on the transmitter output, or
replacing the transmitter aerial with a dummy load will reduce the range to manageable proportions
(or, if available, a signal generator feeding an aerial can replace the transmitter-end of the link).

Monitor the receiver audio: You can tell a lot about a link by looking at (on a scope) or better,
listening to (with a small audio amplifier and speaker) the analogue AF output of the receiver.
The output of a simple FM receiver in the absence of a signal should be white noise. Anything else is
symptomatic of problems. Warbles, tones or burbling noises should suggest either digital noise from
a cpu, or another transmitter in your vicinity, while the absence of noise (quieting) when you remove
your wanted signal indicates an un modulated interfering carrier (typically a harmonic of a stable
clock oscillator).

Monitor the channel: Use an inexpensive scanning receiver set to your operating channel frequency
(or, better, build yourself a monitor receiver from another, identical radio module and aerial. Add a
voltmeter on the signal strength output, and an audio amp and speaker on AF out. Power it from
batteries)
Check that there are no unexpected signals present on your channel.
Conduct this survey both in the lab, and at your final installation location if possible, and be
aware that external interferers are frequently intermittent.

Eliminate your own hardware: Observe the output of your receiver when your entire design is
operating, and when only the receiver itself is powered (preferably from a temporarily connected
battery).

By replacing the monitor receiver aerial with a small search coil ( 2-4 turns of 1mm diameter wire,
wound around a 5mm former), it can be used at shorter range to sniff out an interfering device.

If the signal is found to have an external origin (another radio system, or a piece of equipment over
which you have no control) then you have very few options:
Accept the reduced range, or change frequency to a clear channel.
Module manufacturers now often provide several frequency versions of even the simplest
modules, in identical pin outs, while many of the more sophisticated units are multiple-channel
by design.
Occasionally, the radio design itself will be found to have a blocked channel, when the
channel frequency falls onto a harmonic of one of the internal reference oscillator frequencies.
Your module manufacturer should be able to advise you of this, although all too few will
actually admit it.

If the interferer originates in your hardware, then there are certain modifications and tests that may
help:

Improve general decoupling and filtering: This ought to be general good practice, but ensure:
HF decoupling (47p-1n, surface mount) caps are provided adjacent to all fast logic devices.
PCB tracks carrying fast digital or high frequency analogue signals are short and direct
Good grounding practices (groundplanes or low-impedance earths) are used.
The interface connections to the radio module are clean, and add filtering networks if
necessary

Conduct an audit of all the clock sources in your design: If the harmonic of a processor clock or
other oscillator falls on the channel frequency (even if it is a very high order multiple: Ive seen
trouble from the 35th harmonic of a crystal clock!) then this is a possible culprit. Temporarily
stopping the clock should be seen to remove the interferer, and changing to a different frequency
clock (if system design permits) can be a cure (But beware: the interferer is still present: its just
moved, and if serious enough could still be a general EMC problem).

Radiometrix Ltd.
Application note 009

Page 3
Locate the circuitry inside a shielded enclosure: if the mechanical design constraints permit, locate
the circuitry inside a shielded enclosure (i.e. metal, or treated plastic) with the aerial outside.
Sometimes, the addition of internal shield cans over especially active circuits can be effective

Carefully examine external connections: Power supply or interface cables are frequently found to
radiate interference generated inside a unit, so it is good practice to limit the slew rate of external
signals, and include filtering (RC, LC or ferrite) wherever possible

If the interference originates from the hardware and no easy cure can be found, then there are some
considerations for the (inevitable) redesign.

Identify noisy sub-assemblies (displays, power supply modules). Conduct radio interference
tests of alternative units.
When re-draughting the pcb, put series stopper resistors on fast logic lines (100 ohms is
frequently sufficient to reduce radiation without corrupting the logic waveforms), include a
groundplane, and filter all connections at the board edges
'Sleep' as much of the digital hardware as possible. Modify the operation of the software so as
little as possible that could cause noise (such as updating displays, or reading e2proms)
happens during the receivers operating time window.


And, as always: test everything.


Good luck


Footnote: Range testing the radio alone.

Sometimes, it will seem that a radio link isnt quite right (range is not quite what was hoped for,
but isnt unusably poor) but no absolute cause can be identified.

In such a case it is useful to conduct a range test on the radio/aerial/environment that is independent of
the possibly interference producing customer hardware. This will give an estimate of what the
modules themselves are capable of. The easiest way to conduct this test is to use a proprietary
encode/decoder chip pair to replace the customer hardware. The coder/decoder should be operated at
approximately the same data-rate as that used in the final design, and a simple push button/flash
light interface

Suitable devices for such a test would be the Holtek H12 type coder, the Radiometrix CTR44, or its
evaluation kit: http://www.radiometrix.co.uk/products/nbek.htm
Radiometrix Ltd.
Application note 009

Page 4


Radiometrix Ltd
Hartcran House
231 Kenton Lane
Harrow,
Middlesex
HA3 8RP
ENGLAND
Tel: +44 (0) 20 8909 9595
Fax: +44 (0) 20 89