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Posted by on May 27, 2005, 5:49 am
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What type of signals are able to produce RFI (Radio Frequency
Interference) on Wireless Alarm Systems and will jeopardize they're
operation?
Not all Radio Frequency (RF) transmitters are able to generate RFI,
they have to be in accordance to certain rules described hereafter.
Find here with more detail the list of Radio Frequency (RF)
transmitter sources generating RFI mentioned here above:
1. RF transmitters operating "ON" the Wireless Alarm Systems
frequency.
This is the most annoying RFI, it require only a very low power to
do so (in my REAL case, 1 milliWatt was enough);
2. RF transmitters "SATURATING" the input stage of the Wireless Alarm
Systems RF receiver
A transmitter "close" to the operating frequency may have enough
power to saturate the RF receiver input stage.
This signal overrides the normal incoming RF sensor signals.
This can happen with permanent installed RF transmitter stations or
temporary operated (CB, Radio Ham, ++) mobile or fix.
3. RF transmitters causing "INTERMODULATION"
"In gross" its the mix of two signals that result in an on
the frequency signal of the Wireless Alarm Systems due to the
non-linearity of wireless RF receiver input stage.
It can be defined as a beat tone generated by two signals where
the mixed beat part is on the Wireless Alarm operating frequency.
Realize that quite a fair amount of combinations may produce
that sort of RF signal.
4. "HARMONICS" of a RF transmitters falling in the Wireless receiver
frequency bandwidth.
All transmitters generate harmonics, those harmonics have to
be suppressed (EMC requirement) to a certain degree,
nevertheless there are still attenuated harmonics and when you
have to deal with a powerful RF transmitter they may still be
sufficient to "muzzle" the RF receiver sensor inputs of the
Wireless Alarm Systems.
5. a RF transmitter signal not directly on the frequency who is "NOT
ATTENUATED" enough by the RF "receiver bandpass filters".
The RF receiver's use already since approx. 20 years SAW filters.
Those filters attenuate the beside the frequency incoming signals,
however those signals are still coming true in an attenuated manner
but they may be of a sufficient amplitude to "interfere" the RF
receiver sensor inputs.
6. RF transmitters on the IF (intermediate frequency)
Modern wireless receivers use frequency conversion techniques
who uses and IF stage to narrow the bandwidth (easier on lower
frequencies).
One drawback with them, if the IF frequency is not attenuated
enough by the input stage, a RF transmission on that frequency will
pass and generate RFI.
As you see there is a quite some impressive number possible of RF
transmitter sources that may generate RFI on Wireless Alarm System
receivers.
The major problem being that specific RFI avoidance techniques, used
in military and by similar instances, are not affordable for wireless
alarm systems even if the FCC (or like instances) would allow this
type of avoidance techniques in wireless alarm systems.
RFI is expectedly most of the time (and lucky enough) of the
involuntary nature and not persistent but in the meanwhile the system
is deaf and when it comes to security realize how easy it is to have a
Wireless Alarm Systems disturbed... dead.
Silent key, NO ALARM, NO DETECTION.
Paul
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