JAMMING DETECTOR
provides a logic 0 on pin 10 when a strong signal of greater than 10µV is being received. The detector
may be set to give a delayed output by connecting an electrolytic capacitor between pin 12 (JAM TC)
and 0V. The delay is approxiamtely0.7s/µF ie a 10µF capacitor will need the jamming signal to be
present for 7s before pin 10 goes low. The delay time will be subject to the electrolytic’s tolerance so
may vary widely. For accurate/long delays it is recommended that a delay of 7s (10µF) to be used and
the jam signal be fed to a digital timer to determine the required delay.
AF OUTPUT
This output is the FM demodulator’s output after buffering and de-emphasis. Since it is taken before
the data slicer in the module, it may be used to drive external data slicers/demodulators in cases where
the internal data slicer is not suitable. This is the case where an analogue subcarrier is being employed
eg 2 tone AFSK or DTMF tones. In these cases the AF Output is used to drive the FSK/DTMF decoder
directly.
The AF Output is also a very useful test point for listening for signals or interference. The
output will drive low impedance headphones via a 10µF DC blocking capacitor for monitoring purposes.
The phones should not be left connected during normal system operation as their low impedance will
cause a certain amount of audio distortion which may upset the on board data slicer, if permanent audio
monitoring is required a Hi-Z (>1kΩ) buffer should be used to drive the headphones.
The AF Output is DC coupled to the FM demodulator thus the DC level varies with the
frequency of the incoming signal and may be used to check frequency shifts / drifts between
the transmitter and receiver.
Note - the polarity of this signal is different on different frequency versions of the module,
check the specific data sheet for polarity.
RSSI (Signal Strength) OUTPUT
This is also very useful for monitoring the performance of the radio link. It is a 0V to 3.3V signal which
increases logarithmically with increasing incoming signal strength. There is an internal 6.8kΩ resistor
is series with this output so that a 0.5mA fsd meter may be connected directly to this output for
monitoring purposes. In more sophisticated systems this signal may be fed to an A-D converter to
automatically monitor the integrity of the radio link.
Additional Reading
BS 6799
BS 4737
- British Standard for Wire-free intruder alarm systems
- British Standard for Intruder alarm systems in
buildings from British Standards Institution -
0171 629 9000
MPT1340
- DTI type approval specification for 418MHz
Telemetry from Department of Trade and Industry
0171 211 0502 or 0171 211 0505
e-mail: library@ra.gtnet.gov.uk
ARRL HANDBOOK
- Excellent radio engineering text
ARRL ANTENNA BOOK
- Practical antenna design book from Radio Society
of Great Britain - 01707 659 015 or 01707 663 279
Radiometrix Ltd, RXM-UHF Data Sheet
page 8
