RX Received Signal Strength Indicator (RSSI)
The LMR1 has wide range RSSI that measures the strength of an incoming signal over a range of 60dB
or more. This allows assessment of link quality and available margin and is useful when performing
range tests.
The output on pin 5b of the module has a standing DC bias of up to 0.5V with no signal, rising to 2.5V
at maximum indication (RF input levels of -40dBm and above). ∆Vmin-max is typically 2V and is
largely independent of standing bias variations. Output impedance is 40kΩ. Pin 5b can drive a 100µA
meter directly, for simple monitoring.
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
-140 -130 -120 -110 -100 -90 -80 -70 -60 -50 -40
RF level (dBm)
Figure 6: typical RSSI level with respect to received RF level at LMR1 antenna pin
Antenna requirements
Three types of integral antenna are recommended and approved for use with the module:
A) Whip
This is a wire, rod ,PCB track or combination connected directly to RF pin of the module.
Optimum total length is 410mm (1/4 wave @ 173MHz). Keep the open circuit (hot) end
well away from metal components to prevent serious de-tuning. Whips are ground plane
sensitive and will benefit from internal 1/4 wave earthed radial(s) if the product is small
and plastic cased
B) Helical
C) Loop
Wire coil, connected directly to RF pin, open circuit at other end. This antenna is very
efficient given it’s small size (20mm x 4mm dia.). The helical is a high Q antenna, trim
the wire length or expand the coil for optimum results. The helical de-tunes badly with
proximity to other conductive objects.
A loop of PCB track tuned by a fixed or variable capacitor to ground at the 'hot' end and
fed from RF pin at a point 20% from the ground end. Loops have high immunity to
proximity de-tuning.
A
whip
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***
*
B
helical
**
**
***
C
loop
*
*
**
Ultimate performance
Easy of design set-up
Size
Immunity proximity effects
*
**
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Radiometrix Ltd
LMT1/LMR1 Data Sheet
page 8
RADIOMETRIX [ RADIOMETRIX LTD ]
