Radiometrix Ltd

LMT1/LMR1 Data Sheet

page 8

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.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

-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

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.

C) Loop

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.

FEATURE

A

B

C

whip

helical

loop

Ultimate performance

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Easy of design set-up

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Size

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Immunity proximity effects

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