Radiometrix Ltd., TX1 & RX1 Data Sheet

page 8

RX1 Received Signal Strength Indicator (RSSI)

The RX1 receiver incorporates a wide range RSSI which measures the strength of an incoming signal

over a range of 80dB or more. This allows assessment of link quality and available margin and is useful

when performing range tests.

Please note that the actual RSSI voltage at any given RF input level varies somewhat between units.

The RSSI facility is intended as a relative indicator only - it is not designed to be, or suitable as, an

accurate and repeatable measure of absolute signal level or transmitter-receiver distance.

The output on pin 5 of the module has a standing DC bias of 0.15V-0.45V (0.25V typ.) with no signal,

rising to 0.9-1.3V (1.15V typ.) at maximum indication. Output impedance is 10k

Ω. Pin 5 can drive a

100

µA meter directly, for simple monitoring.

Typical RSSI characteristic is shown below (this is for indicative purposes only and is not a guarantee

of actual RSSI characteristics):

0

0.2

0.4

0.6

0.8

1

1.2

RF Input Level (dBm)

Fig.8: Typical RSSI response curve

To ensure a reasonably fast response the RSSI line has limited internal decoupling of 11nF to ground.

This results in a small amount of audio ripple on the DC output at pin 5 of the module. If this is a

problem further decoupling may be added at the expense of response speed, in the form of a capacitor

from pin 5 to ground. For example, adding an extra 0.1

µF on this pin will increase the RSSI response

time to around 4ms.

Expected range

Predicting the range obtainable in any given situation is notoriously difficult since there are many

factors involved. The main ones to consider are as follows:

•

Type and location of antennas in use (see pages 10-12)

•

Type of terrain and degree of obstruction of the link path

•

Sources of interference affecting the receiver

•

“Dead” spots caused by signal reflections from nearby conductive objects

•

Data rate and degree of filtering employed (see page 9)

The following are typical examples – but range tests should always be performed before assuming that

a particular range can be achieved in a given situation:

Data rate

Tx antenna

Rx antenna

Environment

Range

1.2kbps

half-wave

half-wave

rural/open

10-15km

10kbps

half-wave

half-wave

rural/open

3-4km

10kbps

helical

half-wave

urban/obstructed

500m-1km

10kbps

helical

helical

in-building

100-200m

Note: The figure for 1.2kbps assumes that the receiver bandwidth has been suitably reduced by

utilising an outboard audio filter/data slicer or similar arrangement.

If the RX1 is used “as is” the range will be similar to that for 10kb/s.