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 10kΩ. Pin 5 can drive a
100µA 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):
RSSI Output Voltage (V)
1.2
1
0.8
0.6
0.4
0.2
0
-6
0
-1
25
-1
20
-1
15
-1
10
-1
05
-1
00
-9
5
-7
0
-6
5
-5
5
-7
5
-5
0
-4
5
-8
0
-9
0
-4
0
-8
5
-3
5
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 0.1µF 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
1.2kbps
10kbps
10kbps
10kbps
Tx antenna
half-wave
half-wave
helical
helical
Rx antenna
half-wave
half-wave
half-wave
helical
Environment
rural/open
rural/open
urban/obstructed
in-building
Range
10-15km
3-4km
500m-1km
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.
Radiometrix Ltd., TX1 & RX1 Data Sheet
page 8
RADIOMETRIX [ RADIOMETRIX LTD ]
