Radiometrix Ltd

USX2 Data Sheet

page 7

RX Received Signal Strength Indicator (RSSI)

The USX2 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 9 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 4.7kΩ. The pin has a 1nF cap to ground.

Digital RSSI output

In serial mode, RSSI can be read back as a digital value on pin P3. The RSSI read back command initiates a

read of the analogue RSSI value (as present on pin 9) by an internal 8 bit A/D converter.

This converter uses the 3.0v rail as it's reference (so an RSSI voltage of 3v will give a read back of 255,

while a voltage of 0v will read back 00).

Practically, the RSSI circuit produces about 0.5 - 2.5v (read back value 40 - 212) and this voltage is related

(approximately) to the receiver input level by the graph below.

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 4: typical RSSI level with respect to received RF level at USX2 antenna pin

The table below relate the receiver input level to the typical analogue voltage output at pin 9 and the

corresponding digital RSSI value read back at pin 3 (in serial mode).

RF power (dBm)

Analogue RSSI (V) at pin 9

Digital RSSI value (on pin 3)

No signal

0.45

36 (24h)

-120

0.65

53 (35h)

-110

0.85

75 (4Bh)

-100

1.2

101 (65h)

-90

1.4

123 (7Bh)

-80

1.65

141 (8Dh)

-70

1.95

166 (A6h)

-60

2.2

187 (BBh)

-50

2.3

198 (C6h)

-40

2.35

200 (C8h)

Please note, the RSSI graph and the table values above are for indicative purposes only and is not a

guarantee of actual RSSI characteristics.