Radiometrix Ltd, RX2A Data Sheet

page 4

Power supply requirements

The RX2A incorporates a built-in regulator which delivers a constant 2.8V to the module

circuitry when the external supply voltage is 2.85V or greater, with 40dB or more of supply

ripple rejection. This ensures constant performance up to the maximum permitted supply rail

and removes the need for external supply decoupling except in cases where the supply rail is

Note, however, that for supply voltages lower than 2.85V the regulator is effectively inoperative

and supply ripple rejection is considerably reduced. Under these conditions the ripple/noise on

doubt, it is recommended that a 10

µF low-ESR tantalum or similar capacitor be added between

the module supply pin (Vcc) and ground, together with a 10

Ω series feed resistor between the

Vcc pin and the supply rail.

Received Signal Strength Indicator (RSSI)

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

over a range of approximately 60dB. 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 3 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. The RSSI output source

impedance is high (~50k

Ω) and external loading should therefore be kept to a minimum.

To ensure a fast response the RSSI has limited internal decoupling of 1nF to ground. This may

result in a small amount of ripple on the DC output at pin 3 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 3 to ground. For example, adding 10nF here will increase RSSI response time from

100µs to around 1ms. The value of this capacitor may be increased without limit.

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

guarantee of actual RSSI characteristics):

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

-130 -120 -110 -100 -90

-80

-70

-60

-50

RF level (dBm)

Figure 4: Typical RSSI response