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
extremely poor (ripple/noise content >0.1Vp-p).
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
the supply rail should be below 10mVp-p to avoid problems. If the quality of the supply is in
doubt, it is recommended that a 10µF low-ESR tantalum or similar capacitor be added between
the module supply pin (Vcc) and ground, together with a 10Ω 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 (~50kΩ) 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):
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
-130 -120 -110 -100 -90 -80 -70 -60 -50
RF level (dBm)
Figure 4: Typical RSSI response
Radiometrix Ltd, RX2A Data Sheet
page 4
