In applications such as longer range fixed links where data speed is not of primary importance, a
significant increase in range can be obtained by using the slowest possible data rate together with
filtering to reduce the receiver bandwidth to the minimum necessary. In these circumstances the AF
output can be used to drive an external filter and data slicer. The AF output waveform on pin 13 is in
the same sense as that originally fed to the transmitter, i.e. no inversion takes place.
Received Signal Strength Indicator (RSSI)
The BiM3B receiver 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 11 of the module has a standing DC bias in the region of 0.25V with no signal, rising
to around 1.15V at maximum indication (RF input levels of -50dBm and above). For any given RF input
level, absolute RSSI voltage is likely to vary somewhat between individual units - please refer to
specifications table on p4.
The RSSI output source impedance is high (~50kΩ) and external loading should therefore be kept to a
minimum.
Typical RSSI characteristic is shown below (this is for indicative purposes only and is not a guarantee of
actual RSSI characteristics):
Fig.4: Typical RSSI response curve
To ensure a reasonably fast response the RSSI has limited internal decoupling of 1nF to ground. This
may result in a small amount of audio ripple on the DC output at pin 11 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 11 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.
Radiometrix Ltd, BiM3B Data Sheet
page 6
RADIOMETRIX [ RADIOMETRIX LTD ]
