BIM-418-10 Radiometrix, BIM-418-10 Datasheet - Page 15

• Manufacturer Part Number: BIM-418-10
• Description: Low Power UHF Data Transceiver Module
• Manufacturer: Radiometrix

In many applications the receiver need not be always waiting for a signal (i.e. drawing 15mA). Often it
is only required to turn the RX on after a transmission to receive handshake data , thereafter it may be
deselected (i.e. <1µA leakage current).
In applications where a receiver needs to respond to a call, duty cycle power saving is very effective. For
example selecting the receiver 3 times a second for 1ms and sampling the CD output for the presence of
a signal will give an average current drain of < 50µA. In this example a 700ms preamble "wake up"
would be used.
Interface logic
The logic control / data lines in and out of the BiM all have 10kΩ series EMC isolation resistors internal
to the BiM (see BiM block diagram). We recommend that RXD and CD outputs be used only to drive
CMOS logic inputs and no more than 5 cm of PCB track. Care should also be taken in the routing of the
RXD , TXD , CD & AF tracking to minimise the cross talk between these high impedance lines. In some
applications it is desirable to mute the continuos noise output on the RXD line when no signal is
present, simple CMOS logic gating with the CD signal may be desirable.
There is a dc path of 20 kΩ from the TXD input to the internal switched TX supply.(see block diagram),
it is desirable to hold TXD low whilst TX select is high (i.e. when not transmitting data).
The CD output is designed to be fast acting (< 1 ms), and can under conditions of weak signal or
interference exhibit fast spurious pulses. It can be beneficial to drive a Schmitt trigger CMOS gate with
this output and to include an additional R-C time constant between the CD output and the Schmitt
input gate. The R should be 100 kΩ or greater and the additional time constant delay must be allowed
for in the control software (i.e. preamble times etc.).
Signal Propagation
Three predominant effects are observed in the propagation of short range VHF / UHF signals in and
around obstacles :-
1. Signal reflection:
2. Signal shadowing: This occurs behind large sheets of metal e.g. trucks, foil backed
3. Signal absorption: Principally observed when signals pass through thick damp stone
Radiometrix Ltd, BiM Data Sheet
This gives rise to multiple paths between the transmitter
and the receiver. Since these paths will be of different lengths, the
arriving signals will have differing phases and strengths leading
to signal cancellation at specific points in space. i.e. null points are
observed. These nulls are physically small i.e. moving either the
transmitter or receiver a few centimetres will be enough to take
the signal out of the null. They are more frequent in situations of
weak signal and where lots of large metal items are present, they
are totally absent in open ground situations.
plasterboard , steel reinforced floors etc. In such areas, signals are
received predominantly by reflection from other objects. The
shadow areas are of similar dimensions to the shielding object and
show as areas of weaker average signal level with
an increased occurrence of nulls due to multi path (see 1. above).
walls, the effects are similar to 2. above but there is less reflected
signal.
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