The transmitter portion of a wireless remote control system usually
consists of an encoder that automatically generates serial data that contains both address
bits and data bits and an RF transmitter module that sends the serial data by wireless as
described on our "How encoders and decoders
work" web page. The receiver portion consists of an RF receiver module and a
decoder that deserializes the received data, checks to see if the received address bits
match its own bit settings and sends valid data bits to output
terminals or drivers. Address bits are
used to give an identity to transmitters and receivers so that only those with identical
address settings can process data.
Lets assume, for example, that we have two
transmitter-receiver pairs, A and B, each with their own unique address settings and
operating at the same RF frequency. If we initiate a transmission from transmitter A by
applying an appropriate input level to the encoder, the RF signal will be
received by the receiver modules in both receivers A and B but the transmitted address
bits will only match those of receiver decoder A and data will only appear at the outputs of
A. The same signal will be received by the RF module in receiver B but will be rejected by
the decoder in B because the addresses do not match. No data will be transferred to
receiver B outputs. A signal from transmitter B will be accepted by the decoder in
B and be rejected by the decoder in receiver A.
The above example can be expanded for more complex control
functions. One transmitter can control many receivers if all receivers are set for the
same address as the transmitter. Also many transmitters can control one receiver if all transmitters are set
for the same address as the receiver.
However, we cannot transmit from more than one transmitter at a
time if they are both within range of a receiver regardless of the address
settings in the transmitters and receiver. If we do, the data from both transmitters will
mix together and we will have a data collision. The resulting data will be corrupted
and will be rejected by the receiver decoder. Multiple transmissions are only
allowed when each receiver is within range of only one transmitter. Federal
Communications Commission rules limit power output of these unlicensed transmitters so
that the maximum range is about 1000 feet.
The above applies to transmitters and receivers operating at the
same frequency. But in the case of relatively inexpensive modules having limited
selectivity that are designed for wireless remote control applications, it may also apply
to modules operating within a few hundred MHz of each other.
A high priced double conversion communications receiver is capable
of receiving a signal only a few KHz away from an unwanted one without interference. But a
receiver designed for remote control must be very small, run on low power and be
inexpensive. Many receivers in devices such as garage door openers and wireless doorbells
use a superregenerative circuit for lowest possible cost. This type of receiver has high
sensitivity but very poor selectivity. Our Glolab receiver modules
use a superhetrodyne circuit with a Surface Acoustic Wave (SAW) filter for superior
selectivity and stability, still, the selectivity of these modules cannot compare with
that of an expensive receiver and they cannot reject a strong signal even many MHz away
from their design center.
Radio frequency energy at high frequencies can be reflected by
buildings, metal objects and by other structures and objects. When a transmitter sends a
signal, both the direct signal from the transmitting antenna and a reflection of that
signal can arrive at a receiver. If a reflected signal arrives at a receiver 180 degrees
out of phase with the direct signal from a transmitters antenna, the reflected
signal will partially cancel the direct signal. It will not completely cancel the direct
signal since a reflected signal is weaker but the result is that the received signal will
be weaker than if no 180 degree reflection was received.
The wavelength of an RF signal at 418 MHz is about 27 inches so
moving the position of either the transmitter or the receiver by a few inches can affect
the phase of a reflected signal and therefore the strength of the received signal. The
range of a remote control system is usually specified under ideal conditions, which are
line-of-sight and outdoors with no reflecting objects within range.