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Four Channel wireless system

Basic four channel wireless

Relays, The Electromechanical amplifier

How Encoders and Decoders work

Encoder and Decoder Selection Guide

Characteristics of wireless systems

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Characteristics of a wireless system.

1)

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.

Let’s 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 receiver 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 receiver 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.

2) .

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.

3)

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 transmitter’s 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.

 

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