8 Signal transmission

 8.4 Optical wireless telemetry

Wireless telemetry allows signal transmission to take place without laying down a physical link in the form of electrical or fibre-optic cable. This can be achieved using either radio or light waves to carry the transmitted signal across a plain air path between a transmitter and a receiver.

Optical wireless transmission was first developed in the early 1980s. It consists of a light source (usually infrared) transmitting encoded data information across an open, unprotected air path to a light detector. Three distinct modes of optical telemetry are possible, known as point-to-point, directed and diffuse:

Point-to-point telemetry uses a narrowly focused, fine beam of light, which is commonly used for transmission between adjacent buildings. A data transmission speed of 5 Mbit/s is possible at the maximum transmission distance of 1000 m. However, if the transmission distance is limited to 200 m, a transmission speed of 20 Mbit/s is possible. Point-to-point telemetry is commonly used to connect electrical or fibre-optic ethernet networks in adjacent buildings.

Directed telemetry transmits a slightly divergent beam of light that is directed towards reflective surfaces, such as the walls and ceilings in a room. This produces a wide area of coverage and means that the transmitted signal can be received at a number of points. However, the maximum transmission rate possible is only 1 Mbit/s at the maximum transmission distance of 70 m. If the transmission distance is limited to 20 m, a transmission speed of 10 Mbit/s is possible.

Diffuse telemetry is similar to directed telemetry but the beam is even more divergent. This increases the area of coverage but reduces transmission speed and range. At the maximum range of 20 m, the maximum speed of transmission is 500 kbit/s, though this increases to 2 Mbit/s at a reduced range of 10 m.

In practice, implementations of optical wireless telemetry are relatively uncommon. Where optical transmission is favoured because of its immunity to electromagnetic noise, fibre-optic transmission is usually preferred since optical wireless transmission is susceptible to random interruption when data is transmitted across an open, unprotected air path. This preference for fibre-optic transmission exists despite its much greater cost than optical wireless transmission. Similarly, when the difficulty of laying a physical cable link determines that wireless transmission is used, it is normal to use radio rather than optical transmission. This preference arises because radio transmission is much less prone to interference than optical transmission, since radio waves can pass through most materials. However, there are a few instances where radio transmission is subject to interference from neighbouring radio frequency systems operating at a similar wavelength and, in such circumstances, optical transmission is sometimes a better option.

 

8.5 Radio telemetry (radio wireless transmission)

Radio telemetry is normally used over transmission distances up to 400 miles, though this can be extended by special techniques to provide communication through space over millions of miles. However, radio telemetry is also commonly used over quite short distances to transmit signals where physical electrical or fibre-optic links are difficult to install or maintain. This occurs particularly when the source of the signals is mobile. The great advantage that radio telemetry has over optical wireless transmission through an air medium is that radio waves are attenuated much less by obstacles between the energy transmitter and receiver. Hence, as noted above, radio telemetry usually performs better than optical wireless telemetry and is therefore used much more commonly.

In radio telemetry, data are usually transmitted in a frequency modulated (FM) format according to the scheme shown in Figure 8.9. This scheme actually involves two separate stages of frequency modulation, and the system is consequently known

as an FM/FM system. Eighteen data channels are provided over the frequency range from 0.4 kHz to 70 kHz, as given in Table 8.1. Each channel is known as a subcarrier frequency and can be used to transmit data for a different physical variable. Thus, the system can transmit information on 18 different variables simultaneously.

A voltage-to-frequency converter is used in the first FM stage to convert each analogue voltage signal into a varying frequency around the centre frequency of the subcarrier assigned for that channel. The 18 channels are then mixed into a single signal spanning the frequency range 0.4 kHz to 70 kHz. For transmission, the length of the antenna has to be one-quarter or one-half of the wavelength. At 10 kHz, which is a typical subcarrier frequency in an 18-channel system, the wavelength is 30 km. Hence, an antenna for transmission at this frequency is totally impractical. In consequence, a second FM stage is used to translate the 0.4 kHz to 70 kHz signal into the radio frequency range as modulations on a typical carrier frequency of 217.5 MHz.Ł At this

frequency, the wavelength is 1.38 m, and so a transmission antenna of length 0.69 m or 0.345 m would be suitable. The signal is received by an antenna of identical length some distance away. A frequency divider is then used to convert the signal back to one across the 0.4 kHz to 70 kHz subcarrier frequency spectrum, following which a series of band-pass filters are applied to extract the 18 separate frequency bands containing the measurement data. Finally, a demodulator is applied to each channel to return each signal into varying voltage form.

The inaccuracy of radio telemetry is typically ±1%. Thus, measurement uncertainty in transmitting a temperature measurement signal with a range of 0–100°C over one channel would be ±1%, i.e. ±1°C. However, if there are unused transmission channels available, the signal could be divided into two ranges (0–50°C and 50–100°C) and transmitted over two channels, reducing the measurement uncertainty to ±0.5°C. By using ten channels for one variable, a maximum measurement uncertainty of ±0.1°C could be achieved.

In theory, radio telemetry is very reliable because, although the radio frequency waveband is relatively crowded, specific frequencies within it are allocated to specific usages under national agreements that are normally backed by legislation. Interference is avoided by licensing each frequency to only one user in a particular area, and limiting the transmission range through limits on the power level of transmitted signals, such that there is no interference to other licensed users of the same frequency in other areas. Unfortunately, interference can still occur in practice, due both to adverse atmospheric conditions extending the transmission range beyond that expected into adjoining areas, and also due to unauthorized transmissions by other parties at the wavelengths licensed to registered users. There is a legal solution to this latter problem, although some time may elapse before the offending transmission is successfully stopped.

 

8.6 Digital transmission protocols

Digital transmission has very significant advantages compared with analogue transmission because the possibility of signal corruption during transmission is greatly reduced. Many different protocols exist for digital signal transmission, and these are considered in detail in Chapter 10. However, the protocol that is normally used for the transmission of data from a measurement sensor or circuit is asynchronous serial transmission, with other forms of transmission being reserved for use in instrumentation and computer networks. Asynchronous transmission involves converting an analogue voltage signal into a binary equivalent, using an analogue-to-digital converter as discussed in section 6.4.3. This is then transmitted as a sequence of voltage pulses of equal width that represent binary ‘1’ and ‘0’ digits. Commonly, a voltage level of C6 V is used to represent binary ‘1’ and zero volts represents binary ‘0’. Thus, the transmitted signal takes the form of a sequence of 6 V pulses separated by zero volt pulses. This is often known by the name of pulse code modulation. Such transmission in digital format provides very high immunity to noise because noise is typically much smaller than the amplitude of a pulse representing binary 1. At the receiving end of a transmitted signal, any pulse level between 0 and 3 volts can be interpreted as a binary ‘0’ and anything greater than 3 V can be interpreted as a binary ‘1’. A further advantage of digital transmission is that other information, such as about plant status,

can be conveyed as well as parameter values. However, consideration must be given to the potential problems of aliasing and quantization, as discussed in section 6.4.3, and the sampling frequency must therefore be chosen carefully.

Many different mediums can be used to transmit digital signals. Electrical cable, in the form of a twisted pair or coaxial cable, is commonly used as the transmission path. However, in some industrial environments, the noise levels are so high that even digital data becomes corrupted when transmitted as electrical pulses. In such cases, alternative transmission mechanisms have to be used.

One alternative is to modulate the pulses onto a high-frequency carrier, with positive and zero pulses being represented as two distinct frequencies either side of a centre carrier frequency. Once in such a frequency modulated format, a normal mains electricity supply cable operating at mains frequency is often used to carry the data signal. The large frequency difference between the signal carrier and the mains frequency prevents any corruption of the data transmitted, and simple filtering and demodulation is able to extract the measurement signal after transmission. The public switched telephone network can also be used to transmit frequency modulated data at speeds up to 1200 bits/s, using acoustic couplers as shown in Figure 8.10. The transmitting coupler converts each binary ‘1’ into a tone at 1.4 kHz and each binary ‘0’ into a tone at 2.1 kHz, whilst the receiving coupler converts the tones back into binary digits.

Another solution is to apply the signal to a digital-to-current converter unit and then use current loop transmission, with 4 mA representing binary ‘0’ and 20 mA representing binary ‘1’. This permits baud rates up to 9600 bit/s at transmission distances up to 3 km. Fibre-optic links and radio telemetry are also widely used to transmit digital data.