Transmitting by Radio

One of the problems with long wires is that the resistance of the wire can cause quite a large voltage drop, so the aircraft's computer will not receive the data properly, and therefore will not respond to them. 10 000 metres up this could be catastrophic. An in a big brute like the Antonov 225, this could be a big problem.

The way out is to combine the data with a carrier wave. This is done in the same way as broadcasts are combined with a carrier wave before transmission from a transmitter.

In Amplitude Modulation (AM) transmission, the signal wave and the carrier wave are superposed as shown:

The amplitude varies. At the other end the signal wave is removed by a process of demodulation.

The process is relatively easy (hence cheap) but can be prone to noise. AM radio is particularly prone to interference by electric motors, or by waves travelling by different paths to the receiver.

Frequency modulation uses variations in the frequency of the carrier wave, as shown in the picture below:

If the signal voltage is raised, the frequency is raised and vice versa. The carrier wave is still removed by the receiver (demodulation) to recover the information signal. The advantage of FM over AM is that the signal is less prone to noise.

Both of these methods use analogue signals (signals that vary continuously). With analogue signals, there can be problems if the signal has been changed. There might be distortion making a broadcast unpleasant to listen to. In an aeroplane, it could result in a computer interpreting data incorrectly.

Data are more reliably transmitted if they are converted into digital form (pulses that are ON or OFF).

Pulse Code Modulation (PCM)

The process of pulse code modulation involves an analogue signal being translated into a series of binary numbers. This in turn is represented by a series of ON and OFF pulses (1 and 0). The advantages of this are:

• As long as the voltage in the wire is above a "cut-off" value, the signal is interpreted as a 1.

• The computer can read the data directly with out having to turn them into digital form.

• Checks can be built into the number stream.

The picture shows the idea. The voltage is sampled at regular intervals to give an output shown by the blue lines. There are a couple of problems:

• The sampling may not faithfully reflect the shape of the wave. Increasing the sampling rate reduces this problem.

• Spurious frequencies may arise. This is called aliasing. This again is reduced by increasing the sampling rate.

Sampling should be done at at least twice the frequency of the highest frequency.

Quantisation

In coding we need to assign a value to each sample. However, binary numbers are whole numbers; we cannot do fractions in binary. Each voltage is assigned a binary whole number value. This is called quantisation. The values are discrete, not continuous.

If you look carefully at the arrows on the picture above, you will see that some of the lines have not reached the value of the analogue signal. They are slightly above or slightly below. This problem can be reduced by increasing the number of levels available. In 4-bit (bit = binary digit) code, there are 8 levels. 8-bit codes give 256 levels. Many computers use 32-bit or even 64-bit codes.

The picture shows how samples are quantised into 8 levels:

In binary we can't have negative numbers. So we put the zero line half way up the quantum levels, at level 0100 (4).

We can now turn these samples into a series of binary numbers.

And then turn them into pulses:

Low amplitude signals are particularly prone to distortion as the "rounding factor" is a greater proportion of their value. We get around this by to have more steps in the lower voltage ranges. The signals are compressed.

When they are decoded the signal are expanded. The whole process is described by the ghastly word companding.

Conversion

We see, hear and sense with analogue signals. The position of the control surfaces in an aeroplane is a continuous variable. It would be highly undesirable to have them in discrete steps, otherwise the plane will be climbing or descending, with a very brassed-off pilot. So we need a way of converting analogue signals to digital and back again.

Analogue to Digital

Analogue signals are converted to digital signals with an analogue to digital converter (A-D converter). A simple way of doing this is by generating a series of voltage steps (a ramp voltage) and comparing the analogue voltage with this with a circuit called a comparator.

Digital to Analogue

An operational amplifier (op-amp) is used to convert digital signals back to analogue. The op-amp is not a very spectacular component, but is very useful. It can be used in a number of configurations. The op-amp has two inputs:

• Inverting input (marked "-");

• Non-inverting input (marked "+").

It has a single output. It amplifies the difference between the two inputs.

The ideal op-amp has:

• Infinite input impedance;

• Infinite gain (amount by which it amplifies).

It is most commonly used in a configuration called an inverting amplifier. The basic circuit is shown below:

In this case the input is fed into the inverting input. A couple of key points:

1. Since the non-inverting input is connected to the zero volt line, and the voltage difference between the two input is very small indeed, we say that the inverting input is at zero volts. We call it a virtual earth.

2. Since the input resistance of the non-inverting input is extremely high, the current going in is negligible. So we can say that all the current flows from Rin to Rf.

The current will be the same through Rin and Rf. Therefore:

• Current through R_in, I = V_in/R_in

• Current through R_f, I = -V_out/R_f

The minus sign tells us that the voltage is climbing up from the zero point. We can therefore say:

Therefore the voltage ratio is equal to the ratio of the resistances.

If we add a second parallel resistor to the inverting input, we can get the sum of the two voltages:

Since:

Itot = I1 + I2

we can use Ohm's Law to write:

-Vout = V1 + V2

Rf R1 R2

If the values of the input resistors are the same, we get the sum of the two voltages (with a negative sign). This is called a summing amplifier.

For digital to analogue conversion, we need to use a number of parallel input resistors which are in the ratio 1: 2: 4: 8: 16 etc.

Multiplexing

In aeroplanes the saving of weight is critical. If we can have one wire feeding several sensors, that saves a lot of weight. We use a technique called multiplexing to splice the output of each sensor before it is sent down the wire to the computer.

In time division multiplexing (TDM) the data from each sensor are split up into equal sections of time. They are then sent down the wire to a demultiplexer, where they are reassembled , giving as many outputs as there were inputs. TDM is often used in trains where a single wire carries the control signals from the remote driving cab to the locomotive.

Frequency division multiplexing (FDM) adds the data to a carrier wave of a certain frequency. A number of carrier waves are sent down the wire. At the other end the correct signals are filtered out using electronic filters. The data are demodulated, before being fed to the control circuitry.

Digital Sensors

More recent developments include digital position sensors. A Perspex strip is set between a range of several light emitting diodes(LED) and corresponding photodiode. There are clear and opaque regions on the strip. Light gets through the clear strip to turn on the photodiode, giving a 1. The dark regions prevent the light getting through, so the photodiodes give out a 0.