Analogue is any electrical signal which has a similarity with the original quantity it represents. The properties such as amplitude, frequency, or phase of an analogue signal are therefore variable with time. Audio signals generated by a microphone are a good example for they vary in all three of these parameters in sympathy with the sound wave which originally generated them.
A pulse is a sudden burst of energy and a digital pulse implies two states: on/off, 1/0, high/low. These two states represent bits, or binary digits and the burst of energy has significance (1) when it has maximum value and it has significance (0) when it has zero amplitude. A train of pulses or bits is given meaning through a variety of coding schemes. Some of the most important terms associated with pulses are:
- Amplitude - the height of a pulse; it is a measure of the strength of the pulse.
- Width - the time the pulse remains at its full amplitude.
- Rise time - the time to go from 10% to 90% of its full amplitude.
- Fall time - the time taken to go from 90% to 10% of the amplitude.
Rise time is perhaps the single most important characteristic in high-speed digital applications, for it determines how many pulses per second are possible. When pulses are used to represent bits then speed is a bit rate or so many bits per second (bits/sec).The rising and falling of a pulse is akin to the rising and falling of a sine wave and bit rate is likened to frequency. They are related and the frequency is roughly ten times the bit rate. If the frequency of the radio wave is 100MHz then the bit rate is 10Mbits/sec.The amount of information it is possible to transfer in a digitised waveform depends on the bit rate.
The number of binary digits transmitted per second. Very high speed data requires a very high bit rate and a wider channel bandwith.
To be transmitted undistorted, a pulse must maintain its fast rise and decay times. The frequency components that contribute to the pulse shape cover an infinte bandwidth and in the limit with no harmonics transmitted. the pulse emerges as a sine rather than a square wave. The practical bandwidth is that which ensures that two adjacent pulses can be distinguished. However errors cannot be eliminated entirely, and error checking is usually put in place in the system. It only requires a single pulse representing a binary 1 to be interpereted as 0 and a binary 0 to be interpereted as a binary 1 for the code word to be read as entirely different. By the use of various error correction and coding systems, reliable communicaton can be ensured where normally it would not he possible.
A system is therefore rated by its BER (bit error rate) which is the ratio of the number of received in error to the total transmitted. Several techniques are used to detect errors, the simplest being a parity check. In this method the 7 bits in length used to express a single character leaves one bit spare in the normal 8 bit word (byte). This eighth position can then be used to insert a bit to check the parity of the word to make sure that all is well or not.
An odd parity generator adds a single logical 1 to make the number of 1's in the byte odd. An even parity generator makes the number of 1'as even. The number 78 would then be transmitted as 1 (parity) followed by 10011110 for the odd parity and as 0 (parity) followed by 10010000 for even parity. The parity bit could also follow the code instead of preceding it. At the receiving end every byte is tested for the chosen parity. A positive result in the test is a good indication that of a successful transmission. The addition of a "redundant" symbol for parity checking reduce the information flow but is outweighed by the decrease in errors.
There are three basic methods of modulating a carrier by binary pulses or digits and where keying is the making and breaking of an
- Amplitude-shift keying (ASK). The carrier is present for a digital signal and absent for a digital signal 0. This means that the
keying systems used for making and breaking a circuit must have certain bandwidth for the pulses to be transmitted accurately. This is
because anything which is done to a sine wave (the carrier) such as distorting the wave or even switching it on or off generates
harmonics. These are multiples of the fundamental frequency, decreasing in amplitude as the frequency increases. If the maximum baseband
signalling frequency is designated by f s (the maximum occurs when the pulses alternate regularly between 1 and 0 ) and the carrier on
which the pulses are impressed is f c , then the modulated carrier extends over a frequency range (f c-f s) to (f c +f s). For a baseband
signalling frequency of 10MH z modulating a carrier of 4GH z then the transmission will extend over the range 3.99 to 4.01 GH z.
In a digital system the rate of information flow is expressed as the bit rate (the number of binary digits transmitted per second) not the
frequency as we have shown.
- Frequency- shift keying.(FSK). The carrier is switched rapidly between the two frequencies which represent 1 and 0. The frequency is
lower for a 1 than 0. The bandwith required for FSK is twice that for ASK because two frequencies are changed. However it is less affected
by noise and therefore has less errors compared with ASK.
- Phase-shift keying (PSK). As the he binary signal is changed from 1 to 0 or from 0 to 1, the carrier voltage reverses its direction,
which is equivalent to a 180 degrees phase change. As with ASK it is a single carrier system and it can be shown that a PSK system
requires the narrowest bandwidth compared with ASK and FSK for the same rate of data flow. PSK also has the lowest probability of error
of all three systems. A more advanced form of phase shifting is QPSK, the Q denoting for quadrature. This handles two binary digits at
once and allocates each pair to a carrier advancement, as shown below:
- 45 degrees = binary 00.
- 135 degrees = binary 01.
- 225 degrees = binary 11.
- 315 degrees = binary 10.
In this way each state of the carrier contains two bits of information and also caters for all binary numbers. It can also be
demonstrated that QPSK requires only half the bandwith needed by a PSK system. The equuipment required is however more complex.
From basic studies carried out in the Basic Physics Division the parameters required for
three-level maser action in a Ruby crystal were determined. To avoid the complication of using a conventional copper wire water-cooled
solenoid an integral superconducting solenoid was designed. The microwave components for amplifying the signal bounced off the moon using a
12 ft parabolic microwave aerial were installed in a modified W.D. 2 ton trailer. The gas from the liquid helium refrigerant used to cool the
maser crystal and the superconducting solenoid was collected in gas balloons manufactured at SRDE.
Message switching is the routing of a message by using information contained in the message itself. The message is stored, the address read and then the message is routed accordingly. The whole process is computer controlled and is known as Store-and-Forward.
- Frequency-Division Multiple Acess (FDMA) where each transmission is allocated a specific carrier frequency within the bandwidth of the transponder and the wanted transmission occupies only a small fraction of the total bandwith. The ground terminal receiver has only to distinguish the wanted signal by its carrier frequency.
- Time-Division Multiple Access (TDMA) each ground terminal is allocated a certain time interval during which it has exclusive use of the transponder on the satellite. Corresponding time intervals are allocated to other terminals. There are no intermodulation products but some system complexity results from the need maintain accurate timing of all the transmissions.
- Code-Division Multiple access (CDMA) all transmissions are present simultaneously in the satellite transponder and each of them occupies the whole available bandwidth. Each transmission is identified by a unique code. At a receiving terminal the wanted signal is extracted by a correlation process which requires the local generation of a code which is identical to that used by the transmittting terminal. Unwanted transmissions appear as noise-like interference.
A satellite-borne transponder in "geostationary" orbit acts a common relay for a number of independent point-to-point transmission links.
Because the transponder has a non-linear power characteristic, special care has to be taken to minimise interference between the various
transmissions whilst at the same time ensuring that the transponder output power is used in the most efficient manner possible. To ensure that
a link only contains the information for a particular ground station a multiple access
system was devised.
When a live satellite link via the SRDE earth station was established it was shown that no intermodulation products were present in the
To incorporate the latest digital communication technology into the Net Radio system, SRDE embarked on a programme of work to replace the LARKSPUR range of radios. The CLANSMAN Tactical Combat Net Radio equipment was one of the new generation of front line manpack digital HF and VHF radios using pulse code modulation techniques researched and developed at SRDE. The design of military radios posed many technically conflicting problems not associated with standard commercial radios these included:
- Minimum size and weight.
- High reliability and performance.
- High standard of electromagnetic compatibility.
- Ease of operation.
- Flexibility of frequency selection.
- Mechanical and electrical ruggedness.
- Ease of maintenance and repair.
- Standardisation of accessories.
The fibre consists of a glass core surrounded by a cladding of glass of lower refractive index. It is flexible and has high tensile strength, and when provided with a covering to prevent crushing, small lightweight cables can be constructed and used in the same way as electrical cables, e.g. drawn along ducts, made up into multi-core cables etc. The loss of signal power in a cable is due to scattering and absorption within the glass core and can be influenced by the type of propagation mode in the fibre.
When a light ray enters the end of a glass fibre its mode of propagation will depend on several factors. The fibre is constructed with a cladding that has a different refractive index from the core and this difference together with the wavelength of the light and the diameter of the fibre determines the number of modes of propagation along the fibre.
In pulse-rate modulation the rate at which pulses of equal amplitude are generated varies with the changes in the modulating signal. In pulse-width modulation the width (duration) of the pulses increases (or decreases) with an increase in the amplitude of the modulating voltage. In pulse-position modulation (ppm) the position of a pulse or series of pulses with respect to a reference clock is varied in accordance with the modulating signal.
Radio performance is effected by changes in the profile of large parabolic and spherical reflectors. Deviations induced by high winds and
low elevation need to be known under working conditions. Commonly used and accepted methods utilise revolving templates and dial testing
indicators. These indicate deviations from the true profile but can only be used with the axis vertical. Variations in the profile produced
by the changing weight distribution cannot be be gauged in this manner.
A taut wire was stretched between the features to be monitored with the wire tension kept constant by means of a constant-torque spring.
The movements of a reel potentiometer measured the changes in the distance between the attachment points. The correct balance between wire
diameter and tension was chosen to produce negligible catenary in the wire. The remote reading device permitted several centimetres of variation
to be measured as the the antenna pointing was varied from zenith to horizon.
Measurements taken with the Test Laboratory included the effective radiated power of the spacecraft under various access condition, the
receiver sensitivity and transfer characteristics, the performance of the beacon transmitter and of the spacecraft power-sharing circuitry as
well as recordings of amplitude and phase jitter. The phase delay through the sapcecraft transponder was also examined.
The accuracy of the in-orbit tests was of the same order as that obtained by the spacecraft manufacturer prior to launch. The degree of
accuracy was achieved by continuous calibration and adherence to a carefully designed test protocol. The test facility was also used to:
- Measure the radiated power of remote earth terminals, using any convenient satellite as a link.
- Carry out tests on other nations spacecraft on a repayment basis.
- Support the R&D work of associated groups.
Communication satellites are geostationary and to track them a ground station does not need to have a fully steerable antenna. A fixed
reflector can be used with the aerial beam steered by movement of the the feed system used to illuminate the reflector. By eliminating
the need for a complex steerable mount for the reflector a cost reduction was obtained. Also much of the associated station equipment was
A reflector with a spherical profile was used in this way and as all principal axes through the centre of curvature are principal axes,
the beam was steered without the loss of efficiency when feed system was pivoted about that point. The reflector was fitted with a broad-band
feed system consisting of a concave sub-reflector illuminated by a corrugated "scalar" horn. The sub-reflector corrected the inability of the
spherical reflector to bring received microwave signals to a focus. A small added sub-reflector illuminated by a narrow-angle corrugated horn
via an expanded polystyrene horn "Dielguide" gave an increased gain and lower sidelobe level.
A simple multimode tracking system was devised to simplify the tracking of a conventional
satellite earth terminal which often consisted of a complex waveguide and receiver system. The method developed extracted the tracking
error signal "labelled" by passing it through a continuously variable phase shifter and then reinserted it into the main waveguide signal path.
Both tracking and communication signals were amplified in a low noise receiver.
A dish transmits a beam of radiation with the power falling off each side of the centre of the beam. The beamwidth is defined as the angle from the central axis at which the transmitted or received signal is reduced to half its power, hence the measure is known as the half-power beamwidth. As the angle is increased further from the central axis a point is reached when the power falls to zero and beyond this point the power starts to increase again at a lower level in what is known as a sidelobe.
Step-index fibres have a cladding around the core that is made out of more than one material. There is a sharp line of demarcation between the core medium and the cladding media. Graded-index fibres have a unique method of manufacture that causes the refractive indexes to decrease continuously with radial distance from the fibre axis. The effect of the graded-index is to cause light rays trying to leave the axis to increase its speed the more distant it is from the axis. The ray is also refracted back to the axis with the end result that this ray which travels a longer distance, will arrive at the end of the fibre at the same time as a ray travelling along the axis. This has the effect of decreasing the broadening of pulses applied at the input to the fibre.
Circuits were set up between SRDE and Singapore via satellite and between SRDE and Mauritius via HF radio. Error data loggers were connected
to each circuit to measure the number of digit errors detected in each minute of transmission.
The satellite circuit which had a delay of half a second was found to be extremely stable, and suffered only minor variations due to atmospheric
absorption. The HF radio path by ionospheric layers was subject to flat fading, selective fading and multipath propagation.
Time-division multiplexing (TDM) equipment was used to assemble 12 telegraph channels into a single 1200 bits-per-second data stream which
was then transmitted over a suitable communication link. Each channel was capable of working at between 50 and 110 bauds (transmission speed
of data) in either start-stop or synchronous modes. "Bit-stuffing" techniques (the addition of recognisable extra bits) were used to for
handling synchronous traffic channels so that channel rates needed not to be related either to each other or to the aggregated rate. Research
on 8-channel sampling TDM and an adaptive code-division TDM using Walsh function sequences was carried out.
When a light pulse is injected into a fibre it is propagated by many different guided modes. The light pulse is then split into many pulses
which arrive at the end of the fibre at different times (time dispersion) due to differences in the delays of the modes (modal delay distortion).
This effect tends to limit step-index fibres to relatively low bandwidth systems.
An electron has angular mometum and a charge giving it a magnetic moment which causes it to precess in a steady magnetic field with an angular frequency called the Larmor frequency, equal to the gyromagnetic constant times the magnitude of the steady magnetic field. If an alternating field is applied at this frequency then magnetic resonance conditions are obtained and energy is absorbed from the system. For an unpaired electron there are two possible energy levels in the presence of the steady field magnetic field. The lower energy state is when the magnetic moment is parallel with the magnetic field and the higher energy state is when the magnetic moment is anti-parallel with the magnetic field. The separation of these two energy levels for the single unpaired electron corresponds to 2.8 Mhz per oersted of magnetic field applied to the sample. In some paramagnetic crystals two unpaired electrons contribute to the magnetic moment and three unequally spaced energy levels are present rather than two for the single unpaired electron. Appropiate separation of the three energy levels is achieved by adjusting the value of the magnetic field. The number of electrons in the three energy levels are n1, n2, and n3 where n1 > n2 > n3 and the population n1 corresponds to the number of electrons in the ground state.
Microwave energy at a frequency corresponding to the transition n1-n3 (pumping power) is applied to equalise the populations in these two energy levels, which causes the population levels in the other two levels to be inverted ie. (n3 is now greater than n2) and when microwave energy is applied to the sample placed in a microwave cavity resonating at the frequency corresponding to this transition, stimulated emission is produced which is the maser action required for amplifying weak signals.
Tracking a satellite by a ground station requires the determination of both direction and distance away from the station. It is usually accomplished by use of a special carrier wave generated in the telemetry system of the satellite. Directional measurements are performed on this wave by the ground station using the established direction finding methods. The range is measured by phase modulating an uplink (ground to satellite) carrier by two or more frequencies. The tones are received by the satellite and on board they are detected and then used to modulate a downlink (satellite to ground) carrier. The net overall phase shift in each tone is then measured at the ground station and translated into distance.
A transmission system carrying multichannel speech as an analogue signal has to maintain a faithful reproduction of the original signal at all points along the route. This requires an extremely linear and noise free transmission path - a situation rarely encountered in a battlefield communication system.
To overcome this problem the analogue signal is converted into binary pulses or digits by digital signal processing. Over a complete channel, digital transmission has the advantage over an analogue system in that it is possible to use regenerative receivers which accepts a relatively poor signal and transmit onwards a signal identical to the original. The analogue system amplifies noise and distortions picked up on the way and after several amplification stages the analogue signal is quite different to the digital signal which is as "new".