8:Transmitters

The Radar Transmitter

The Radar Transmitter:!!Generates high power RF radiation which illuminates targets and scenes of interest.!!Includes three basic elements; (i) The power amplifier, (ii) the modulator and (iii) the power supply.!!Peak powers can range from milliwatts to gigawatts.!!Frequencies can range from MHz to a hundred GHz.!!Can be continuously operated or pulsed.!!Can be a single device or many!!!

Block diagram of a typical pulsed radar system

Block diagram of a typical phased array pulsed radar system

Radar Transmitter parameters

The average power of a radar system determines the maximum detection range. The average power Pav is given by:!!

Pav = Pp. .PRF = PP.dc(watts)!!

Where !Pp = Peak Power (watts).! ! = maximum pulse duration (secs).!

!PRF = Pulse repetition frequency (Hertz).!!dc = Duty Cycle!

!Duty cycles for high power amplifiers such as Klystrons, TWTs and magnetrons typically vary between 1% and 30%.!!The power aperture product is given by:!!

Pav.Ae!!

Where Ae = the effective area of the antenna.!!The power-aperture product is a measure of the search performance of a radar system.!

Τ

Τ

Radar Transmitter parameters

!Transmitter efficiency is defined by:!!

= Pav/PDC!!Where PDC is the DC prime power (watts)!!The overall radar efficiency is the ratio of RF power actually radiated by the antenna to the DC prime power input:!!

= Pav/PDCLmL!!

Where !Lm = the transmitter to antenna loss factor > 1.0!!L = the antenna ohmic loss factor > 1.0!

η

ηr

ηr Ω

Ω

Microwave Tube Taxonomy

Linear Amplification

The Magnetron

Magnetron Tube Oscillators!!Developed during World War II by Randall and Boot of Birmingham University in the UK!!Magnetrons produce high pulsed powers in short pulses but are NOT coherent (i.e. they allow measurement of amplitude of echoes only).!!Magnetrons are low cost (as low as tens of dollars) and are found in simple maritime and coastal radar systems (and in the microwave oven!!)!!The fast rise and fall times of pulses tend to help mitigate clutter!!Attempts have been made to make magnetrons phase coherent using injection locking, however, this has not yet found widespread use.!

The Magnetron

The Magnetron

•  Magnetrons consist of a hot cathode with a high negative potential provided by a high-voltage, direct-current power supply. !

•  The cathode is built into the center of an evacuated, lobed, circular chamber. !•  A magnetic field parallel to the filament is imposed by a permanent magnet. The

magnetic field causes the electrons, attracted to the (relatively) positive outer part of the chamber, to spiral outward in a circular path rather than moving directly to this anode. !

•  Spaced around the rim of the chamber are cylindrical cavities. The cavities are open along their length and connect to the common cavity space. As electrons sweep past these openings, they induce a resonant, high-frequency radio field in the cavity, which in turn causes the electrons to bunch into groups. !

•  A portion of this field is extracted with a short antenna that is connected to a waveguide. !

•  The waveguide directs the extracted RF energy to the antenna.!•  The sizes of the cavities determine the resonant frequency, and thereby the

frequency of emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in load impedance, with changes in the supply current, and with the temperature of the tube. !

An X-Band Magnetron

The Klystron

Klystrons are linear beam tubes!!The interaction between the RF field and the electron beam occurs longitudinally over the length of the tube!!They consist of a series of high Q cavities through which the electron beam passes, exchanging energy to create high power at RF. The weak electric field causes the electrons to bunch by speeding them up and slowing them down. This bunching is re-enforced by the high Q cavities.!!Each cavity acts as a resonant circuit tuned to a chosen frequency!!Klystrons can be tuned to a broad range of operating frequencies (HF to W band), have high efficiency (around 50%) and have high gain (around 50dB)!!Klystron are narrow band and hence are not used widely in modern radar systems!

The Klystron

Klystrons

The Travelling Wave Tube

The Travelling Wave Tube (TWT) is a linear beam device.!!The TWT supports measurement of amplitude and phase!!It is in common use as a high power amplifier in modern radar systems (as well as in satellite comms, ECM and other applications)!!TWTs can support wide band widths and a wide variety of modulation types!!Efficiencies are relatively low (approximately 20%)!!!!

•  A TWT is an elongated vacuum tube with an electron gun (a heated cathode that emits electrons) at one end. !

•  A magnetic containment field around the tube focuses the electrons into a beam, which then passes down the middle of an RF circuit (wire helix or coupled cavity) that stretches from the RF input to the RF output.!

•  The electron beam finally strikes a collector at the other end. !•  A directional coupler, which can be either a waveguide or an

electromagnetic coil, is fed with the low-powered radio signal that is to be amplified!

•  The directional coupler is positioned near the emitter, and induces a current into the helix.!

•  The RF circuit acts as a delay line, in which the RF signal travels at near the same speed along the tube as the electron beam. !

The Travelling Wave Tube

•  The electromagnetic field interacts with the electron beam, causing bunching of the electrons (an effect called velocity modulation)!

•  The electromagnetic field due to the beam current then induces more current back into the RF circuit and the current builds up and is thus amplified!

•  A second directional coupler, positioned near the collector, receives an amplified version of the input signal at the far end of the RF circuit. !

•  Attenuator(s) placed along the RF circuit prevents reflected waves from traveling back to the cathode.!

•  Higher powered Helix TWT’s usually contain beryllium oxide ceramic as both a helix support rod and in some cases, as an electron collector for the TWT because of its special electrical, mechanical, and thermal properties!

The Travelling Wave Tube

The Travelling Wave Tube

The Travelling Wave Tube

TWT major components!!1.  The electron gun that emits a beam of electrons!2.  The input port into which RF energy is injected!3.  Magnets that keep the electron beam focused as it traverses the

tube!4.  Attenuators to reduce backward flow of the wave!5.  The helix coil (or equivalent) that bunches the electrons!6.  The RF output port collects the amplified radiation!7.  The vacuum tube encloses the components of the TWT!8.  The collector which dissipates the thermal energy collected in the

beam!

Beam control is accomplished by pulsing a modulating anode or grid!

The Travelling Wave Tube

Solid State Amplifiers

Solid state amplifiers should have improved, reliability, maintainability, modularity and overall performance.!!In some cases this holds true in others they are out performed by tube devices!!They tend to have a restricted frequency range, wide bandwidths, lower powers and variable efficiency!!Development of high power solid sate amplifiers is still a subject of current research!!Most are based on Gallium Arsenide technology whilst Gallium Nitride is being increasingly employed.!!Silicon Carbide offers a possible route to high powers with high efficiencies!

Solid State Amplifiers

No Hot cathode is required and therefore no delay for device warm-up!!Much lower operation voltages, therefore, cheaper, smaller and easier to handle!!Much higher reliability (longer Mean Time Between (MTB) failures!!Wide bandwidths can be supported (typically up to 20% and exceptionally up to 50%)!!Can supported distributed power based designs and therefore well suited to phased array antenna based radar systems!!Many different amplifiers class circuit designs can be used. A, AB, B and C being the most common!!!

Solid State Transmitter/Receiver Modules

Solid-State T/R modules consists of:!!An actuator for control of receive gain and receive antenna side-lobes!!A low power phase shifter for beam steering control!!A solid-State amplifier!!A circulator to provide isolation between transmission and reception!!Receiver protection to prevent burn out (e.g. from close in clutter etc.)!!GaAs produces powers in region of watts and GaN improves on this by a factor of 5 to 10!

Solid State Transmitter/Receiver Modules

Solid State Transmitter/Receiver Module example

•  The Cobham Sensor Systems Dual S-Band 100W Tx/Rx Module is designed to operate in active Phased Array Radar systems. !

•  It utilizes a 40V supply to drive the 100W LDMOS amplifier, which operates in class AB, with local storage capacitance to supply the peak currents for 100us pulse lengths at duty cycles up to 20%.!

•  A circulator provides the duplexer function with a T/R switch to protect the receiver from reflected power.!

•  The 6 BIT phase shifter and attenuator are positioned in the common arm so are available in transmit and receive. They are controlled with parallel TTL logic derived from a FPGA that is driven by a 3-wire serial data stream.!

•  It is configured to be suitable for mounting in the antenna array, directly behind the antenna element.!

•  Control of the digital phase and amplitude trimmers is by parallel TTL logic derived from an internal FPGA, as is the T/R switch control.!

•  Minimum 100W Output Power per channel using high-efficiency LDMOS transistors!

•  Low receiver noise figure <3.3dB @ +55°C !•  Up to 100μs pulse Width, 20% Duty Cycle!•  6 BIT phase and attenuation control!•  Independent control of each channel!•  Large Internal Storage Capacitance for Optimum Pulse Shape!•  Integral power supply sequencing and conditioning!•  Integral digital control and messaging logic!

Solid State Transmitter/Receiver Module example

Solid State Transmitter/Receiver Module example

30W L-band module - NASA!

Solid State Active Aperture Arrays

Solid state active aperture arrays can be low or high power density!!Low density have low powers per element. They maintain power on target through large array sizes. They are lower cost, easier to cool and less vulnerable to intercept.!!High density increase the amount of output power per element to reduce the array size. They can be more expensive, require cooling!!There is a balance between the two with design trade-offs that require careful consideration for any given application.!

Solid State Active Aperture Arrays

Solid State Active Aperture Arrays

Transmitter Design and Spectrum Issues

There is great pressure on the use of the Electro-Magnetic spectrum. !!Commercial communications systems are demanding ever more sections of bandwidth. !!Radar systems can transmit high powers and care has to be taken not to inadvertently interfere with communication systems!!There have been cases of the radar systems being switched off to avoid interference.!!Management of the spectrum is placing increased demand on careful control of radar emissions!

Transmitter Design and Spectrum Issues

Transmitter Design and Spectrum Issues

Transmitter Design and Spectrum Issues

Transmitter Design and Spectrum Purity

Errors in spectrum purity can be time varying or non-time varying, i.e. they are either constant from pulse to pulses or vary from pulse to pulse.!!Amplitude and phase can be sensitive to transmitter operating voltage!!Some transmitters can have non-linear phase characteristics which leads to undesirable phase dispersion (and degrades pulse compression performance). They tend to ne non varying and can be calibrated out.!!Power supply voltage ripples that can result in “paired echoes” that degrade the impulse response function!!Digital processing facilitate easier correction!!!

Side-lobe Response to Phase Errors

Further Reading

1.  G. Ewell, Radar Transmitters, McGraw-Hill, New York, 1981!2.  E. Ostroff, M. Borowski and H. Thomas, Solid-State radar

transmitters, Artech house, 1985!3.  A.S. Gilmour, Microwave tubes, Artech House, 1986!4.  A.S. Gilmour, Principles of travelling wave tubes, Artech House

1991!