The radar transmitter produces the short duration high-power rf pulses of energy that are radiated into space by the antenna. The radar transmitter is required to have the following technical and operating characteristics:
- The transmitter must have the ability to generate the required mean RF power and the required peak power
- The transmitter must have a suitable RF bandwidth.
- The transmitter must have a high RF stability to meet signal processing requirements
- The transmitter must be easily modulated to meet waveform design requirements.
- The transmitter must be efficient, reliable and easy to maintain and the life expectancy and cost of the output device must be acceptable.
The radar transmitter is designed around the selected output device and most of the transmitter chapter is devoted to describing output devices therefore:
- One main type of transmitters is the keyed-oscillator type. In this transmitter one stage or tube, usually a magnetron, produces the rf pulse. The oscillator tube is keyed by a high-power dc pulse of energy generated by a separate unit called the modulator. This transmitting system is called POT (Power Oscillator Transmitter). Radar units fitted with an POT are either non-coherent or pseudo-coherent.
- Power-Amplifier-Transmitters (PAT) are used in many recently developed radar sets. In this system the transmitting pulse is caused with a small performance in a waveform generator. It is taken to the necessary power with an amplifier followingly (Amplitron, klystron or Solid-State-Amplifier). Radar units fitted with an PAT are fully coherent in the majority of cases.
- A special case of the PAT is the active antenna.
- Even every antenna element
- or every antenna-group
is equipped with an own amplifier here
Pictured is a keyed oscillator transmitter of the radar unit.
The picture shows the typical transmitter system that uses a magnetron oscillator and a waveguide transmission line. The magnetron at the middle of the figure is connected to the waveguide by a coaxial connector. High-power magnetrons, however, are usually coupled directly to the waveguide. Beside the magnetron with its magnets you can see the modulator with its thyratron. The impulse-transformer and the pulse-forming network with the charging diode and the high-voltage transformer are in the lower bay of this rack.
Pseudo-coherent Radar Review
Figure 1: The principle of a pseudo-coherent radar.
A requirement for any Doppler radar is coherence; that is, some definite phase relationship must exist between the transmitted frequency and the reference frequency, which is used to detect the Doppler shift of the receiver signal. Moving objects are detected by the phase difference between the target signal and background clutter and noise components. Phase detection of this type relies on coherence between the transmitter frequency and the receiver reference frequency.
If the transmitter output stage is a self oscillating device, the pulse to pulse phase is random on transmission. In coherent detection, a stable cw reference oscillator signal, which is locked in phase with the transmitter during each transmitted pulse, is mixed with the echo signal to produce a beat or difference signal. Since the reference oscillator and the transmitter are locked in phase, the echoes are effectively compared with the transmitter in frequency and phase. This phase reference must be maintained from the transmitted pulse to the return pulse picked up by the receiver.
Pseudo-coherent Radar sets are sometimes called: „coherent-on-receive”
The duplexer alternately switches the antenna between the transmitter and receiver so that only one antenna need be used. This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver.
The function of the mixer stage is to convert the received rf energy to a lower, intermediate frequency (IF) that is easier to amplify and manipulate electronically. The intermediate frequency is usually 30 or 60 megahertz. It is obtained by heterodyning the received signal with a local-oscillator signal in the mixer stage. The mixer stage converts the received signal to the lower IF signal without distorting the data on the received signal.
After conversion to the intermediate frequency, the signal is amplified in several IF-amplifier stages. Most of the gain of the receiver is developed in the IF-amplifier stages. The overall bandwidth of the receiver is often determined by the bandwidth of the IF-stages.
Automatic Frequency Control
As in all superheterodyne receivers, controlling the frequency of the local oscillator keeps the receiver tuned. Since this tuning is critical, some form of automatic frequency control (afc) is essential to avoid constant manual tuning. Automatic frequency control circuits mix an attenuated portion of the transmitted signal with the local oscillator signal to form an IF signal. This signal is applied to a frequency-sensitive discriminator that produces an output voltage proportional in amplitude and polarity to any change in IF-frequency. If the IF signal is at the discriminator center frequency, no discriminator output occurs. The center frequency of the discriminator is essentially a reference frequency for the IF-signal.
The output of the discriminator provides a control voltage to maintain the local oscillator at the correct frequency.
Stable Local Oscillator
As the receiver is normally a super heterodyne, a stable local oscillator known as the StaLO down converts the signal to intermediate frequency.
Most radar receivers use a 30 or 60 megahertz intermediate frequency. The IF is produced by mixing a local oscillator signal with the incoming signal. The local oscillator is, therefore, essential to efficient operation and must be both tunable and very stable. For example, if the local oscillator frequency is 3,000 megahertz, a frequency change of 0.1 percent will produce a frequency shift of 3 megahertz. This is equal to the bandwidth of most receivers and would greatly decrease receiver gain.
The power output requirement for most local oscillators is small (20 to 50 milliwatts) because most receivers use crystal mixers that require very little power.
The local oscillator output frequency must be tunable over a range of several megahertz in the 4,000-megahertz region. The local oscillator must compensate for any changes in the transmitted frequency and maintain a constant 30 or 60 megahertz difference between the oscillator and the transmitter frequency. A local oscillator that can be tuned by varying the applied voltage is most desirable.
The IF-signal is passed to a phase sensitive detector (PSD) which converts the signal to base band, while faithfully retaining the full phase and quadrature information of the Doppler signal. This means, the phase-sensitive detector produces a video signal. The amplitude of the video signal is determined by the phase difference between the coho reference signal and the IF echo signals. This phase difference is the same as that between the actual transmitted pulse and its echo. The resultant video signal may be either positive or negative.
The signal processor is that part of the system which separates targets from clutter on the basis of Doppler content and amplitude characteristics.
The directional coupler provides a sample of the transmitter output on every pulse. This signal adjusts the STALO frequency via the AFC but more importantly, it adjusts the phase of the COHO, locking it to the phase reference from the magnetron. The phase synchronization of the COHO by means of a sample of the magnetron output is mandatory because there is no phase correlation between two successive RF pulses of the magnetron.
The function of this mixer stage is to convert the sample of the transmitter output to the intermediate frequency. This coho lock pulse synchronize the coho to a fixed phase relationship with the transmitted frequency at each transmitted pulse.
The second local oscillator known as the coherent oscillator (COHO) enables the down conversion process into the phase sensitive detector, whilst maintaining an accurate phase reference. The coho lock pulse is originated by the transmitted pulse. It is used to synchronize the coho to a fixed phase relationship with the transmitted frequency at each transmitted pulse.
The COHO takes over the phase of the transmitter tube and provides it to the receiver part of the system. This is the reason why the pseudo-coherent radar is also called „coherent on receive”.
The oscillator tube of the transmitter is keyed by a high-power dc pulse of energy generated by this separate unit called the modulator.
Disadvantages of the pseudo-coherent radar
The disadvantages of the pseudo-coherent radar can be summarized as follows:
- The phase locking process is not as accurate as a fully coherent system, which reduces the MTI Improvement factor.
- This technique cannot be applied to a frequency agile radar. Frequency change in a magnetron relies on the mechanical tuning of a cavity and it is essentially a narrow band device.
- It is not flexible and cannot easily accommodate changes in the PRF, pulsewidth or other parameters of the transmitted signal. Such changes are straightforward in a fully coherent radar because they can be performed at low level. It is also impossible to perform FM modulation (which is mandatory for a pulse compression radar) with this type of system.
- Second time around echoes are returns from large fixed clutter areas located a long distance from the radar. Because they originate from a large distance, such echoes are returned after a second magnetron pulse has been transmitted. However, they pertain to the first pulse transmitted by the magnetron. Such echoes are range ambiguous but, in addition, second time around clutter will not cancel. This is due to the fact that the phase locking of the COHO applies only to the last transmitted pulse.