Pulse compression

The pulse compression method is a technology in modern radar equipment, which is mostly due to the use of semiconductor components in the radar transmitter. In pulse compression, the energetic advantage of very long pulses with the advantages of a very short transmission pulses are combined.

Operation of the classical radar

For an impulse radar pulse compression without extremely short pulses with power emitted in the megawatt range. With increasing pulse power greater range can be achieved, because only then the reflected very weak signal is not masked by the noise floor of the receiver. The shorter the duration of the transmit pulse, the better the range resolution and different objects is almost the same distance can be displayed separately. In practice, this means huge power pulses in a few microseconds duration of transmission, which places high demands on the respective transmitting tubes.

To illustrate the resolving power of the transmitted signal is shown in red on the left side in the following pictures, the echoes of two targets are shown in blue. In the two left images of the background noise ( noise floor of the receiver) has been omitted which would be almost as large as the weak received signals. Because the amplitude response of the transmission signal is known, the signal -to-noise ratio can be obtained by a matched filter matched ( engl: matched filter ) are considerably improved. As a result, the originally rectangular envelope is replaced by a triangular shape, so it is difficult to separate targets similar distance. The strong background noise is significantly reduced.

Operation of a radar pulse compression

With the use of amplifier modules in semiconductor technology such high -power pulses can not or very difficult to be achieved because the tiny crystal flakes are quick to overheat because of their low thermal capacity and limits also exist with respect to the maximum current density and dielectric strengths. Therefore, much longer transmission pulses are generated whose frequency is changed specifically within the transmitted pulse duration. This frequency modulation allows a time reference in the transmitter pulse, similar to how it is carried out at the frequency modulated continuous wave radar ( FMCW ). The timing of the frequency change can be controlled linearly or non-linear with voltage-controlled oscillators, including coded pulse - phase modulation, it is used.

The significant extension of the duration of the transmitted pulses have kastastrophale effects on the resolving power of the radar unit, when there is no way to reduce the relatively long pulse duration ( some microseconds) significantly ( compress ) so that the duration of the radar signal can be accurately measured. For this particular pulse compression filters were developed: surface acoustic wave filter with a special arrangement of " finger " as a hardware solution and FIR filter as a software solution.

The starting point is the dispersion function of the signal transit time of the frequency. This effect is the basis of many optical effects such as refraction, allowing, for example, separating white light into its spectral colors. Pulse compression is in a sense the reverse: A bunch of spectral colors will be reunited.

The technical implementation is as follows:

• In an up-chirp a low frequency is sent first, this is increased pursuant to a program within a few microseconds up to a limit, then the program is terminated.
• In the same order, the high frequency "package", arrives at the receiver, is amplified, and - depending on the frequency - programmed delays:
• The filter allows low frequencies to pass slowly while high frequencies pass through this filter with minimal time delay.
• With proper design of all delays all individual frequencies simultaneously reach the filter output and produce constructive interference there by a very short-term " giant pulse " that can be uniquely identified. The following images show the signal improvement.

Pros and cons of the pulse compression

• The pulse maximum of the "giant pulse " can be temporally locate considerably more accurate than that of the triangular signal at the output of the matched filter.
• It can be isolated targets, the distance is less than would permit the duration of the transmit pulse. The reason for this is the signal propagation time in the filters: This can begin with the processing of the low frequencies of the pulse sequence, while the processing of the high frequencies of the previous pulse is still in progress. The different frequencies interfering with each other.
• The reduction of peak power has the essential for air surveillance radars advantage that an enemy reconnaissance radar is complicated and often is only possible if the exact image of the modulation is known to the enlightening system. That is why there is often talk of " Silent Radar ", ie from a "silent radar ".

• Since the noise of the frequency- synchronous component of the noise in comparison with the echo signal as a result of the statistical distribution is more broadband and relatively low, this type of filter reduces the amount of noise such that still a clear output signal is obtained even when the input signal is as small that in a classical radar long ago goes down in the noise and lost for a simple demodulation is thus.

• The radar receiver is considerably more sensitive for pulse compression and a suitable choice of pulse duration and frequency deviation, the power of the received signal is displayed by a factor of (usually in the range 20 to 30) is enhanced. In the pictures below show how the received signal is disturbed before the pulse compression filter by a strong additive noise and how insignificant this noise after the pulse compression filter is still.

The main disadvantage of the pulse compression method is the deterioration of the minimum possible distance, as long as is sent, nothing can be received ( "dead zone "). For the duration of the transmit pulse, the radar is thus blind. Since this is a major disadvantage, especially for air traffic control radars, these tend to work alternately with both methods. Between the frequency- modulated pulses for long range small and very short pulses are emitted, which only need to cover the short-range and therefore do not require large pulse power.

Example of a radar application

As an example of a technical realization following data are to serve: The transmission frequency is within the duration of a transmission pulse increased (1000 microseconds ) with the rate of change of 0.2 MHz / microseconds ( up -chirp ) of the starting frequency 1175 MHz to 1375 MHz. Parallel to this, the oscillator frequency of the superheterodyne receiver of 1665 MHz is changed to 1865 MHz, which is why incoming receive signals are mixed down to the intermediate frequency 490 MHz. According to a further reduction to 60 MHz and more powerful amplification, the sampling is performed by two analog-to- digital converter to provide the necessary signals for the I and Q process. The further processing is performed by digital signal processing.

Technical Details for frequency modulation

Linear frequency modulation

At this pulse compression method of transmission pulse is linearly frequency modulated. This has the advantage that the circuit can still be kept relatively simple. In this case, the transmit pulse accepted in a number of time intervals with constant frequency is divided. Special filters for exactly the frequency of each time interval provide a respective output signal, which is added in a cascade of delay lines, and a summation output pulse.

The linear frequency modulation has the disadvantage that disturbances can be generated relatively easily by the so-called " sweeper ". As an example of an application of the linear frequency modulation, the RIP 117 may be mentioned. The disadvantage of susceptibility to interference is compensated for by the emission of two different carrier frequencies, each with linear frequency modulation.

In the adjacent circuit example, the principle is represented in terms of five present in the transmit pulse frequencies. The high circuit complexity is quite manageable with today's integration capability. There are really two basic ways to implement this method technically:

• A processor-controlled data processing ( after A / D conversion)
• With a SAW filter

A surface acoustic wave filter

SAW filters, SAW filters also (german Surface Acoustic Wave) called, are often used in radar systems with pulse compression and compress frequency modulated echo signal in an analogous manner. They operate on the piezoelectric principle.

On a piezoelectric crystal, a broadband transducer is deposited, which converts the electric oscillation into mechanical vibrations in the crystal. However, these mechanical vibrations spread out with much smaller velocity than the electrical signals on a line up. Therefore, relatively high deceleration times can be achieved. Also at the same frequency-dependent crystal transducers are deposited, the back convert the mechanical energy back into electrical signals.

By different distances of the various transducer necessarily to the exciter system to receive the different frequency components of the input signal, a different time delay, such that all frequency components of the input signal are shifted to a same time and are thus interpreted by the radar device at the same distance. The sum of the delay must be taken into account when calculating distance, so be deducted from the total run time.

Time - Side Lobes

Since the frequency-dependent transducer ( like all filters ) can be excited by harmonics arise in addition to the sharp output pulse interfering sidelobes, so-called "time- sidelobes ", which often have to be compensated by cumbersome procedures.

Since both the temporal as well as amplitude of distance are constant, these sidelobes can be reduced to an acceptable value with a weighting of the signal amplitudes. If this amplitude weighting is carried out on the receive path, but it also causes a deterioration of the filter and reduces the signal-to- noise ratio. The size of these sidelobes are an important quality criterion for pulse compression methods and can be lowered by this amplitude weighting to a value in the range of -30 dB.

Non-linear frequency modulation,

The pulse compression with non-linear frequency modulation, has several distinct advantages. For example, it requires for the suppression generated during the compression time sidelobes no more, because the function of the otherwise necessary amplitude weighting is already fulfilled by the shape of the modulation amplitude weighting. Thus, a filter matching with steeper edges is possible. In this way, the losses that would otherwise occur by the amplitude weighting in the signal -to-noise ratio can be avoided. There are two ways of non-linear frequency modulation:

• A symmetrical shape, and
• An asymmetrical shape.

The symmetrical shape of the modulation has during the first half of the transmit pulse duration an ascending ( or descending ) and frequency change in the second half of a falling (or rising ) frequency change. A non-symmetrical shape of the modulation obtained when is used by the symmetric form only one half.

The disadvantages of the pulse compression by non-linear frequency modulation is:

• A very complicated circuit construction and
• A complicated modulation, so that each transmit pulse exactly the same properties obtained in the amplitude weighting.

Generally, this type of frequency modulation is generated by special waveform generators which generate a processor-controlled pulse shape.

Pulse compression with phase modulation

The phase-coded waveform is different from the frequency-modulated waveform is that of the total pulse time is divided into smaller sub- pulses of the same frequency. These sub - pulses always represent the smallest resolvable distance. These sub - pulses all have the same length and within this pulse duration, the phase is constant. A phase jump can be programmed between the sub- pulses. Most of this phase jump is associated with a binary code. With a total number of 13 sub- pulses in the transmitted pulse ( Barker code as shown in the adjacent picture ) have the time sidelobes size of -23 dB. ( This pulse pattern is used by the radar device AN/TPS-43. )

Binary code is a sequence of logic states. In response to this binary code, the phase of the transmission signal between 0 and 180 ° is switched. In contrast to the picture shown and greatly simplified the transmission frequency is not necessarily a multiple of the frequency of the switching pulses. The encoded transmission frequency is switched to the phase reversal points generally discordant.

Reference