SAW Pulse Compression
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Technical Notes
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Pulse Compression is a complex topic and COM DEV International has close to 20 years' experience in interpreting the needs of radar system designers to optimise their specific system's performance.

Processing Gain (PG)

Processing gain is the term given to the signal to noise improvement achieved by pulse compression. It can be expressed as: PG 10log10 (TB). In practice, this factor is reduced by the mismatch loss. Incidentally, the mismatch loss can be measured fairly precisely, but not so the processing gain since bandwidth can be somewhat arbitrarily defined in a Non-linear FM pulse compression system.

Spectral Shape

SAW Pulse Compression Spectral ShapeMost pulse compression radars require a drive signal for the power amplifier which has an essentially rectangular envelope. For a linear FM (chirp) signal the associated frequency spectrum is also approximately rectangular, as shown in Figure 5a, for a chirp with TB equal to 56. The deviation from the nominal 14MHz bandwidth is due to Fresnel ripple associated with linear FM waveforms. This in turn introduces gating sidelobes at a level -20log10(TB), which for this case are at a level of -35dB. The processes of weighting and ripple compensation in the compressor can significantly reduce both close-in and gating sidelobes. Figure 5b shows the spectrum of a non-linear chirp with similar pulse compression characteristics. With this type of spectrum, low sidelobe levels can also be achieved without the introduction of mismatch loss and some ripple compensation may enhance performance at the expense of a minor mismatch loss penalty. The spectral shape of the chirps defined by linear and non-linear design rules can differ significantly and, although these shapes can be accurately computed in the course of design procedure, it is sometimes difficult to specify reference points for bandwidth measurement, e.g. there may be a 2dB amplitude ripple at the band centre. This needs to be recognised when defining bandwidths for test specification purposes.

Pulse Broadening and Mismatch Loss

In most radars it is necessary to employ a weighting function to achieve adequate sidelobe suppression. A wide range of suitable functions is available and COM DEV International would normally select the one best suited to a customer's requirement. The effect of weighting in both linear or non-linear coded systems is to produce broadening of the compressed pulse for any given use of bandwidth. For a specified pulse width it is common to select the required bandwidth accordingly. The introduction of the weighting function in linear coded systems creates a mismatch loss of approximately 1.2dB for 40dB sidelobe suppression. Where non-linear coding is used, mismatch loss can be zero, even at this sidelobe level, although there is no advantage in terms of overall bandwidth which remains the same as for linear FM.

 

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