SAW Pulse Compression
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The Expander or Transmit section of the Pulse Compression Subsystem comprises the digital interface circuits which control and define the transmitted (or expanded pulse), the SAW expansion dispersive delay line (DDL or chirp filter), the impulse generator which drives the SAW DDL and the device matching and amplification circuits. The key subsystem parameters are explained below.

Centre Frequency (IF)

The range of centre frequencies is 30-600MHz. The frequency should be chosen to give a fractional bandwidth of less than 33%. Very high centre frequencies give poorer sidelobe levels - higher percentage bandwidths give poorer system SNR and dynamic range. A maximum practical bandwidth of approximately 200MHz is possible using current techniques.


The bandwidth, B, will normally be determined by COM DEV International since it depends on:

  • Compressed Pulse Width (specified by customer)
  • Weighting Function (selected by COM DEV International to achieve the required time sidelobe level)

Any restriction on the spectral shape of the transmitted (expanded) pulse should be specified by the customer, as this may affect the available performance.

Waveform Coding and Coding Sense

non_linear_chirp_signals_fig1_2.gifThe coding sense (i.e. upchirp or downchirp) is not normally important to the radar performance, but COM DEV Internationalrecommend a downchirp for the IF waveform. The most important waveform decision is between Linear or Non-Linear FM coding. In general, linear coding is recommended for applications where high Doppler shifts are expected. Non-linear coding is recommended where mismatch loss is critical and low Doppler shifts expected. However, we can also design hybrid non-linear systems which optimise mismatch loss against Doppler sensitivity for any given application. Figure 1 gives a schematic of the typical FM coding used for Linear and Non-Linear applications, and, as expected, Hybrid designs lie within these extremes.

Pulse Length

The range of pulse lengths, T, available is from 0.5µs to 100µs and beyond. The signal is normally accurately internally gated to match the receiver characteristics. Sidelobe performance can be degraded if the radar system does not allow the full pulse length to be transmitted.

Time Bandwidth Product

A common figure which characterises pulse compression devices is the time bandwidth product expressed as T(µs)B(MHz). In MESL subsystems values of TB between 5 and 1000 can be achieved. For low values of TB, say between 5 and 15, MESL have successfully developed techniques which allow sidelobe suppression exceeding 35dB, which is considerably better than expected. For TB between 15 and 500 (the highest "off-the-shelf" TB), sidelobe levels can vary from 35dB to 45dB, depending on such things as the Doppler shift v mismatch loss trade-off, and the selected IF.

Signal Delay

Signal Delay Timing Expander unit

The timing of the expanded pulse is generally derived from the parent system. Inherent in the SAW device is a delay, t1, to the leading edge of the expanded pulse as shown in Figure 2. It is often more convenient, however, to define time, t0, as the delay from trigger reference to the "centre frequency" of the expanded pulse. This delay, when added to the corresponding delay in the compressor, gives the minimum total delay between the trigger input and compressed pulse output, in the absence of any Doppler shift. Commonly known as the loop delay, this parameter is specified with a tolerance of 100ns in most systems, but will track to typically better than 5% of the loop delay.

Output Signal Level

COM DEV International provides a nominal output signal level in the range 0 to +6dBm. Other values can be accommodated on request.

Amplitude Ripple

Amplitude ripple on the output waveform generally results from its rapid frequency sweep and is typically ± 0.25dB. In order to preserve the sidelobe level, the system designer must ensure that no additional ripple is introduced within his system.

Signal-to-Noise Ratio (SNR)

The SNR at the output of the expander subsystem varies typically between 40dB and 60dB for wideband and narrowband subsystems respectively. Because of processing gain, this parameter is not as critical as it may at first seem. Our Digital Waveform Generators have practically immeasurable in-band noise, even using sophisticated spectrum analysers.

Output Impedance

The impedance of a COM DEV International subsystem is typically 50W with a VSWR of 1.5. On special request COM DEV International will provide, where possible, different impedance and VSWR levels.


COM DEV International, Lochend Industrial Estate, Newbridge, Edinburgh, EH28 8PL, Scotland
Tel: 0131 333 2000 | Fax: 0131 333 3607 | COM DEV Europe Limited Registered England and Wales Company Number: 1863723