Power Matters™

Dispersive Filters

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Dispersive Filters (DF), also called Dispersive Delay Lines (DDL), are linear filters with group delay that varies by T over their passband B. In other words, the impulse response of a DF is an FM pulse whose instantaneous frequency varies by B over its duration T. The FM is usually chosen to be linear (LFM), with the amplitude response being flat or weighted to suppress sidelobes. Nonlinear FM (NLFM) can also be implemented, the tradeoff being improved signal to noise versus greater Doppler sensitivity. Several techniques are available to manufacture such filters. All consist in varying the propagation path length with frequency.

Interdigital Dispersive (ID) Filters

This technique includes the dispersion effect one or both transducers on a quartz or lithium niobate substrate. The transducer is then composed of a great number of electrodes whose effective periodicity varies from one extremity to the other. As with bandpass filters, position and length of the electrodes set the phase and amplitude of the response. With in-line transducers, BT to 500 are possible. With slanted transducers, BT to 12000 and post-photolithography phase error correction are possible.

Reflective Dispersive (RD) Filters

The RD, also called RAC or Reflective Array Compressor, is the dominant LFM DF technique for large T. It uses a quartz or lithium niobate substrate (usually heated) with short transducers and obtains dispersion with two long oblique acoustic reflector arrays which are frequency selective due to their non-uniform period. The reflectors are metal deposited at the same time as the transducers. The RAC allows post-photolithography phase error correction by insertion of a metallic film pattern between the arrays.

DF Performance

Product Code: ID RD
Bandwidth B MHz 1000 500
Dispersion T us 25 100
BT 12000 12000
Center frequency MHz 1500 1500
Relative bandwidth % 150 60
Sidelobes dB 30-40 30-40
Insertion loss dB 20-40 30-50