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.
RADAR pulse compression was the first DF application. A transmitter expander DF can be used for FM pulse generation, although digital methods are also common. A receiver matched filter DF compresses the return signal from length T to 1/B, giving a TB increase in signal to noise ratio, or processing gain. DFs used for transmission generally have a flat impulse response. DFs used for matched filtering in receivers are generally amplitude weighted with Taylor or Hamming functions to reduce time sidelobes. RADAR altimetry is a related application. A DF is used to generate a LFM chirp. Mixing the return signal with the transmitted reference signal supplies a CW signal whose frequency is a linear function of altitude. Compressive receivers use DFs to perform high speed spectrum analysis for RADAR and communications signal analysis applications. They provide a BT speed enhancement over a standard scanning superhet receiver of resolution 1/T. Real time spectrum analyzers and Fourier transformers use sets of DFs to process arbitrary signals with 100% probability of intercept. They analyze in real time a band B with resolution 1/T. A broad range of adaptive signal processing functions are made possible with these modules.
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 (maximum values):
|Bandwidth B MHz||1000||500|
|Dispersion T us||25||100|
|Center frequency MHz||1500||1500|
|Relative bandwidth %||150||60|
|Insertion loss dB||20-40||30-50|