Digital Waveform Generator for Weather Radars

Modern digital signal processing technology is now allowing the development of advanced weather radar features like recovery of second-trip echoes, Doppler-tolerant pulse compression of klystron and solid-state transmitter systems, optimization of the occupied bandwidth and predistortion of exciter signals in order to improve the subclutter visibility of the radar. An indispensable basis for these developments is a programmable signal source, which is capable to synthesize IF waveforms with low phase noise.

A dual-channel waveform generator (WGen) has been developed by Lassen Research for use with the Lassen/Gematronik digital receiver AspenDRX. This generator can provide arbitrary IF waveforms for driving coherent transmitters and for receiver calibration. The generator features a 6-MHz pass band (-1 dB), a wide-band SNR of 78 dB, 80 dB on/off ratio, and phase noise >70 dB below the carrier. The unit is implemented using 14-bit D/A converters, a large waveform memory (2 x 256k samples), and a SHARC DSP host for pulse-to-pulse waveform management. The high update rate and wide dynamic range of the D/A converters allows arbitrary intra-pulse amplitude and phase tailoring to compensate for klystron modulator droop or other transmitter artifacts. The extensive waveform memory allows compressed waveforms of any type to be implemented in pulses up to 6 ms in length. When used as a receiver calibrator the generator can provide Doppler waveforms of controlled spectral width, at multiple arbitrary multiple in range.

This paper provides the technical specifications of the Wgen together with details of the hard-and software design. Preliminary test results will also be presented.