A Wide Dynamic Range Playback System for Radar Signals

I mprovements in the speed and resolution of both digital to ana-log converters (DACs) and analog to digital converters (ADCs) have resulted in a continual push to move more func-tionality into the digital signal processing arena. RF and analog signal processing methodologies such as filtering and frequency translation are being handled increasingly in the digital domain, where near-ideal filters are achievable and analog errors are eliminated. However, as discussed in the article entitled " A Wide Dynamic Range Radar Digitizer, " [1] converting to the digital domain introduces errors which limit overall system performance. One of the most important limitations is dynamic range, which is the range of signal amplitudes that can be captured by an ADC. This is determined by the number of conversion bits as well as by the signal-to-noise ratio (SNR) of the analog components (amplifiers, mixers, etc.) which precede the ADC. This article describes the use of a dual high-speed 16-bit DAC for reproducing a Doppler weather radar signal. The signal is played back from a digital recording produced using the digitizer described in the preceding article. A simplified block diagram for the system is shown in Figure 1. Note that the dual 16-bit DAC is used to effectively emulate a 20-bit DAC by the means described in this article. This system is required to digitally record and reproduce an analog reflection-return radar signal down-converted to an IF of 30 MHz in a 1-MHz bandwidth. A key requirement is the ability to accommodate a dynamic range of at least 105 dB between the maximum capable and minimum-detectable amplitudes that may occur in the course of a single radar trace. The actual system operates above 5 GHz and includes RF mixers, filters, amplifiers, tunable frequency sources, and other analog devices that are not shown in the figure. However, the dynamic range and SNR are set primarily by the IF devices in this diagram. The receiver in the radar itself as originally designed used analog AGC to compress the Here is a technique for reproducing a digitally recorded radar signal (or other high frequency analog signal) with 105 dB amplitude dynamic range and 16-bit resolution Figure 1 · Weather radar capture and play-back system.