Design Ideas: March 31, 1994
A typical DSP system first converts an analog signal to digital, then performs windowing, and finally does a fast-Fourier transform. However, you can accelerate this process by swapping the converting and windowing steps and implementing the windowing in hardware. The multiplying DAC in Fig 1 performs the windowing of an input signal and produces the following output signal, where the digital code N ranges from 0 to 4095:
The DAC7545 and 80C51 can easily implement an appropriate window function that ranges from 0 to 1. The 80C51 simply stores the digital codes for the window function (Hamming, Hanning, triangle, for example) and writes these codes to the DAC. IC1A is simply a front-end buffer. The DAC connects with IC1B for unipolar operation, and C1 prevents glitches. You may want to add an inverter in the system to compensate for the fact that this circuit inverts VIN. A low output from the NAND gate enables the system's ADC. Listing 1's C program produces digital codes that implement the Hamming window. EDN BBS /DI_SIG #1386