See figure 10 14. In recent times, integrated circuits called, multiplier-accumulator circuits (MAC) have been employed in Doppler-filter banks to determine Doppler shift in mtd systems. The chapter on mtd, in this book, explains the operation of these filters in some detail. The Doppler-filter operation can also be performed by computer software, provided a representation of the transmitter output has been retained in a memory, so that there are no time limitations. In the ASR-9, a special system stability test provides a means to observe the spectrum of an echo, in the receiver, to verify that the spectrum is repetitive and consistent. A window permits the receiver to operate only at a given range, so that the only received data is from a target of interest. The phase-detector digital data for that window is then stored in memory for several hits. The information may, then, be sequentially read from memory, as data for a computer program, which will test the data for all harmonics within the expected spectrum band. If the spectrum is unstable, it is most likely an indication of poor transmitter performance; the crystal oscillators used for the stalo or coho are rarely unstable.

Figure 10 14: System Stability Display, Received Spectrum.