| 摘要 |
<p>A programmable digital frequency synthesizer for providing an output signal at a selected frequency has a very low level of spurious signals in the output. The synthesizer uses a programmable Walsh function generator having a predetermined basic period. The generator is arranged to provide a Walsh function signal having a sequency such that the sequency divided by the basic period is equal to n times the selected frequency, n being an integer greater than one. A scale-of-n frequency divider is connected to the Walsh function generator for counting down in response to the Walsh function signal, thereby providing a square wave at the selected frequency. A low-pass filter is connected to the frequency divider for accepting the square wave and providing the output signal at the selected frequency by attenuating harmonics of the selected frequency. The frequency divider serves a novel function, since it is used to "clean up" the frequency spectrum of the Walsh function signal by attenuating spurious sideband signals. The spectrum can be cleaned up to any degree desired, since the attenuation of spurious sideband signals (as a voltage ratio) is approximately proportional to the countdown ratio n of the frequency divider.</p> |
