In this case, the harmonics are the integral multiples of frequency-divided signal frequency. If you find noise in any of these frequencies, it is considered to be caused by a frequency-divided signal not the original signal.ĭigital circuits often operate in synchronization with the clock signal, and many of them operate at a frequency of 1/N of the clock signal (called “frequency division”). ![]() For example, 100MHz signal will never generate noise of 20MHz, 50MHz or 90MHz. In addition, a cyclic waveform does not have any frequency component that is lower than the fundamental frequency. (4) Contains no frequencies other than integral multiples This is because neither signal of 33.3MHz nor 34MHz contains the harmonic of 330MHz. 2-4-2, we can suspect that it is caused by the circuit with 33.0MHz not 33.3MHz. For example, if noise exists at 330MHz in Fig. ![]() Therefore, this noise is considered to be caused by a circuit that operates in synchronization with the 33MHz clock.Įven if this electronic device concurrently uses circuits with a near-by cyclic frequency such as 33.3MHz or 34MHz, such a frequency can be separated if you can accurately measure the noise frequency and interval. The interval of the frequencies with strong noise seems to be at 33MHz. For example, let's say we have observed noise such as shown in Fig. Measuring the noise spectrum interval allows us to analogically think the cyclic frequency of the signal that is causing noise. The above mentioned nature of harmonics comes in useful when looking for a noise source based on the noise frequency. (3) How to find noise source from noise frequency The bottom part around 40dB or lower indicates the background noise of the spectrum analyzer. You can see that the odd harmonics and even harmonics have a different trend. The sections that are projecting upwards like a needle are the harmonics and are observed accurately at the interval of 33MHz. 2-4-1 shows an example of the harmonics of a 33MHz clock signal measured by a spectrum analyzer. (2) Measuring the harmonics of a clock signalįig. Therefore, when the frequency distribution (called “spectrum”) is measured, it is precisely broken down to harmonics, showing a discretely distributed spectrum. Many of digital signals have a repeating waveform. In the case of accurately repeating waves, there is no other frequency component apart from these. The multiple for the fundamental wave is called harmonic order. Generally, all waves with a constant cycle period can be broken down into a fundamental wave with the cyclic frequency and the harmonics that have frequencies of integral multiples of the cyclic frequency. Nature of harmonics in terms of noise (1) Digital signal is made out of harmonics This section will describe the basic nature of the harmonics contained in digital signals.Ģ-4-1. If you can control harmonics well, you can efficiently implement noise suppression for digital circuits. As described in Section 2-3, one type of the noise sources generated by digital circuit is harmonics.
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