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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> phase distortion - the root cause of jitter? https://designers-guide.org/forum/YaBB.pl?num=1135843857 Message started by river on Dec 29th, 2005, 12:10am |
Title: phase distortion - the root cause of jitter? Post by river on Dec 29th, 2005, 12:10am I read from some material that phase distortion is the root cause of jitter. If so, let's think about the following example. Assume we input a square wave to a filter which has phase distortion, because this filter has phase distrotion and also because square wave has plenty of hamonics, so from that theory, we should see jitter in the output signal. But the fact is that we can not see the expected jitter. What's wrong? |
Title: Re: phase distortion - the root cause of jitter? Post by sheldon on Jan 2nd, 2006, 8:34am River, Not sure that this is a good analogy since a square wave is a pathological case, that is, it has frequency components at all frequencies and always suffers from phase distortion. There is some discussion about this issue in Razavi's book,"RF Microelectronics" in the intersymbol interference section(?). Don't confuse systematic and random jitter. One way to look at this is to consider the following data patterns 01010101 and 00001111. Due to the finite bandwidth of the channel, the output signal from the channel for these two signals will have different amplitudes and zero crossings. When you plot the eye diagram, the difference in zero crossings causes the "eye" to narrow and this is equivalent to jitter. That is for different data patterns the zero crossings "jitters", changes. However, this is a determinstic phenomena, that is, for the same data patterns you see the same "jitter". You have to account for the jitter due to the difference in the data pattern in the overall system design. In addition to the systematic jitter there will also be random jitter due to thermal, shot, 1/f, ... noise in the system. Best Regards, Sheldon |
Title: Re: phase distortion - the root cause of jitter? Post by neoflash on Jan 2nd, 2006, 7:42pm Very good question. Since the group delay is different. 3rd order harmonics should experience different delay from base harmonics. Intuitively, the waveform should be distorted. Yes, in deed it is distorted. However, it is still periodic signal. It is repeating itself in distorted way. |
Title: Re: phase distortion - the root cause of jitter? Post by sheldon on Jan 2nd, 2006, 10:39pm Neo_flash, You are correct that if the signal was periodic then there would be no pulse distortion. However in general signals is not truly periodic, they are random. This is why standards specify pseudo-random data, PRBS, data for testing bit error rates. So if there is a system with a pulse input, the output pulse will be distorted unless the group delay of the system is constant. The pulse distortion is dependent on the data pattern, for example, the distortion for the data patterns 01010101 and 00001111 are different[if the group delay is not constant for two octaves]. When the outputs for the different data patterns are overlayed, the pulse distortion appears as jitter. Even if the communication channel is perfect, systems usually have internal filtering that causes pulse distortion, for example, the channel filter in Bluetooth/GSM. The result is that systems have systematic jitter that typically comes from the system specification, for example, 1 MHz channel spacing, 20dB of attenuation in the next adjecent channel, Bit error rate of 0.1%, ... Best Regards, Sheldon |
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