# effect of jitter on signal quality

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In this paper we examine the effects of jitter on signal quality as measured by first and second order statistics. We then identify these results with the performance of Analog to Digital Converters (ADC) with regards to an output signal to noise ratio. Our results show that for a baseband ADC sampling random noise with a flat spectral shape, the contribution of jitter is less than previously reported with sinusoidal analysis. We also indicate how wideband and narrowband jitter are effected by the signal system transfer function. In the case of a ∑−Δ ADC, the low frequency phase noise in the Digital to Analog Converter (DAC) output is the main contributor to the jitter.

There are three general ways that this convolution can contribute. In the most usual case the jitter spectrum will be narrowband compared to the signal and I(ω)≈σ2ω2P(ω) yielding our previous results. Another, but unlikely, case that could possibly occur is when the signal is narrowband compared to the jitter for which I(ω)≈Ψ(ω) and we would be integrating over the jitter bandwidth. The third case is when the bandwidths of the jitter spectrum and the signal spectrum are comparable, for which case we would have to integrate over 4Ω instead of 2Ω. This case is also unlikely.

This jitter spectrum is not necessarily the power spectrum of the oscillator. It would appear that the jitter process is influenced both by the phase noise of the oscillator and the jitter in the logic circuitry. Unless we measure the jitter spectrum directly, other sources of jitter behave like phase noise for which a transfer function would have to be known. In the simplest case of a direct phase modulation (PM) of the jitter, we know that it matters what the modulation index is since the transfer function goes as the square of the index. Wideband PM would enhance the noise, while narrowband PM would not.

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