Long term jitter measurements.健美操二级
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R S U 14.1.5 n o i s r e V t c u d o r P 1e t a D e s a e l e R Application Note. Long term jitter
measurements.
The procedures described in this document are deliberately broad and generic. Your specific design might require procedures that are slightly different from tho described here.Purpo
This document illustrates how to u the SpectreRF simulators within the Analog Design Environment (ADE) to measure long term jitter characteristics.
Audience
Circuit designers who are interested to u Cadence’s tools for a jitter performance verification.
Long Term Jitter
It is esntial to be able to predict and verify the timing performance of a circuit in the prence of internally and externally generated jitter and pha noi. Long term jitter characterizes the variation in the accumulated width of a large number of clock periods.If a periodic system has a reference time point, we can consider all other events in relation to that time point. For a driven system it could be the crossing by the periodic input signal a particular threshold value. For an autonomous system which is rather lf-referenced, the first
crossing during the obrvation time could be ud. We expect that the periodic output signal will generate an ideal quence of events, parated by the period . If the first crossing
happened at the time
灭火器的正确使用步骤>学游泳,the quence of threshold crossing by the output in the same direction is expected at times , where we index the events with .(1-1) The actual transition times
of the noisy signal will vary from the expected as:
. )2
-1(°°+{}ν+()°+ ()ν°+()ν
=={
}ν()ν
°()++()ν==
Long term jitter measurements.Application Note. Long term jitter measurements.
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.
(1-4) .
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(){}+––}()+–()
徐志摩的散文《落叶》
Long term jitter measurements.Application Note. Long term jitter measurements.
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In our ca, we tup PSS simulation and PNoi analys, type “jitter” as shown at Figure 1-2
PNoi frequency sweep has maximum t for the half of PSS fundamental. In most cas this will be the highest frequency one need for the measurements - beyond that value, certain assumption in pha noi computation of SpectreRF could break down. The minimum frequency is usually defined to account for the longest expected obrvation time or maybe limited by the ur knowledge of the system bandwidth.
Figure 1-2 VCO jitter measurement tup.
The PNoi simulation allows us to plot pha noi power spectral density and we will u it to compute the long term jitter.
It is helpful to plot pha noi prior to beginning jitter measurements. One can verify the noi frequency content. It also helps to decide about which integration limits to u. Since at lower frequency offts, the pha noi will grow boundless, it could be confusing at first. One can plot th
e pha noi using Direct Plot functionality, e Figure 1-3. In addition, to determine the prence of the flicker noi, one can u the “-20dB/dec” curve which could be placed on the pha noi curve at any frequency.
Long term jitter measurements.Application Note. Long term jitter measurements.
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Figure 1-3 VCO pha noi on Direct Plot form.
Once we know the pha noi, there are two important parameters that have to be lected for the long term jitter computation: the number of cycles and the minimum frequency of the integration. In most cas, a maximum frequency will be t once. It does not affect the long term jitter computation since the high frequency content of pha noi PSD will be much lower then at small frequency, especially in the prence of the flicker noi.
The time of obrvation or the loop bandwidth of PLL are the two typical factors that are ud to lect the “Number of Cycles” k and the “Start Frequency” . For lowest frequency of pha noi that can be obrved, we have to u either PLL loop bandwidth frequency or the jitter measurement (obrvation) time . The same information is ud to define the number of cycles for jitter computation:
(1-5) (1-6) .
浏览器首页In our example, for the PLL loop bandwidth of 10kHz, the numbers will be and
. The plot of the pha noi indicates the prence of the flicker noi, just below the , e Figure 1-5. Using the Direct Plot, the jitter value is plotted for the nominal and a few other values of the bandwidth: 20k, 100k and 500k. The noi is mostly white PM noi and the jitter increas as a square root of obrvation time (which is inver to the PLL loop bandwidth), according the to theory [4]. In addition to the long term jitter, the single period jitter is plotted. It confirms that the slow, low frequency components of the pha noi do not affect its value, making the lection of the minimum integration limit less of a problem.
Pha Noi plot.Slope “assistant”
----
=---------==()()⁄==
Long term jitter measurements.Application Note. Long term jitter measurements.
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Figure 1-4 VCO jitter, Direct Plot form.
Figure 1-5
VCO jitter and pha noi results.The reason that the number of cycles affects the jitter computation so dramatically is becau we extract the broadband noi characteristic, jitter, from the narrowband pha noi PSD. The shape of the equivalent frequency responce for the sampling which is ud to detect the Period jitter with
different integration limits
K-cycles Jitter
1/f^3 flicker noi region
Long Term Jitter.
