Chapter 4 All-Digital Spread Spectrum Clock Generator Design
4.2 The Proposed ADSSCG Design
4.2.2 Spread Spectrum Algorithm
Since triangular modulation is easy to be implemented and has good performance in reduction of radiated emissions, it becomes the major modulation method for SSCG [6], [28]. In triangular modulation, the EMI attenuation depends on the frequency-spreading ratio and center frequency, and it can be formulated as
DCO_N
DCO OUTPUT FIN
DCO CODE[17:0]
Pre-Divider
(M[7:0])
PFD
Divider (N[7:0])FeedbackFIN_M
LAG LEAD
DCO
ADSSCG Controller *: BASELINE CODE[17:0]
DCO Code Generator (DCG)
Fig. 4.1: Architecture of the proposed ADSSCG.
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AdB =I+Jlog(SR/100)+Klog(FC) (4.1)
where
A
dB is the EMI attenuation, SR is the frequency spreading ratio,F
C is the center frequency, and I, J, K are modulation parameters [27]. Based on (4.1), under the same center frequency, EMI can be reduced further by increasing spreading ratio.Tmax
Tmin
TC
(a)
Tmax
Tmin
TC
(b)
Tmax
Tmin
TC
(c)
Fig. 4.2: (a) Conventional triangular modulation. (b) Division triangular modulation.
(c) Rescheduling division triangular modulation.
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In addition, under the same spreading ratio, the higher center frequency has better EMI attenuation performance.
Fig. 4.2(a) illustrates the conventional triangular modulation with digital approach [33]. Since the output frequency can be changed by the DCO control code, the output clock frequency can be spread by tuning DCO control codes with triangular modulation within one modulation cycle. In the beginning of the conventional spread spectrum, it will start at center frequency (Tc) and take one-fourth of the modulation cycle time to reach the minimum frequency (Tmax), and then takes half of the modulation cycle time to reach the maximum frequency (Tmin). Finally, it will return to the center frequency in the last one-fourth modulation cycle time.
Because the upper half and lower half in the triangle have the same area, as shown in Fig. 4.2(a), the mean frequency of the spreading clock is equal to center frequency and the phase drift will be zero in the end of each modulation cycle.
However, in the conventional triangular modulation, the ADSSCG controller can only perform phase and frequency maintenance based on the PFD’s output in the end of each modulation cycle. Hence due to the frequency error between reference clock and output clock, reference clock jitter and supply noise, the phase error will be accumulated within one modulation cycle, leading to induce the loss of lock and stability problems.
Thus, in order to enhance phase tracking ability, the division triangular modulation (DTM) is proposed as shown in Fig. 4.2(b). DTM divides one modulation cycle into many sub-sections (for example in Fig. 4.2(b), modulation cycle divides into 16 sub-sections) and updates DCO control code for phase tracking in every 4
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sub-sections. As a result, the ADSSCG controller can perform four times phase and frequency maintenance in one modulation cycle when modulation cycle divides into 16 sub-sections. Because DTM can provide the frequency spreading function and keep phase tracking at the same time, it is very suitable for ADSSCG in µP-based system applications. However the disadvantage of DTM is when the frequency changes to different sub-sections; it will induce large DCO control code fluctuations (7S) as shown in Fig. 4.2(b), where S is the spreading step of DCO control code in spreading modulation.
In order to reduce the peak-to-peak value of DCO control code changing in DTM, the rescheduling DTM (RDTM) is proposed as shown in Fig. 4.2(c). By reordering the sub-sections in DTM, the peak-to-peak value of DCO control code changing can be reduced to 5S. As a result, the peak-to-peak value of cycle-to-cycle jitter can be reduced while the period jitter is kept the same. Compared with DTM, the reduction ratio of peak-to-peak jitter by RDTM is related with number of sub-section, and it can be formulated as
where JR is the jitter reduction ratio, COUNT is number of sub-sections. For example, if there are 16 sub-sections, the jitter reduction ratio is 29% ((7-5)/7), and if the number of sub-section is 32, the jitter reduction ratio is 40% ((15-9)/15). Although the order of sub-sections of DTM is rescheduled by RDTM to reduce the peak cycle-to-cycle jitter, the average cycle-to-cycle jitter still keeps the same as DTM.
Besides, because the phase drift of the opposite direction in DTM and RDTM remains the same, the equivalent phase drift is zero. As a result, it will not induce an extra
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phase drift while the mean frequency remains the same. The results of frequency spread of DTM and RDTM are the same as the conventional triangular modulation.
Table 4.1 summarizes the jitter and timing comparisons of DTM and RDTM with 16 sub-sections within one modulation cycle.
With two control signals, spreading step (S) and number of sub-sections (COUNT), the proposed RDTM can provide a flexible spreading ratio for different system requirements. Spreading step is the difference of DCO control code between two consecutive sub-sections. Number of sub-sections determines how many sub-sections in one modulation cycle. COUNT and S decoded from SEC_SEL and
STEP by the modulation controller, respectively. Based on the definitions, the
frequency-spreading ratio equation can be given as% Table 4.1: Jitter and Timing Comparisons of DTM and RDTM
DTM RDTM
Cycle-to-Cycle Jitter (S)
(1+1+1+2+1+3+1+4+ Peak Cycle-to-Cycle Jitter (S) 7 5
*: S times Period of Sub-Section
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where SR is the spreading ratio, RES is the finest time resolution of DCO, and