General Technical Information; Pll Bandwidths; Table 7 - Timing Jitter Variation With Pll Bandwidth - DCS 992 User Manual

dCS 992 User Manual
dCS Ltd
G T
ENERAL ECHNICAL

PLL bandwidths

Manual part no: DOC063992 iss 2A1
Contact on + 44 1799 531 999
dCS
(inside the UK replace + 44 with 0)
I
NFORMATION
The lower the bandwidth of a PLL, the more slowly it tracks an incoming signal
and, effectively, the more it averages in the incoming frequency. The lower the
bandwidth, the less the PLL responds to small variations in the input, and the
more filtering it applies. So, jitter rejection should be better as the bandwidth
goes down.
However, all clock sources (GPS, Rubidium, Crystal, etc – everything) have a
small amount of close in phase noise. This produces small but noticeable low
frequency (slow) timing variations. Outside the PLL bandwidth, a slaved clock
cannot track these variations in the input. As the PLL bandwidth reduces, more
of these slow variations remain untracked. Surprisingly, perhaps, this means
that as the PLL bandwidth is reduced, the low frequency timing variations (jitter)
between the input and the slaved tracking generator increase. They may arise
in the source, or in the slaved clock, but they are variations that the slaved clock
no longer tracks.
As PLL bandwidths move down into the mHz area, these timing variations
become significant. If the purpose of the slaving is to match frequencies, and
phase does not matter – they are not important, and very low bandwidths can be
used. If the purpose of the slaving is to also lock phase, there is a limit below
which the PLL bandwidth should not be reduced. This depends on the input
clock, but for two VCXO based clocks, the figures below may be useful.
PLL Bandwidth
(mHz)
1000
250
62.5
Table 7 – Timing Jitter variation with PLL Bandwidth
From these figures, 250 mHz is a good compromise between reasonable low
frequency timing accuracy (±5 nsecs peak) and the modulating signal
suppression of narrow PLL bandwidths. Scope shots showing this data are
given as Figure 24, Figure
250 mHz is the default PLL bandwidth for a dCS 992.
It should be remembered that this jitter is very low frequency, and corresponds
to very slow and gradual timing changes. It is very substantially below the audio
band.
Manual for Software Version 2.0x
Very Low Frequency Timing
Jitter (nsecs, p-p, one unit
relative to another)
25, and Figure 26 below.
Page 54
January 2001
2.2
10
53
file 063992ma2a1.pdf available from website
email to: more@dcsltd.co.uk
web site: www.dcsltd.co.uk