Digital Signal Processing Reference
In-Depth Information
Although it is beyond our scope here, we note that the hardware measurement
step can be quite extensive. For example, statistical techniques may be employed
to extrapolate the measured behavior for the purpose of validating that the design
will meet the timing requirements at the extremes of the manufacturing process
[Norman, 2003].
13.4 NOISE SOURCES AND BUDGETS
As discussed in Chapter 12, voltage noise limits the maximum achievable per-
formance of a high-speed signaling system by reducing the signal-to-noise ratio
according to the Shannon-Hartley theorem:
1
B
=
2
log
2
(
1
+
SNR
)
(12-3)
In addition, bit error rates are also a function of noise on the signal [Buchs et al.,
2004]. Following an approach similar to the one that we used to develop the
BER as a function of timing jitter, we can produce the following equation, which
expresses bit error rate as a function of deterministic and random noise sources:
0
.
5 erfc
v
−
DN
max
/
2
√
2
σ
RN
BER
(v)
=
(13-21)
where DN
max
is the maximum deterministic noise (V) and
σ
RJ
is the root-
mean-square Gaussian noise (V). Finally, voltage noise is a chief source of tim-
ing noise (jitter) and therefore limits the maximum data rates that we can achieve
in practice. We describe the main sources of voltage noise in the next section.
13.4.1 Noise Sources
The primary sources of noise in digital signaling systems include crosstalk, inter-
symbol interference, supply noise, circuit input offsets and resolution, thermal
noise, and shot noise. We discuss crosstalk and ISI extensively elsewhere in the
topic, so we focus on the remaining sources in this section.
Supply Noise
As discussed in Chapter 10, power supply noise as it applies to
high-speed signals arises from two sources. The first is externally generated sup-
ply noise, which occurs in the distribution system between the voltage regulator
and the local I/O circuits. This external supply noise is typically on the order
of 5 to 10% of the nominal supply. Current-mode transmitters typically have a
large-impedance connection to the power supply, effectively isolating them from
the supply noise.
The second source is local supply noise generated by transient current in the
I/O circuits. Because it depends on the transient current demand according to
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