Sifos Technologies PhyView PVA-3000 User Manual

PVA-3000 Reference Manual
December 2, 2019 Sifos Technologies
Page
15
1.3.4. Signal-to-Noise Ratio
Signal-Noise Ratio (SNR) is a measure of residual distortion on a transmitted signal. Residual distortion refers to non-
correctable distortions that occur either because of imperfections in a transmitter or because of problems in a link.
Examples of non-correctable distortion include amplitude noise (or alien crosstalk), phase noise (or severe jitter),
clipping, signal compression (or non-linear gain/loss), and severe inter-symbol interference (ISI) that would occur
given non-linear or notch-like filtering.
Because residual distortion is not correctable, it will for certain adversely affect bit error rate and packet flow in a
receiver. SNR is reported in units of dB, that is, the power of the signal in ratio to the power of non-ideal or distortion
components. 36 dB SNR would mean that the ratio between true signal power and distortion components is 10
(36/10)
=
3981. (Note: SNR = 10*LOG[P
SIGNAL
/ P
NOISE
]).
Modern, DSP-based twisted pair PHY receivers are
designed to correct for predictable distortions
including the spectral shaping done in 1000BaseT
and the effects of insertion loss and linear phase
shifting that are characteristic of transmission
media, namely cabling. The PSD measurement
discussed earlier in Section 1.3.3 is designed to be
sensitive these types of impairments or distortions
and will be affected by both “correctable” and
certain residual distortions. To the extent that a
receiver successfully compensates the predictable
distortions, the bit error rate, and ultimately packet
flow, is unaffected by the presence of those
distortions.
One special case of residual impairment is jitter.
While jitter is random, it is to a certain extent, correctable. Receivers are designed to acquire signal timing from the
received signal and thus have some ability to track out, or nullify, randomly varying signal timing. That ability is very
much receiver implementation dependent and will have limitations both in jitter magnitude and jitter rate. Lower
magnitude and lower rate jitter is easily tracked out while high rate (or high frequency) jitter may easily flow through
the receiver and turn up as residual noise. This is why the 1000BaseT standard places a 5KHz bandwidth on the
specification of peak-peak jitter.
In 1000BaseT, receivers have to deal with a special form of quasi-random noise in the form of echoed transmitted
signals and local crosstalk from adjacent transmitting pairs. 1000BaseT receivers are designed to use matched filters
to remove these quasi-random interferers, however, to the
extent that those filters are not perfect or that the hybrid echo
cancellation is not perfect, those interfering signals will show
up as residual noise and affect the bit error rate. While the
PhyView Analyzer as a measuring instrument compensates for
this internal error, this effect also places an upper bound on the
maximum SNR (36 dB) that the meter can report.
The relationship between SNR, Bit Error Rate, and Packet Flow
will generally be design implementation dependent. Figure
1.10 diagrams this using numbers that are realistic but show the
spread of potential performance. From this diagram, it is
evident that 100BaseTx is more tolerant of lower SNR than
1000BaseT in general. It is also evident that SNR in the 30’s
will seldom result in any packet loss at any receiver while SNR
in the low 20’s is a problem in many implementations for
1000BaseT and in the mid-teens will likely be a problem for
100BaseTx on most implementations.
Finally, it should be noted that low transmit signal amplitude will adversely impact measured SNR. This relationship
is essentially in the definition of SNR since lower signal power means that the ratio of signal power to ambient noise
power must decline. This fact should be considered when creating impaired receiver tests as will be discussed
elsewhere in this manual. It also means that SNR measurements can be artificially reduced simply by adding fixed loss
in line with a transmitter-under-test.
Jitter,
Severe ISI
Crosstalk/Noise
Clipping/Compression
Severe ISI
Alien Crosstalk
Severe ISI
Jitter,
Severe ISI
Crosstalk/Noise
Clipping/Compression
Severe ISI
Alien Crosstalk
Severe ISI
Figure 1.9 Sources of Residual Distortion
Figure 1.10
SNR vs Packet Flow

Sifos Technologies PhyView PVA-3000 Specifications

General

Brand	Sifos Technologies
Model	PhyView PVA-3000
Category	Measuring Instruments
Language	English