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Actually, we evaluate nonlinear distortion, which is integrated in parameters : THD … Total Harmonic Distortion IMD … Inter-Modulation Distortion SINAD … SNR plus Distortion THD + N... THD plus Noise SFDR … Spurious Free Dynamic Range In addition, in DMT signal area: MTPR... Multi-Tone Power Ratio SNR... Signal-to-Noise Ratio… we know this already - all parameters are in units [dB] (logarithmic area) or clear number (ratio) or in [%]
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DSL lines diagnostics
- Parameters of analog part of xDSL transceiver
Access Networks 2010/11
Ľ. Maceková, KEMT- FEI – TU - Košice
Describing of DMT in time domain (DMT is transport method in ADSL and VDSL systems):
2
1
)cos()( 0
n
nkkk tkUtu
Uk ... amplitude of k-th carrier (it is modulated by QAM)
φk ... phase of k-th carrier
ω0 = 2π. 4312,5 s-1 ... angle speed
Fig. Typical time domain DMT signal look with limiting areas (signal mustn’t exceed their boundaries)
...
limiting areas
Actually, we evaluate nonlinear distortion, which is integrated in parameters :
THD … Total Harmonic DistortionIMD … Inter-Modulation DistortionSINAD … SNR plus DistortionTHD + N ... THD plus NoiseSFDR … Spurious Free Dynamic Range
In addition, in DMT signal area:MTPR ... Multi-Tone Power RatioSNR ... Signal-to-Noise Ratio… we know this already
- all parameters are in units [dB] (logarithmic area) or clear number (ratio) or in [%]
THD … Total Harmonic Distortion
P1; f P1, P2, P3, ...
power of higher harmonics; result of devices nonlinearities
(because of nonlinear transfer characteristics of amplifiers) W] W,[dB; log10
1
2
P
PTHD k
k
power of basic frequency signal
We measure all higher harmonics which can be distinguished from noise
inputoutput
5
IMD … Inter-Modulation Distortion
V] V, [dB;
UUlog20
2
1212
2
2ff
f
knfnf
UIMD
- it is product of nonlinearities of transfer characteristics when actuated by two frequencies signal
- it is sum of intermodulation products of 2 input harmonics (sums, differencies and multiples)
f1,f2,; f2>f1 f2±f1, f2 ±2f1,f2 ±3f1,...
- the measurement: by modulation method or by differencial method [1]
input output
6
SINAD – SNR plus Distortion
W] W,[dB; log10 in
distnoise
distnoisesig
PPPPP
SINAD
- parameter which contains the noise and nonlinear distortion
total output signal
Psig in Psig, P2, P3, ..., noise
distortion
power output signal after filtration of input signal
7
SFDR – Spurious Free Dynamic Range
W] W,[dB; log101
max
PPSFDR IM
- it is about nonlinear properties of devices again
- they measure partial harmonic elements and another distortion elements; we are interested in distortion element with highest amplitude
power of the distortion element with highest amplitude
power of basic signal (1-st harmonic frequency) (measurement signal)
[1]
(hence SFDR < 0 dB)
8
MTPR … Multi – Tone Power Ratio- this parameter is established for DMT
average subchannel power (applied; transmitted)
max. power of distortion signal in the position of unused subchannel (‘empty bin’)
[1], [2]
W] W,[dB; )max(
log10k
T
PPMTPR
… and another measurements:
-measurement of impulse noise robustness
-measurement of ADSL splitters
-measurement of influensis of ADSL in voice (telephone) band
- etc.
References:[1] J. Vodrážka, M. Havlan: Přístupové přenosové systémy, Cvičení – Měření na přípojkách xDSL, ČVUT Praha, 2003.[2] http://www.analog.com/static/imported-files/tutorials/MT-053.pdf