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