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VDSL Emission Measurements at BT Adastral Park

VDSL Emission Measurements at BT Adastral Park. 2-6 April 2001 A collaborative project by RA & BT Radiocommunications Agency. VDSL Spectrum Allocation - Plan 998. Four frequency blocks used in two pairs Downstream: .138 to 3.75 MHz Upstream: 3.75 to 5.2 MHz

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VDSL Emission Measurements at BT Adastral Park

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  1. VDSL Emission Measurements at BT Adastral Park 2-6 April 2001A collaborative projectbyRA & BTRadiocommunications Agency

  2. VDSL Spectrum Allocation - Plan 998 • Four frequency blocks used in two pairs • Downstream: .138 to 3.75 MHz • Upstream: 3.75 to 5.2 MHz • Downstream: 5.2 to 8.5 MHz • Upstream: 8.5 to 12.0 MHz

  3. BT VDSL Spectrum - Plan 998

  4. BT Plan 998 VDSL Deployment • 1.2 to 3.3 MHz – VDSL downstream • 4.0 to 5.0 MHz – VDSL upstream • 5.8 to 7.8 MHz – VDSL downstream • 9.5 to 11.0 MHz – VDSL upstream

  5. VDSL Equipment Under Test • One pair of ,998 compliant, four band VDSL modems with a nominal power Spectral Density of –60 dBm / Hz • The PSD was not measured due to uncertainty over whether power backoff for short lines was enabled • It is believed that the modems did have power backoff enabled but the nature and extent of this feature was not established

  6. BT’s Test Network • Six physically identical test routes with each route comprising an overhead section of two-pair dropwire connected to a multi-pair underground cable giving twelve test circuits. • The end to end length of each circuit was about 250 metres • All twelve circuits were characterised by BT and the results are expected shortly

  7. Measurement Considerations • Measurements were made using a peak detector using the standard 9 kHz CISPR bandwidth • To facilitate regression measurements tuned loop antennas with antenna factors approximately 30 db lower than a standard CISPR loop were used • These sensitive loop antennas are manually tuned so swept measurements were not possible and two‘quiet’ measuring frequencies were selected within each of the four BT VDSL frequency bands

  8. VDSL Measuring Frequencies • 1.65 & 3.10 MHz – in the downstream band • 4.03 & 4.83 MHz – in the upstream band • 6.55 & 6.98 MHz – in the downstream band • 9.64 & 10.5 MHz – in the upstream band

  9. Radiated Measurement Issues • Field strength regression over a 1 to 10 metre range using both loop and dipole antennas • VDSL radiated emission levels and the degree of variation from apparently identical wire pairs • Effect of POTS overlay and operation on VDSL radiated emission levels

  10. 1 to 3 Metre Field Strength Regression Using Loop Antenna • Text book value (20 log d1/d2) = 9.5 db • Variation in measured values up to 20 db • Results of 24 measurements at frequencies between 1.6 & 11 MHz gave an average 1 to 3 metre regression of 7.6 db

  11. 1 to 10 Metre Field Strength Regression Using Loop Antenna • Text book value (20 log d1/d2) = 20 db • Values between 4.5 and 21.2 db were measured using a single wire pair over a range of 8 frequencies between 1.6 and 11 MHz • The average value obtained was 15.6 db

  12. Apparent Field Regression From a 3 Metre Loop Measurement to a 10 Metre Dipole Measurement • Textbook value for 3 to 10 metre regression using loop antennas is 9.5 db • Tests at 3 frequencies in the 3.5, 7 and 10.1 MHz bands using a single wire pair with a loop for the 3 metre measurement and a dipole for the 10 metre measurement gave values between 6.4 and 23.2 db • The average value obtained was 17.2 db

  13. VDSL Radiated Emission Levels • Significant variations were noted at all 8 test frequencies • Single frequency spread between the two pairs in a single dropwire averaged 5 db but was as much as 18 db • Single frequency spread between apparently identical dropwires averaged 17 db but was as much as 23 db

  14. Effect of POTS Overlay on Emission Levels • It was found that VDSL overlaid on a working POTS line radiated no more than if the same line was used in isolation • This was unexpected and it is conjectured that this may have due to BT using a non-standard POTS termination at the ‘exchange’ whereby both legs of the pair were floated above earth • This issue is currently under investigation by BT

  15. Observations on VDSL Emission Levels • RA & BT have agreed that until the uncertainty surrounding the modem PSD and representative nature of the POTS ‘exchange line are answered it would be pointless to publish emission levels • The large emission level variations encountered are well in excess of the power sum addition to be expected from the cumulative emissions from 12 pairs of modems

  16. Variations in Radiated Emission Levels From Apparently Identical Circuits – Contributory Factors ? • Variations in: • Circuit balance – common mode current • Inductance of the twisted pairs and hence its choking effect on the common mode current • The electrical length – common mode current maximum displacement between different pairs • Crosstalk coupling factor to adjacent pairs

  17. Outstanding Issues to Resolve • The PSD of the BT test VDSL modems together with the nature and extent of any power back off employed • The representative nature of the POTS line used during the tests in respect of the exchange end earthing arrangements • Examine the BT line characterisation results and establish whether these can account for the large spread in VDSL radiated emission levels seen

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