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3.5 C/N Threshold

The carrier to noise threshold is one of the basic system measurements that is repeated during most of the test program measurements. It forms the basis of determining the margin above the error threshold that the system is operating. The carrier to noise threshold was measured using a HP3708 C/I test set that was inserted in the DTTB signal path before the D/U combiner.

The test set measures the power of the incoming DTTB signal and then mixes in broadband white noise to achieve the set carrier to noise ratio. The broadband noise source is 200 MHz wide allowing the measurements at channel 8 to fit within the baseband capabilities of this instrument. The test set determines the broadband noise level to apply by measuring the applied signal power and by being told what system bandwidth is being assumed. For the testing of both DTTB systems a system bandwidth of 7 MHz was selected. Although the 8VSB system is a 6 MHz system during the testing we regarded each system as a black box that was to be accommodated within a nominal 7 MHz band plan. To scale the C/N measurements from a nominal 7 MHz to 6 MHz bandwidth a 0.7 dB correction can be applied to the absolute C/N measurements.

Test procedure for C/N Threshold was

  1. Set C/N set to a high C/N above 50 dB
  2. Set wanted signal level to 27.6 dBm (0 dB)
  3. Set C/N step size to 12.8 dB decreasing
  4. Measure receiver error rate
  5. If error rate is less than threshold then step C/N 12.8 dB
  6. Measure receiver error rate
  7. If error rate has not crossed threshold then step C/N repeat step 6
  8. If error rate exceeds threshold then
    halve C/N step and reverse step direction
  9. Repeat from step 6 until step size is less than 0.1 dB
  10. Compare final and previous measurements and
    report C/N level closest to the threshold error rate.

DTTB System

COFDM

8-VSB

Test Rig Beginning (7 MHz)

19.15 dB

14.3-14.4 dB

NEC Tx 180 W

19.3 dB

14.5 dB

Harris Tx 180 W

19.2 dB

14.5 dB

Harris Tx 600/900 W

19.9 dB

14.3 dB

Test Rig End (7 MHz)

19.4-19.6 dB

14.35 dB

Test Rig (6 MHz)

NA

15.1 dB

Table 3.5.1 - Summary of DTTB C/N Thresholds

Table 3.5.1 above summarises the C/N Thresholds that were measured for each system during the majority of the measurement program.

During the main COFDM system tests the C/N threshold remained at around 19.2 dB. Sometime after the Parliament house demonstration, which occurred in May 1997, a system software upgrade and re-earthing of the tuner area to stabilise the receiver noise figure the C/N threshold increased by up to 0.4 dB. None of these events can be directly attributed to this change however it may be due to a combination of these factors.

An 0.1 to 0.2 dB increase in the C/N threshold was observed for measurements where the transmitters were operating. The Harris 600 W COFDM case was an exception to this. In this case a 0.7 dB shift was observed. The Harris 600 W COFDM measurement was done during the early COFDM testing before the Harris transmitter pre-equaliser had been optimised.

A measurement of the 8-VSB C/N threshold with the test set programmed for 6 MHz operation is included to confirm the 0.7 dB theoretical shift between the 6 MHz and 7 MHz channel cases.

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