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3.17 BER vs Signal Level

A measurement of the BER vs minimum signal level characteristic was performed using the following procedure for both DTTB systems. As outlined in section 3.18 the curves for the two systems are not directly comparable and separate parameters were used during the measurement to allow for the differing system error measurement points.

The measurement was performed as follows:

  1. The DTTB receiver had its signal supplied from the test rig.
  2. A Minimum signal level measurement (Section 3.6) was made to determine the threshold error level due to signal level.
  3. The signal level was then decreased below the threshold level by:
    5.0 dB for COFDM and 2.0 dB for 8-VSB
  4. The Bit Error Rate was measured in 109 data samples for accuracy.
  5. The signal level was increased in the following increments.
    0.5 dB for COFDM and 0.1 dB for 8-VSB
  6. The measurement was repeated (steps 4-6) until the signal level exceeded the original minimum signal threshold by:
    5.0 dB for COFDM and 0.5 dB for 8-VSB.
  7. The data was then plotted.


Figure 3.17.1 - BER vs Receiver Signal Level for DTTB

Figure 3.17.1 shows the performance of the DTTB systems with the rig and both transmitters around the 180 W output level.

These plots are typical of the normal performance measured. During the course of the laboratory testing the COFDM VHF receiver had some variation in it’s minimum signal performance which was isolated to the tuner input section. Some additional chassis earthing around the tuner was installed and it is also suspected there may be an intermittent RF input cable. The variation observed caused a degradation of up to 2 dB in tuners front end performance.

Figure 3.17.1 also shows the cliff edge output performance of the DTTB systems. This is demonstrated in the 8-VSB plot but will be similar for both systems data output.

The failure point of the 8-VSB system occurred at around 2 dB higher signal level than the COFDM system.

 

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