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3.19 C/N Threshold vs Signal Level

The C/N margin for varying signal levels was plotted for all COFDM modulation modes as well as 8-VSB. The plots obtained from this measurement give a visual representation of the C/N and noise figure performance of each of the digital receivers.

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 low signal level.
  3. The minimum signal level was then applied and noise injected using the C/N test set.
  4. The additional noise level was adjusted to find the error threshold with a resolution of 0.1 dB.
  5. The C/N level was recorded and then the signal level was increased by 0.5 dB for COFDM and 0.1 dB for 8VSB.
  6. The measurement was repeated (steps 4-6) until the signal level reached normal operating levels. As the signal level was increased above the minimum signal level the step size of each increment increased up to a maximum of 10 dB steps.
  7. The data was then plotted.

The bit error rates for this measurement were measured using 107 data samples for the initial noise level searches increasing to 109 data samples when within 0.2 dB of the final result for accuracy.


Figure 3.19.1 - DTTB Noise Figure Plot - C/N vs Min Receiver Level

Figure 3.19.1 details the results of this measurement for all the various modulation types. It can be seen that the 8-VSB system performs around the level of COFDM 16QAM 3/4 & 5/6 FEC rates at signal levels above 37 dBuV however below this level 8-VSB degrades around 6 dB earlier than the 16 QAM rates which have comparable C/N thresholds.


Figure 3.19.2 - DTTB Noise Figure Plot

Figure 3.19.2 shows the plots for COFDM 2/3 FEC and 8-VSB annotated with C/N threshold levels for the measurement points. Both systems have a similar degradation with signal level to the 1 dB excess C/N margin point with 8-VSB achieving 35 dBuV and COFDM achieving 34 dBuV. The 8-VSB system degrades with a steeper slope from this point, this is due to it’s higher noise figure. Curves were plotted for varying COFDM guard intervals, however no variation was in performance was observed.

Although the 8-VSB system has a better system C/N against the COFDM system being evaluated the higher noise figure of the 8-VSB receiver offsets this performance gain in the low signal performance area.

 

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