Section 3.15

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3.16 Transmitter Shoulder performance

The shoulder performance of the two transmitters was measured running similar power levels on both transmitters. This provided an indicative shoulder performance and out of band emissions of real on-air operation during the field testing phase.

The transmitter was run with a COFDM signal at the nominal power level indicated on the forward power meter into a dummy load. A HP-70000 spectrum analyser was connected to the test port to observe the channel 8 RF signal with a span of 25 MHz so that the upper and lower adjacent channel space was visible. The video bandwidth of the spectrum analyser was set to 3 kHz to provide an averaged noise spectrum and the reference level set so that the centre of the Digital signal was aligned with the top graticule. The scale was adjusted to 5 dB/div and a delta marker used to indicate the centre channel and upper channel limit positions corresponding to the shoulder measurement.

The spectrum analyser trace was stored in memory and spectrum traces repeated for drive levels of ±3 dB about the nominal output level. The attenuation between the test port and spectrum analyser was adjusted in each case to keep the centre of the digital signal co-incident with the top graticule providing a common reference point for the spectrum comparisons. When all three spectrum plots had been stored they were all simultaneously displayed and plotted. The shoulder was measured as the smallest step from the centre channel level to the highest edge of channel position to the nearest dB.

Figure 3.16.1 and Figure 3.16.2 show the shoulder performance of each of the DTTB systems (violet COFDM & blue 8-VSB) operating at 200 W through each transmitter. These measurements were taken with a 4% bandwidth tunable telonic filter on the transmitter output. This filter was used to simulate adjacent channel filtering that would be applied to normal Australian transmission sites. Figure 3.16.3 through Figure 3.16.6 show individual transmitters and modulation types with (blue) and without (black) this filter. Table 3.16.1 details the measurement of these shoulder levels. It is obvious that without the output filter there are out of band emissions extending into the adjacent channels which may cause problems for adjacent services in some situations.

Note the Harris transmitter is operating over 4 dB below its normal power level during this measurement.

Figure 3.16.1 - Shoulder Plots for 200 W DTTB through NEC Tx

Figure 3.16.2 - Shoulder Plots for 200 W DTTB through Harris Tx

Figure 3.16.3 - Effect of Output Filter on COFDM thru NEC Tx @ 200 W

Figure 3.16.4 - Effect of Output Filter on COFDM thru Harris Tx @ 200 W

Figure 3.16.5 - Effect of Output Filter on 8-VSB thru NEC Tx @ 200 W

Figure 3.16.6 - Effect of Output Filter on 8-VSB thru Harris Tx @ 200 W

Section 3.17

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