Figure 4. FM IBOC transmission mask. Click to enlarge.

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Figure 4 shows the FM IBOC NRSC mask and Figure 5 shows the comparison, at least in a graphical way, to the FCC emission mask. The documents associated with both masks are important to read and understand before doing the compliance measurements. Table 1 provides the NRSC emission mask limits expressed in dBc/kHz values.

Figure 5. Comparison of FM hybrid mask and FCC mask. Click to enlarge.

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The basic transmission system and setup of an emission mask measurement task is shown in Figure 6. For discussion, first assume a combined configuration is used. The hybrid IBOC signal is sampled using a directional coupler after all transmitter filtering to observe the signal. As seen by the diagram, measurements can be made into dummy load or the antenna. In general more reliable measurements are made into a dummy load.

Figure 6. Measurement setup for combined transmitter operation. Click to enlarge.

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Measurement method

If you look back at the separate transmitter configuration in Figure 3, there is no place to monitor the combined signal, only the independent analog and IBOC signals. So what happens if you happen to be one of the lucky people using separate transmitters? Necessity is the mother of invention, and a simple but great idea was introduced: By utilizing a Combined Hybrid IBOC Measurement Package, known as the CHIMP, it essentially makes a combined transmitter measurement possible even though separate transmitters are used. As shown in Figure 7, a directional coupler sample from each transmitter with attenuation to isolate the signals from each other and to ensure the proper ratio is used. Those signals are fed to a 3dB hybrid and the output result is the FM IBOC hybrid signal. This allows accurate measurements that can be made as if a combined transmitter were used. The separate transmitter measurement configuration using a CHIMP is shown in Figure 7.

Figure 7. Measurement setup for separate transmitter configuration. Click to enlarge.

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- continued on page 4