IBOC Mask Compliance
A hybrid signal has challenges, but one method simplifies the process.
The NRSC emission mask limit values in Table 1 are all referenced to a 1kHz measurement bandwidth. But the measurements in the spreadsheet were made in different bandwidths (100kHz, 50kHz, 390kHz, etc.). To convert these measured values to a 1kHz bandwidth for comparison, it is necessary to scale the measured powers. To do that, simply add 10 log * (1kHz/measurement BW) in dB to each measured power to get the values in a 1kHz bandwidth. This scaling will reduce the amplitude values.
Once each measured value is corrected for measurement near the noise floor and for bandwidth, it is subtracted from the analog power. In this example, the power was 0dBm. So 0.0dBm - (-46dBm) yields a value of 46dB. This can be compared to the mask value of 40dB. Since the value is greater than the mask limit, the waveform passes the mask limit. To complete the rest of the spreadsheet for the entire IBOC channel measurement and comparison to the mask, the same process is followed.
Increased digital levels
The above analysis reflects the case for when the standard -20dBc IBOC digital power is used. Now that the FCC has authorized blanket power level increases to -14dBc, you might ask how that affects the mask value. The only mask change (refer to page 16, Table 6 of NRSC G202, at the NRSC website) is with respect to the area of spectrum containing the primary IBOC sidebands. Instead of being 40dB, the limit is decreased 1dB for every 1dB increase in operating power. For example, if the digital power is -17dB relative to the analog power, then the first limit of the mask would be 37dB instead of 40dB. Although the FCC has only authorized experimental FM IBOC stations for operation with asymmetric sidebands, NRSC G202 also addresses how to make these measurements. The purpose of using asymmetrical sidebands is to maximize digital reception while not causing interference to a lower or upper adjacent channel. Figure 9 shows an example of asymmetric sideband operation on an SA.
If you operate a station with asymmetric sidebands, the process is essentially the same with a small twist. The twist is to first determine the total digital power relative to the analog power. NRSC G202 provides a table for doing this for various degrees of sideband asymmetry. Once that detail is known, then the emission mask is measured as if symmetric sidebands of the total digital power were used. For example, assuming that only MP1 mode is used, and one sideband is at -20dB and the other sideband is at -17dB, Table 2 identifies the total digital power relative to the analog carrier as -15.2dB. From there, the emission mask compliance measurements are conducted as if the total digital power was -15.2dB relative to the analog power.
This is quite a bit of information to absorb in one reading, but keeping the key points in spectrum analyzer use and focusing on the process steps will help guide you through these measurements on your own. Hopefully this information will allow you to approach measurement of your hybrid FM IBOC station for emission mask compliance with some confidence. Keep resources at your disposal so that accurate results can be achieved, and digital and analog stations will continue to operate without interference.
Best is president of Greg Best Consulting, Inc., Kansas City.
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