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IBOC Mask Compliance
A hybrid signal has challenges, but one method simplifies the process.
Measurement of your transmitter's hybrid FM IBOC mask for emission mask compliance is not a monumental task but requires some strategy and attention to a few key parameters. This article will walk through the process and identify those key parameters and give examples of steps throughout the process so the mystery is removed. With more than 1,600 FM hybrid IBOC stations on the air and 207 operating above the nominal -20dB power level, emission mask compliance measurements will become more important.
As a primer, it's important to have some understanding of how a spectrum analyzer (SA) measures digital signals. So a small amount of introduction will first be devoted to this. First, let's show the spectrum to be measured. Figure 1 shows a hybrid FM IBOC spectrum with no modulation on the analog signal.
The analog carrier signal level is a coherent carrier (bandwidth ~100Hz) so its displayed amplitude is not dependent on the resolution bandwidth used. However, as the SA sweeps through the IBOC signal, the amount of IBOC signal energy displayed is dependent upon the resolution bandwidth filter employed because the IBOC signal is essentially noise-like. This can be seen in Figure 1, as the blue trace uses a 1kHz RBW filter and the black trace utilizes a 3kHz resolution bandwidth (RBW) filter. It looks like the IBOC signal with the 3kHz RBW filter has a greater amplitude. Notice also that the noise floor of the SA has also risen due to the same reason. To determine the total power of the IBOC spectrum being evaluated, simply multiply the ratio of the total bandwidth of the spectrum to the RBW being used for display on the analyzer. For example, the power of the lower sideband signal of ~70kHz spectrum of the IBOC signal is 70 times the power of the power displayed using the 1kHz RBW filter (or +18.45dB).
There are a couple of other important factors associated with the SA setup. One is the type of detector selected. Most newer SAs have an RMS detector and this is the detector mode that should be used. If an older SA with a logarithmic detector is used, there will be a 2.5dB error introduced that must be accounted for. The video bandwidth (VBW) filter should be at least 10 times the RBW filter selected so that peaks in the baseband signal are accurately reproduced. If the SA has a channel power (CP) mode function, it enables relatively easy measurements to determine the power of the signal in a given bandwidth, which is a fundamental part of making emission mask compliance measurements. So, a word to the wise, use an SA that has both a RMS detector and a CP function and you will get reliable numbers and will not waste a lot of time.
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