A technologically economical explanation of NRSC-5 IBOC compliance
In April of this year the NRSC released NRSC-G201-A, which discusses in detail the NRSC-5 IBOC emissions mask and recommended methodology used to measure compliance. Many IBOC systems have been up and running for five years now (or more) and so re-measuring compliance with the RF mask, while making use of the recommended methods, represents good engineering practice.
This is a rather long document -- 102 pages. Although I recommend reading the entire thing yourself, the purpose behind this article is to concentrate the information found within NRSC-G201-A, and to present the methods described therein in a technically economical fashion. I'll discuss the following aspects of the measurement methodology:
The measuring instrument
An RF spectrum analyzer is obviously the measuring instrument of choice. While older units can be used, it is necessary to check their specifications and abilities to see whether or not they are appropriate for the job at hand. Unfortunately some of the old “classic” analyzers (the Tek 2710 comes to mind) are not up to the task.
When looking at the analyzer's specifications, note especially the "1dB compression point" as well as the DANL (displayed average noise level).
The 1dB compression point is important because if you exceed this level of power into the first mixer, you could not only damage the unit, but you'll generate intermod products that could possibly show up in the displayed spectrums. It's also critical to consider all the RF sources that make up the sample. If you were to sample on the wideband output of the combined system (for example) you would need to consider the total power from all carriers heading out to the antenna.
On the other hand, the document recommends (at minimum) that the DANL be at least 10dB below the lowest mask limit. So, as the instrument user, you need to be sure that you have a strong enough sample signal so that you don't bring the analyzers' own noise floor up too high in the reading – otherwise the lowest limit of the emissions mask (-80dBc) will be buried by the instrument's own internal noise.
The final line under this heading in the NRSC document reads as follows: "There is no better way to evaluate the instrument than to try it out on signals whose performance is already known." I take this to mean that if the DANL reads low enough, based on a given RF sample level, and that the instrument's capabilities are correct, then the unit is good enough to take measurements.
Resolution bandwidth (RBW) is one of the most important parameters you will set during your measurement. In this document, the standard RBW used for measuring IBOC in the FM band is 1KHz; in the AM band, 300Hz. The type of RBW filter also affects the readings slightly (check your analyzer specs again). Subtract 0.5dB from your readings if the filter type is 4-pole analog; subtract 0.24dB if the filters are Gaussian/analog. Video bandwidth, can be turned off, or as a rule of thumb, set for 10 times that of the RBW.
The detector type in use by the analyzer is important. It is recommended in this document that the detector be average power (or RMS) type. Again, consult the analyzer specs.
And finally, the number of sweeps taken by the analyzer in the collection of the data should be 100. To comply literally with the recommendations, you must measure at least 100 sweeps and extend the collection time to at least 30 seconds.
So a quick review of our basic spectrum analyzer specs and setup:
-- continued on page 2
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