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A technologically economical explanation of NRSC-5 IBOC compliance
After setting your reference level, change the RBW to 1kHz (for the FM band). As I mentioned previously, use the average power (or RMS) detector in the analyzer, and let it sweep at least 100 times (leave the sweep speed on auto) and for at least 30 seconds.
What can we expect to see for results?
Another common feature in newer analyzers is the ability to set "limit lines" on the display to quickly judge whether or not the emissions you measure are in compliance. Take a look at Table 1 to see those limits – keeping in mind those refer to 1kHz RBW on the analyzer.
|Frequency Offset relative to carrier (kHz)||Level relative to carrier (dBc/kHz)|
|100 - 200||-40|
|200 - 250||(-61.4 - [freq. in kHz - 200]) × 0.260|
|250 - 540||-74.4|
|540 – 600||(-74.4 - [freq. in kHz - 540]) × 0.293|
If your analyzer meets all the other requirements mentioned herein but won't accommodate limit lines, then when printing out the results (or otherwise generating a hard copy of the results) you'll need to add the limit lines yourself.
During the measurement process, I first look at a span of 500kHz. I then go up to 1MHz, and finally 2MHz. See Figures 4, 5 and 6 for examples.
By making your input reference level as close to one of the 10s as possible (like -10dBm) and then adjusting the span so that each division is a multiple of 100kHz you'll find evaluating your performance with respect to the mask is made easier.
IBOC measurements are easy to do if you have the correct instrumentation. Make notes on the particular details of how it is done in your case, and file the results in a well-known spot (one you won't forget about between measurements) and you'll have gotten one more of your job functions completely under control.
Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at email@example.com.
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