IBOC Mask Compliance
A hybrid signal has challenges, but one method simplifies the process.
Table 2 shows an example of an NRSC emission mask spreadsheet with measured results. Where did those numbers come from? The top of the spreadsheet takes steps to ensure that your SA has its noise floor below where you plan the weakest signal measurement. The noise floor can be found by removing all signals to the SA, terminating the SA input with 50ω load, setting the RF attenuation to zero, the reference level to approximately -40dBm, the RBW to 1kHz , the detector mode to RMS, and observing the noise floor in dBm of the SA. Now add a minimum of 80dB to it and that will equal the minimum RF sample level for the analog power reference. If the analog RF sample from the directional coupler is more than that number, then your SA is capable of making the measurement from a dynamic range point of view. To remember this, take the SA noise floor (1kHz RBW) + 80dB + measurement headroom (reasonable value is 10dB) and that is what the transmitter analog RF sample power should be. The same equation applies for combined or separate transmitters but remember to add the attenuators of the CHIMP.
Next measure the analog total power reference with an unmodulated carrier. This was found to be 0.0dBm in the example case. Then apply modulation to the analog transmitter (the digital transmitter is generating its spectrum with or without modulation applied). The recommended modulating signal for the analog transmitter is a 1kHz tone with the transmitter modulation at 75kHz deviation. If the transmitter is equipped with linearity correction, now is the time to engage and optimize it. Once that is done, remember to double check the analog power reference and the digital power ratio to make sure they are correct. The next step is to measure the power in the IBOC primary spectrum defined by the NRSC mask (e.g. +100 to +200kHz) at the frequency midpoint of the required mask level (i.e. +150kHz from Fo - the carrier frequency). The total measurement bandwidth is 100kHz (200-100), so that number is used to set up the channel power measurement (CP) mode of the SA. In the case below, the recorded power is -26dBm. That is the power in a 100kHz bandwidth matching the first line of NRSC mask as listed in Table 1. This measurement reflects the "plus" side of the center frequency so the measurement needs to be repeated for the "minus" side of center frequency. The next mask section starts at 200kHz and goes to 250kHz so the measurement frequency is the midpoint, or 225kHz, and the CP bandwidth will be set at (250-200), or 50kHz. In this case, the value measured was -65.2dBm. Because the mask limit line is sloped at the measurement frequency, the limit value must be calculated using Table 1 to determine the pass/fail threshold value. Again, the same measurement should be made for the "minus" side of Fo. The next section of the mask goes from 250kHz to 540kHz so the measurement point is the midpoint, or 395kHz + Fo, and the bandwidth is 290kHz so the SA CP mode is reconfigured for 290kHz, the value measured was -61.4dBm.
This process is repeated until all measurements have been made. As long as all measurements were at least 10dB above the SA noise floor, good engineering practice would indicate that the noise floor has not impacted the data and no correction for measurements near the noise floor is necessary. If the noise floor is 6dB or less compared to the measured level, good engineering practice would dictate that the noise floor power should be subtracted from the measured value. Remember that the only way to do this is to find the equivalent linear value power (not dBm) of the measured value and subtract the equivalent linear value noise floor power from it. Then convert the net power back to a logarithmic (dBm) value. If you find that the measured value is equal or lower than 3dB of the noise floor, just use the value that is equal to 3dB above the noise floor measurement. The reason for this is that it is uncertain which is being measured - signal or noise. Plus, whenever a very small number is subtracted from another very small number, the accuracy of the measurement is in jeopardy.
- continued on page 6
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