The sample point
The place to take measurements will be on the transmission line that carries both the analog and IBOC carriers. In the event that you use space-combining, or separate antenna feeds, the situation is a bit more complicated. We'll discuss those particulars shortly.
Figure 1. Test equipment setup for a single transmission line system
When both types of carriers share the transmission line, you'll obviously use a line section downstream from a high-level or split-level combiner – or simply on the output side of a combined amplifier. Insert an RF sample slug (such as a Bird 553-75) in the line section. The sample level on that particular slug is -55dBc – which seems like quite a bit of attenuation. But, even with a sample at 55dB below the carrier level, you've still got lots of power (relatively speaking). For example, if your TPO is 10kW – otherwise known as +70dBm, 55dB below that is still +15dBm, which is likely way too much power for the input of your analyzer to handle. Be prepared with at least 20dB of outboard attenuation and connect that to the input of your analyzer before even connecting your sample line! I use outboard attenuators in series – typically a couple of -10dB, a single -6dB, a single -3dB and -2dB. Put them physically as close to the analyzer input as possible – thereby minimizing the chances of RF pollution getting into leaky coaxes that may otherwise be used to connect the output of the attenuators to the input of the analyzer. See Figure 1 for more clarity.
Figure 2. Combined hybrid IBOC measurement package (CHIMP) sampling FM signals from separate transmission lines
If your analog and digital transmitters do not share a common transmission line, then the measurement technique is more involved. NRSC-G201-A has a section about CHIMP, otherwise known as the Combined Hybrid IBOC Measurement Package, and it's a very straightforward method for measuring mask compliance in this circumstance. (Randy Mullinax of Clear Channel developed it in 2006.) Basically the technique involves the addition of the analog sample, and the IBOC sample, by means of small combiner (such as a Mini-circuits ZFSC-2-2) that subsequently drives the analyzer input. (See Figure 2.) The reference levels are carefully set (more on that below) and external attenuation is used to ensure that the analog sample is 20dB higher than the digital sample. Once this is accomplished, the spectrum display is equivalent to what you would see if the two signals shared one transmission line.
Taking the measurement
As mentioned earlier, when taking the IBOC measurements, you'll make use of the average power (or RMS) detector in your spectrum analyzer. Set the RBW for at least 300kHz for the reference set – that way you'll be telling the analyzer to include all the RF from your analog carrier – regardless of the modulation. NRSC-G201-A recommends removing all sub-carriers and stereo pilot for this measurement, and modulating the carrier at 100 percent with a 1kHz tone, by the way. The reasons given are twofold: The results are more repeatable and demanding against the mask limits than either pink noise or normal program audio. Obviously this will require the station to be off-air in the middle of the night. (I prefer not to work in the middle of the night unless it just can't be avoided.)
Figure 3. Setting the reference: get the trace as close to the top of the display as possible.
With the RBW set at 300kHz, and external attenuation in place ahead of the analyzer input, connect the sample signal, and take a look at how much power is making it into the analyzer. (Let the analyzer average out this power reading as well – set the reference level after 15 or 20 sweeps). A good rule of thumb is aiming for 10dB below the 1dB compression point. If the input signal is too low, remove some of the external attenuation until you get a level you can easily work with, and change the attenuation in the analyzer (1dB steps) to get the trace as close to the top of the display as you can. (Figure 3 shows an example analyzer display.) Strive to get the power reference right at a value that's a multiple of 10 (like -10dBm) because this will make it a bit easier later on to evaluate your performance with respect to the mask.
Setting reference levels while using CHIMP is a little more involved. First, using the output of the little power combiner, set your analog reference level with the sample with the IBOC signal removed. Make a note of the level. Then take a look at the IBOC-only level, with the analog signal removed. Add or subtract attentuation on the analog sample input to the combiner so the two samples, after being combined, hit the input of the analyzer exactly 20dB apart in level.
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