The debate on the efficacy of the required NRSC measurement continues. The author is chief engineer for the Cumulus Radio Station Group in Colorado Springs, Colo.
I have been told I was incorrect when I stated that the NRSC occupied bandwidth measurement does not include harmonic, intermod and spurious tests. The following is an explanation of these tests and why I was accurate. I will conclude with a discussion as to why these four yearly tests should be put to bed as Larry Langford had suggested.
Today, most engineers and companies making the required yearly AM compliance tests simply refer to them as the NRSC Test, including me. This is a little bit misleading. The yearly AM transmitter tests are a series of four RF tests consisting of the NRSC mask (occupied bandwidth), harmonic, intermod and spurious. I look at these tests as to what a transmission system can do and what it can’t do.
The first one, the NRSC occupied bandwidth test is a measure of what the transmission system (audio modulation) can do. This measurement was instituted to address the second adjacent interference problem. This is the measurement I was referring to in my “Larry was right” comment. The test measures the “occupied bandwidth” of the modulation process in the transmitter, nothing else. Originally, as a method the control of the modulation bandwidth, an external filter was used in the audio path limiting the audio to 10 kHz. All manufacturers quickly added the NRSC filter to their equipment and soon the problem subsided, technology to the rescue. Actually this was a known solution to the problem and only required getting the word out and the stations implementing it.
The NRSC measurement is different than the others and not related but is the most accurate. You are measuring what the transmission system can emit. The other three tests measure what the station can’t emit. The measurements all have, as a lower floor, a level where any further improvement i.e. lower than –80dB down, is deemed insignificant. This level is arbitrary to an extent but is based on real life interference experience.
Normally I would conduct the NRSC “Occupied Bandwidth” measurement first and then measure the harmonics followed by intermod. Spurious emissions where looked for by keeping an eye out for anything that couldn’t be readily identified while making the other measurements and then a careful scan of the band and beyond. This last test was very difficult to perform and is not done with the current automated techniques.
I don’t think many firms conduct the intermod test much anymore either. When I was making these tests the intermod test is where I found most of the problems with closely spaced stations. Intermod problems can usually be remedied by installing a notch filter in the transmission system near the antenna. One of the stations I measured and found not in compliance needed a notch filter to prevent an intermod issue with a nearby station and now, many years later, is one of the stations I am responsible for.
Keep these tests separate so “Occupied Bandwidth” has meaning. The greater than 75 kHz of the NRSC mask means that the modulation process cannot have any emissions greater than –80 dB out to forever but does NOT mean that Occupied Bandwidth includes harmonics which are a very different process than modulation. Flunking a station and just saying you didn’t pass NRSC would not have meaning.
Now we are many years down the road and still measuring the same transmitters and processors for there “Occupied Bandwidth.” HD has been added, DSP chips are in use, and many, if not most, transmitters are solid state. The manufacturers are producing incredible machines. This year I had the privilege of helping three other engineers install a 50 kW AM solid-state transmitter. After a day and a half the transmitter was turned on and there, before my eyes, was a display that blew me away. I could see an expanded Smith chart displaying the varying impedance at the transmitter over the modulation bandwidth. We used this display to further tweak the phasor match to make the transmitter a very happy camper.
These measurements, conducted once a year, are only a brief snapshot of the performance of the transmission system. A one-hour test time, usually automated, only sees the stations parameters for 0.02% of the time. That means that 99.98% of the time no one is “looking.” But again, technology to the rescue. With the internet, that 50 kW transmitter can be looked at by the engineer anytime, anywhere. The measurements are more accurate (technology again) and not subject to the conditions and errors that the field measurements are.
The harmonic, spurious and intermod field measurements are subject to several possible error conditions and require substituting equipment, position and techniques to verify their accuracy. That is not something most engineers today want to get involved in. In fact, a measurement failure may not even be a station problem. It is time to put these tests to bed, technology has obsoleted them.
Yes, Larry is still right.
Radio World invites industry-oriented commentaries and responses. Send to Radio World.
Here is an aggregation page for this topic.