C/N tests are one of the most important, and least understood, of the proof of performance test requirements. My recommendation is to have a thorough understanding of the fundamentals of C/N measurement techniques (using manual measurement methods). Once you have a grasp of the fundamentals, then look at your automated test gear and decide if it is capable of performing the task as you feel it needs to be done. Its important to understand how the various types of automated C/N test equipment work!
There is a lot of debate about how carrier to noise ratio tests should be run. The issues tend to center around tests at the output of set-top converters. In most cases, C/N tests should be conducted at the output of the converter (as specified in the rules). See the technical paper: "To test at the tap, or the converter. THAT is the question" for the details.
If your test equipment measures C/N by measuring the noise level in the guard band between channels be aware that:
1. Technically, the test isn't being performed within the FCC's definition of the noise spectrum: "……. System noise is specified in terms of its rms voltage or its mean power level as measured in the 4 MHz bandwidth between 1.25 and 5.25 MHz above the lower channel boundary of a cable television channel."
2. That test method relies on the filters in the headend equipment to attenuate signals in the guard band far enough to allow accurate C/N test results. Sometimes that's correct, sometimes it isn't.
3. If you run the test at the output of a set-top converter, the frequency response of the converter may roll of in the guard band -- giving really nice, but incorrect, test results.
If you run the tests at the output of a set-top converter and the converter has AGC and/or AFC circuits in it, the converter will increase its gain and/or lock to the lower adjacent aural carrier when you remove the visual carrier. If that happens, you must use a different method for the tests.
This is one of the safer ways to run C/N at the output of the converter. However, check to make certain that the peak of the visual carrier doesn't change when you remove video to the modulator. Some modulators have built in AGC circuits that keep the peak of visual carrier constant, even when video is removed. If that doesn't happen, the visual carrier level will drop, causing the AGC in the set-top converter to increase its gain to compensate, resulting in worse than actual C/N test results.
Some test equipment such as the HP/Agilent 8591C opt 107, Tek 2715, Avantron AT2000R and some Stealth receivers can measure C/N by sampling the RF signal during "quiet line(s)" in the vertical blanking interval. Quiet lines are scan lines that have no video, data, or VITs on them.
Because nearly everything in the signal path (modulator, distribution system & set-top box) is operating under normal service conditions, this is perhaps the best method to perform non-disruptive C/N tests. However, be aware that quiet lines are not necessarily all that quiet -- they will have a certain amount of noise on them dependent on the quality of the video feed and the path to your headend (typically a satellite feed). If the noise on the incoming feed is high enough, it will dominate noise generated in your distribution system. In that case, your C/N reading is more a reflection of the quality of the video being provided to you than of the performance of your system. In one sense, that's good! The measured C/N is what your subscribers are actually receiving. However, when evaluating your system performance, you need to eliminate the effects of incoming noise on your test results. There are several ways to do this:
1. Run test on channels that are received digitally. For digital feeds, the Vertical Blanking Interval is generated in the receiver, resulting in essentially no noise in the VBI.
2. Install VITs inserters at the inputs to the modulator for the channels to be tested. This will provide the necessary quiet line(s).
3. Install a Quiet Line Inserter at the inputs to the modulators for the channels to be tested. This costs much less than using multiple full-fledged VITs inserters and can also provide test conditions for in-service In-Channel Response and Coherent Disturbance tests. For information about such devices, refer to the Products section of this web site.
Links to tech articles:
To test at the tap, or the converter. THAT is the question
Running tests on scrambled channels
How to run non-disruptive tests even if your test gear doesn't do gated measurements