revised: 10/11/01
If your test gear provides automated CSO and CTB measurement capabilities, by all means, use them for CSO and CTB tests. But, if you have any other choice, don't use them for Coherent Disturbance tests. They take too long and don't perform the test properly! See the tech article "Don't use automated CSO and CTB routines for proofs!"
It can sometimes be difficult to find a channel, especially in the low VHF band (2 through 6) that can be used for tests. Often these channels are nearly filled with off-air signals and/or the low pilot is in this range. The problem arises when processors are used for the off-airs. In general, you can't insert your own test signals or quiet lines on processed channels, making it difficult to perform in-service C/N and In-Channel Response tests. If there's at least one modulated channel in this band, you can select it for C/N and ICR but, if it also happens to be the low pilot channel, you can't use it for CTB - unless you used gated CTB tests.
A way around this is to go ahead and run C/N and ICR on the modulated channel but run CTB (actually, coherent disturbances) on a different channel. There's nothing in the rules that require all the tests to conducted on the same selection of channels.
This is a tough one to run at the output of a converter -- unless you use a simple heterodyne converter.
The problem is that the visual carrier must be removed in order to look for coherent disturbances at the visual carrier frequency. When the visual carrier is removed, the converter's AGC circuit will greatly increase the gain, producing wrong numbers. Or, the converter's AFC circuit will lock to the aural carrier of the lower adjacent channel. Or, if you have baseband set-top converters, the visual carrier at the output of the converter doesn't change when you turn off the modulator in the headend.
Here's a way to run the test at the output of a converter (baseband, with AGC, or with AFC).
When you remove the carrier in the headend, replace it with a substitution carrier that is the same amplitude as the original, but offset in frequency by approximately 300 kHz. The converter will lock to the new carrier, allowing you to check for coherent disturbances at the original carrier's frequency.
A convenient way to do this is to use a programmable CW generator in the headend. The generator is programmed for the required frequency and amplitude for each of the test channels, then connected to an unused port on the combiner. To run the test, the visual carrier is measured in the field, then the tech in the headend turns off the modulator and enables the pre-programmed signal from the generator to provide the substitution carrier.
In practice, most sytems run this test at the tap. If you check the specificications for your set-top converters
(or, better yet, run some tests on the ones you use), you will probably find that, if input levels are kept below
approximately 15 dBmV, the converter will contribute little to coherent disturbances.
Links to tech papers:
Don't use automated CSO and CTB routines for proofs!