Hello,
This is a loaded question as many people do not supply a good reference point in the headend to correctly check. This is the same for sweep or modulator balancing to be blunt.
The reference level for ingress depends on what (or where) you are checking. Some testpoint locations (drop, tap seizure probe, input testpoint on amp/output testpoint on amp, return receiver in headend, combiner testpoint in HE, at CMTS or wherever it may be) will yield a lower RF input to the RF meter or spectrum analyzer thus not having enough Carrier-to-Noise (CNR) ratio to perform a valid test. What measures as a -20dB CNR may actually be much higher if the test gear does not have enough level to be tested...typically +18 to +20dBmV input is required for an accurate measurement to get above the noise floor of the test equipment.
Most commonly used checks are to first confirm that the REVERSE optical links has been correctly optimized in the headend with ALL combined nodes to a specific CMTS port as they have been combined and one node can effect the other combined nodes. Confirm that ALL RF AMPLIFIERS have been CORRECTLY balanced in the field. After this has been confirmed, typically there is a common SPARE RETURN RF output testpoint for each node combiner/splitter (in the headend) that can be used for this test that provides both isolation and enough RF level to conduct this test.
The REFERENCE test can be gauged to either a single CW carrier injected at the node at the correct manufacturer reccomended level OR looking at the modem haystack levels. The problem with the modem haystack levels are that if the RF amplifiers and insertion losses in the system are "out of designed level parameters" and the modems are making up for a poorly balanced network, many may be incorrect. In any event, your SPECTRUM ANALYZER (the only correct way to see noise and ingress) will be measured from the noise floor to the peak of the carrier(s) to determine the correct CNR. Only after this point can an accurate noise and ingress test be performed.
If your system has not been optimized in the RETURN with all return RF amplifiers in the field to the respective nodes with a SWEEP, the noise and ingress is not accurately measured as the UNITY GAIN concept is not valid!
Sorry if I ran off too much information but I wanted to try to cover all bases. There are many of other "tricks of the trade" but not knowing your specifics, I just threw this out there. Possibly you already know this! In any event, GOOD LUCK!
Have a HAPPY TURKEY DAY!!!
Greg
