HP 8591C User Manual page 268

I -
Test Descriptions
Carrier-to-Noise Ratio Test Description
Another difference between the non-gated and the gated variations of the test
is the frequency range over which the noise is measured. The non- gated test
measures the noise at the current marker position averaged with 1 position
on either side. In a 6 MHz span, 3 measurement buckets is a range of 45
bandwidth. This assumes the noise floor is flat which is almost always true
when using the method of turning modulation off to make the measurement.
The gated measurement has the added flexibility of being able to measure
over a much wider frequency range. This is necessary when measuring with
a video signal present because the noise floor is very often not flat. Therefore
measuring at a single point and converting to a 4 MHz bandwidth can give
incorrect results. The FCC rules stipulate that when the noise floor is not
flat that it must be integrated or summed. The gated carrier-to-noise test
integrates over as wide a range as it can. It defaults to 85% of the 4 MHz
range. It cannot be 100% because the carrier is always present and cannot
be gated away. However, the noise below the carrier is almost always equal
to or less than the noise in the rest of the channel so it is conservative to
assume the integrated value also applies to the noise under the carrier. You
can change the range of 85 % . Entering 1% replicates the non-gated noise
measurement range.
Refer to "Carrier-to-noise test" in Chapter 2 for information on how to make
a measurement.