Be sure to use the right frequency range when running this test. Its from 500 kHz below to 3.75 MHz above the visual carrier frequency. The wording in part 76 can be a little confusing.
Analyzers that have Gaussian shaped filters, like most HPs, will mask sidebands within 500 kHz of the visual carrier
when using the most common resolution bandwidth setting for in-channel response tests (300 kHz). That's unacceptable
because, more often than not, either the lower 500 kHz or the upper 500 kHz sideband determines the maximum or
minimum level within the specified frequency range. For example, its not unusual for the lower 500 kHz sideband
to be the low point in the frequency response of a modulator. This is determined by the shape of the vistigial
sideband filter.
300 kHz RBW 100 kHz RBW
Fortunately, its a simple matter to change the resolution bandwidth setting to 100 kHz. This will provide the correct result.
Another common problem is that all test signals for In-Channel Response tests are not created equal. Most were
designed to be used in the studio and measured using a waveform monitor. Frequency response test signals for use
with spectrum analyzers should give the same result whether using 300 kHz or 100 kHz resolution bandwidth filters.
Here are examples of Multiburst test signals, as displayed on a Tek 2715, generated by a Tek TSG95 signal generator
and a TVMS 4200 In Service Test Processor. The 4200 was designed specifically for use with a spectrum analyzer.
Notice how the relative levels of the packets stay constant with the 4200, but vary with the TSG95 as the resolution
bandwidth is changed from 300 kHz to 100 kHz.
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