Wavetek 147 Spezifikationen PDF herunterladen (Seite 363)

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101

Application Notes: Proofing and Testing

339

NOTE: Measuring digital "haystacks" with a spectrum

analyzer is very misleading. The level reading is based on

the RBW of the analyzer and there are correction factors

associated with this. A simple calculation is not adequate

because of the shape of the "haystack." Refer to the

"Digital vs. High Speed Data" article that was published in

Communications Technology’s Dec. ’98 issue.

Measure levels of both channels and calculate the difference. This is the S/

N. Because the test equipment noise floor may give faulty readings, a pre-

amp may be warranted.

Perform a "Noise Near Noise Test". Remove the input cable from the field

unit and record the level from the NOIS channel again. Subtract this from the

original NOIS measurement.

Locate the difference between the two readings on the "X" axis of the graph

below and find the corresponding correction factor on the "Y" axis. Add this

correction factor to the S/N value.

Fig. Noise Near Noise Correction Chart

Any correction factor > 6 dB will be less reliable since a very small change in

amplitude causes a large change in correction.

orrection ( m)

measured_above

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