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Chapter 5

Spectrum Analyzer

Measurements

Spectrum Analyzer Fundamentals

Advanced measurements require the use of additional spectrum analyzer functions beyond

frequency, span, amplitude and marker functions. In particular, this section will focus on

resolution bandwidth, video bandwidth, sweep, and attenuator functions.

Effect of Resolution Bandwidth

Resolution Bandwidth is determined by the intermediate frequency (IF) filter bandwidth.

The spectrum analyzer traces the shape of its IF filter as it tunes past a signal. If more than

one IF filter is used in a spectrum analyzer, the narrowest one dominates and is considered

the resolution bandwidth.

The choice of resolution bandwidth depends on several factors. Filters take time to settle.

That is, when a signal first appears at the input of the filter, it will take a while before the

signal appears at the output. Additionally, the output of the filter will take some time to set-

tle to the correct value, so that it can be measured. The narrower the filter bandwidth (reso-

lution bandwidth) the longer the settling time.

The choice of resolution bandwidth will depend on the signal being measured. If two sig-

nals are to be measured individually, then a narrow bandwidth is required. If a wider band-

width is used, then the energy of both signals will be included in the measurement. Thus,

the wider bandwidth does not have the ability to look at frequencies selectively but instead

must measure across their entire frequency range at all times.

Therefore, a broadband measurement would include the fundamental frequency, harmonics,

spurious responses, and noise in the measurement. On the other hand a narrow-band mea-

surement will filter out all but the desired frequency components, resulting in a measure-

ment that includes only the fundamental. There are advantages to each. The ultimate

decision will be dependent on the type of measurement required by the user.

There is always some amount of noise present in a measurement. Noise is often broadband

in nature; that is, it exists at a broad range of frequencies in the frequency domain. If the

noise is included in the measurement, the measured value will be in error (too large) de-

pending on the noise level. With a wide bandwidth, more noise is included in the measure-

ment. With a narrow bandwidth, very little noise enters the resolution bandwidth filter, and

the measurement is more accurate. If the resolution bandwidth is narrower, the noise floor

will drop on the display of the spectrum analyzer. This is because the IF filter of the ana-

lyzer has been made narrower in bandwidth, which lets in less noise. As the measured noise

level drops, smaller signals that were previously obscured by the noise can now be mea-

sured. As a general rule of thumb, most field spectrum analyzer measurements are made at

a resolution bandwidth of 30 kHz.

5-1

SgLabs

www.sglabs.it

email: [email protected]

tel. +39 0755149360

SgLabs

www.sglabs.it

email: [email protected]

tel. +39 0755149360

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