Anritsu S113C Podręcznik Użytkownika Pobierz pdf (Strona 74)

Effect of Video Bandwidth

Spectrum analyzers typically use another type of filtering after the detector called VIDEO

FILTERING. This filter also affects the noise on the display but in a different manner than

the resolution bandwidth. In video filtering, the average level of the noise remains the same

but the variation in the noise is reduced. Hence, the effect of video filtering is a “smooth

ing” of the signal noise. The resultant effect on the analyzer’s display is that the noise floor

compresses into a thinner trace, while the position of the trace remains the same. Thus,

changing the video bandwidth (VBW) does not improve sensitivity; however, it does im

prove discernability and repeatability when making low-level measurements.

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

bandwidth that is a factor of 10 to 100 less than the resolution bandwidth. Thus, for a reso

lution bandwidth of 30 kHz, the typical video bandwidth setting options are either 3 kHz or

300 Hz.

Sweep Limitations

With some spectrum analyzers, the user has control over sweep time (the elapsed time of

each sweep, sometimes called scan time). The analyzer cannot be swept arbitrarily fast

while maintaining its specified accuracy, but will have a sweep rate limitation depending on

the resolution bandwidth, video bandwidth, and frequency range selected. The sweep rate is

not usually chosen by the user but is determined by the frequency range swept divided by

the sweep time.

The limitation on sweep rate comes from the settling or response time of the resolution and

video bandwidth filters. If an analyzer is swept very quickly, the filters do not have time to

respond, and the measurement is inaccurate. Under such conditions, the analyzer display

tends to have a “smeared” look to it, with the spectral lines being wider than normal and

shifted to the right.

Fortunately, the Anritsu Hand Held Spectrum Analyzer has mechanisms designed into it

that unburden the user from having to calculate the sweep rate.

When changing the RBW and VBW, the sweep rate will change accordingly. The sweep

rate will be faster for a wide RBW or VBW and slower for a narrow RBW or VBW.

Attenuator Functions

Attenuation adjusts the hand held spectrum analyzer input attenuator. In

AUTO mode, as the

reference level is increased, the attenuation is increased. In manual (

MANUAL) mode, the

input attenuation can be adjusted by using the Up/down arrow key. The attenuator range is

0 to 50 dB, in 10 dB steps.

IMPORTANT! Attenuation is normally a coupled function and is automatically

adjusted when the reference level changes. The reference level will not change

however, when the attenuator changes. The attenuator should be adjusted so

that the maximum signal amplitude at the input mixer is –30 dBm or less. For

example, if the reference level is + 20 dBm, the attenuation should be 50 dB for

an input signal of –30 dBm at the mixer (+20 – 50 = –30). This prevents signal

compression.

5-2

Chapter 5 Spectrum Analyzer Measurements

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