Spectrum Analyzer Measurement Guide : Spectrum Analyzer : Sweep Limitations
Sweep Limitations
With some spectrum analyzers, the user has control over sweep time (the elapsed time of each sweep, sometimes called scan time). An analyzer cannot be swept arbitrarily fast while maintaining its specified accuracy, but has a sweep rate limitation depending upon 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 too 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 and at a lower amplitude than is correct.
Anritsu products are designed to relieve the user of having to calculate the sweep speed or experiment to discover a sweep speed that yields accurate results. When changing the RBW and VBW, the sweep speed automatically changes to the fastest sweep speed that yields accurate results. The sweep speed is faster for a wide RBW or VBW and slower for a narrow RBW or VBW. The sweep speed can also be changed manually, by pressing the Sweep key and selecting the Sweep Time submenu key. Enter a sweep time from 10 μs to 600 seconds. If the minimum sweep time entered by the user is less than the value needed to assure accurate results, the value that delivers accurate results is used. Regardless of the minimum sweep time setting, the instrument never sweeps faster than the RBW and VBW settings allow. The instrument is designed to ensure that no uncalibrated measurement conditions can occur.
Attenuator Functions
The Spectrum Analyzer includes a step attenuator at the RF input. This attenuator is used to reduce large signals to levels that make best use of the analyzer’s dynamic range. Normally, the input attenuation automatically adjusts as a function of Reference Level. In the Amplitude menu, the Attn Lvl key allows manual adjustment of the input attenuation, using the numeric keypad, the Up/Down arrow keys, or the rotary knob. In Auto Atten mode, as the reference level is increased, the attenuation is increased.
The following actions, listed in decreasing order of effectiveness, can facilitate the detection of low-level CW signals:
1. Decrease the reference level and attenuation. Refer to Amplitude Menu.
2. Turn on the Preamp.
3. Reduce RBW. Refer to BW (Bandwidth) Menu.
4. Decrease VBW (RBW/VBW = 10 is often optimal for this purpose).
5. Use trace averaging if VBW is already set to 1 Hz.