How Spectrum Analyzers Work (2)

1. Attenuator

Placing an attenuator in the RF input path broadens the dynamic range of the input signal level or adds more input protection to the spectrum analyzer.

The attenuator limits the signal level from the mixer (RF middle) to a certain range, which will cause measurement errors or spurious noise if the input signal exceeds the reference level. This is why some spectrum analyzers list instrument specifications for specific signal conditions, including specific signal levels in the mixer.

2. Resolution filter

When the input signal frequency is converted to a lower frequency band and filtered into the detection and display unit, an RBW (Resolution Bandwidth) filter is used in order to distinguish signals with close frequencies.

How does the RBW filter differentiate between two signals at different resolution bandwidths? Two equal amplitude signals are filtered through two RBW filters. The resolution of RBW1 is worse than that of RBW2. When passing through the narrower RBW1 filter, the two signals can be clearly distinguished, but when passing through the wider RBW2 filter, the result Not as good as RBW1. Predictably, if the RBW2 had a wider resolution bandwidth, we would even mistake the result for a signal. This happens when the frequencies of the two signals are very close. Another situation is that when the amplitudes of the two signals are very different, RBW1 can still detect the smaller signal. But RBW2 cannot, as shown in the figure below, so these filters are also called resolution filters.

3. Geophone

After RBW filtering, the detector can detect the energy and convert it into a DC voltage. The display unit draws the spectral distribution using the DC voltage.

4. Video filter

Before the DC voltage enters the display unit, the noise generated by the detector needs to be compressed. This filter is called a video filter, and its bandwidth is called VBW.

The video filter also acts as a post filter and the VBW has an effect on the display output. If the signal under test passes through two VBW filters, where VBW1 is smaller than VBW2, the result shows that the noise floor of VBW2 is larger than that of VBW1, in other words, the video filter averages the noise floor. However, it should be noted that the noise floor level has not changed, the VBW filter only averages the noise level and does not affect the overall amplitude of the signal noise floor.

5. Scan time

The above content introduces the basic structure of the spectrum analyzer, and also explained the RBW and VBW in detail. In general, sweep time is inversely proportional to the frequency resolution of a spectrum analyzer. The faster the scan time, the lower the resolution. The slower the scan time, the higher the resolution. Therefore, if a narrower RBW or VBW is selected, the time to display the signal will be longer. This means that the narrower the RBW and VBW, the longer the scan time. For RBW/VBW sweep time, most spectrum analyzers have automatic and manual selection modes. Auto mode trades off bandwidth, RBW, VBW, and scan time, and usually gets the best combination.

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