6 Common Questions Asked by Spectrum Analyzer Users

Spectrum analyzers are a common tool on electronic engineers' workbenches or in university laboratories. In this article, we have compiled 6 common questions about spectrum analyzer usage, hoping that it can answer your questions.

Q1. How can I set the spectrum analyzer to achieve the best sensitivity for observing small signals?

A: First, set the corresponding center frequency, span and reference level according to the size of the small signal to be measured; then, gradually reduce the attenuation value when there is no overload prompt on the spectrum analyzer; if the signal-to-noise ratio of the small signal to be measured is less than 15dB at this time, gradually reduce the RBW. The smaller the RBW, the lower the noise floor of the spectrum analyzer and the higher the sensitivity.

If the spectrum analyzer has preamplifier, turn it on. Turning on preamplifier can improve the noise factor of the spectrum analyzer, thereby improving sensitivity. For small signals with low signal-to-noise ratio, you can reduce VBW or use trace averaging to smooth noise and reduce fluctuations.

It should be noted that the measurement result of the spectrum analyzer is the sum of the external input signal and the internal noise of the spectrum analyzer. In order to make the measurement result accurate, the signal-to-noise ratio is usually required to be greater than 20dB.

Q2. Is it better to have a smaller resolution bandwidth (RBW)?

A: The smaller the RBW, the better the sensitivity of the spectrum analyzer, but the scanning speed will be slower. It is best to set the RBW according to the actual test needs and find a balance between sensitivity and speed - ensuring accurate measurement of the signal while achieving fast measurement speed.

Q3. How to choose the average detection method (average type): Power? Log power? Voltage?

Log power logarithmic power averaging: also known as Video Averaging, this averaging method has the lowest noise floor and is suitable for low-level continuous wave signal testing. However, it will have certain errors for "noise-like" signals, such as broadband modulation signals W-CDMA, etc.

Power average: also known as RMS average, this averaging method is suitable for total power measurement of "noise-like" signals (such as CDMA)

Voltage averaging: This averaging method is suitable for observing the rise and fall time measurements of amplitude modulated signals or pulse modulated signals.

Q4. Which scanning mode should I choose: sweep or FFT?

A: Modern spectrum analyzers usually have two scanning modes: Sweep mode and FFT mode. Usually, when the RBW is relatively narrow, FFT has a speed advantage over sweep, but when the RBW is relatively wide, the sweep mode is faster.

When the span is smaller than the FFT analysis bandwidth, the FFT mode can measure transient signals; when the span exceeds the FFT analysis bandwidth of the spectrum analyzer, if the FFT scanning mode is used, the signal is processed in segments, and there is a time discontinuity between segments. In this case, useful signals may be lost during the signal sampling interval, and the spectrum analysis will be distorted. This type of signal includes: pulse signals, TDMA signals, FSK modulation signals, etc.

Q5. How does the choice of detector affect the measurement results?

Peak detection mode: select the maximum value in each bucket as the measurement value. This detection mode is suitable for continuous wave signal and signal search test.

Sample detection mode: This detection mode is usually suitable for testing noise and "noise-like" signals.

Neg Peak detection mode: suitable for small signal testing, such as EMC testing.

Normal detection mode: suitable for observing signals and noise at the same time.

Q6. What is the function of the tracking generator (TG)?

A: Tracking source is one of the common options on spectrum analyzers. When the tracking source output is connected to the input port of the device under test, and the output of the device is connected to the input port of the spectrum analyzer, the spectrum analyzer and the tracking source form a complete adaptive frequency sweep measurement system. The frequency of the signal output by the tracking source can accurately track the tuning frequency of the spectrum analyzer. The spectrum analyzer with the tracking source option can be used for simple scalar network analysis to observe the stimulus response characteristic curve of the device under test, such as the frequency response and insertion loss of the device.