Spectrum Analyzer Six Frequently Asked Questions

A logic analyzer is an instrument that uses a clock to acquire and display digital signals from a test device. The most important function is timing determination. Unlike an oscilloscope, a logic analyzer cannot display continuous analog waveforms, but only high and low level states (logic 1 and 0). After the reference voltage is set, the logic analyzer compares the acquired signal with the voltage comparator, which is logic 1 above the reference voltage and logic 0 below the reference voltage. In this way, the measured signals can be displayed in chronological order as continuous high and low level waveforms, which is convenient for users to analyze and debug. Using the logic analyzer, you can easily set the signal trigger condition to start sampling, analyze the timing of multiple signals, capture the interference glitch of the signal, and decode the level sequence according to the rules to complete the communication protocol analysis.
The spectrum analyzer is a commonly used analytical instrument, mainly for detecting RF and microwave signals, and has certain applications in many fields. There are some common problems in the use of spectrum analyzers that require the attention of users. Today, Xiaobian will introduce you to the six common problems in the use of spectrum analyzers. I hope to help you.

Q1: How to get the best sensitivity of the spectrum analyzer to facilitate observation of small signals

A: Firstly set the corresponding center frequency, span and reference level according to the size of the measured small signal; then gradually reduce the attenuation value if the spectrum analyzer does not have an overload indication; if the small signal is measured at this time The signal-to-noise ratio is less than 15db, and the rbw is gradually reduced. The smaller the rbw is, the lower the noise floor of the spectrum analyzer is, and the higher the sensitivity is.

If the spectrum analyzer has a preamplifier, turn on the preamp. Pre-release allows for improved noise figure of the spectrum analyzer, which increases sensitivity. For small signals with low signal-to-noise ratio, vbw can be reduced or trajectory averaging can be used to smooth out noise and reduce fluctuations.

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

Q2: Is the resolution bandwidth (rbw) as small as possible?

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 rbw according to the actual test requirements, to find a balance between sensitivity and speed – both to ensure accurate measurement of the signal and fast measurement speed.

Q3: How to choose the average detection method (averagetype): power? Logpower? Voltage?

Logpower logarithmic power average: Also known as videoaveraging, this averaging mode has the lowest noise floor and is suitable for low-level continuous wave signal testing. However, there will be certain errors in the "noise-like" signal, such as the wideband modulated signal w-cdma.

Power average: Also known as rms average, this averaging method is suitable for "noise-like" signal (eg cdma) total power measurement.

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

Q4: Scan mode selection: sweep or fft?

A: The scanning mode of modern spectrum analyzers usually has a sweep mode and an fft mode. Usually, in a narrower rbw setting, fft has a speed advantage over sweep, but in a wider rbw condition, the sweep mode is faster.

When the span is less than the analysis bandwidth of fft, the fft mode can measure the transient signal; when the span is beyond the fft analysis bandwidth of the spectrum analyzer, if the fft scan mode is adopted, the working mode is to segment the signal, segment and segment There is a discontinuity in time between them, and the useful signal may be lost in the signal sampling gap, and there will be distortion in the spectrum analysis. This type of signal includes: pulse signal, tdma signal, fsk modulation signal, and the like.

Q5: What is the effect of the choice of detector on the measurement results?

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

Sample detection method: This detection method is generally suitable for testing noise and "noise-like" signals.

Negpeak detection method: suitable for small signal test, for example, emc test.

Normal detection mode: suitable for simultaneous observation of signals and noise.

Q6: What is the role of the tracking source (tg)?

A: The tracking source is one of the common options on the spectrum analyzer. When the tracking source is output through 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. Tracking the frequency of the signal output by the source accurately tracks the tuning frequency of the spectrum analyzer. The spectrum analyzer is equipped with a tracking source option that can be used as a simple scalar network analysis to observe the excitation response characteristics of the device under test, such as the frequency response of the device, insertion loss, and so on.

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