What is a Spectrum Analyzer?
A spectrum analyzer is an essential tool in the field of electronic measurements. It allows us to analyze the frequency spectrum or content of a signal (sine and cosine waves), providing valuable insights into their behavior. Here are the key points:
Frequency Domain Analysis
A spectrum analyzer measures the magnitude of an input signal versus frequency within its full frequency range.
The primary purpose is to determine the power of known and unknown signals across different frequencies.
Graphical Representation
By analyzing the frequency spectrum, the spectrum analyzer produces a graph that shows the amplitude (in decibels, dB) versus frequency (in hertz).
This graphical representation helps us visualize how the signal’s power varies across different frequency components.
This is different from an oscilloscope, where the trace travels from the left side of the screen to the right side of the screen, displaying the voltage of a signal as time progresses.
The trace of a spectrum analyzer moves from the left side of the screen to the right side of the screen, just like the oscilloscope, but it is moving from a lower frequency to a higher frequency. It shows the amplitude of the signal at the various frequencies that make up the signal.
The vertical axis on a spectrum analyzer is specified in dB as opposed to volts found on an oscilloscope. The horizontal axis of a spectrum analyzer is frequency as opposed to time, which is displayed on an oscilloscope.
The nature of the spectrum analyzer’s display and its ability to show the frequency content of a signal allows us to see perturbations of the signal that cannot normally be seen when viewing a time-domain signal with an oscilloscope.
Identifying Signals and Noise
By comparing the amplitude and frequency measurements, we can identify:
- In-band signals: These are the desired signals within the specified frequency range.
- Out-of-band signals: These are unwanted signals or noise that may be present outside the desired frequency range.
Key Parameters for Spectrum Analyzer Operation
To operate a spectrum analyzer effectively, we need to understand and configure the following parameters:
- Center and Span
- Reference Level
- The reference level represents the maximum expected power at the spectrum analyzer input.
- We adjust the reference level so that the highest signal level is slightly below it.
- Setting it too low affects measurement accuracy, while setting it too high reduces dynamic range.
- Resolution Bandwidth (RBW)
- RBW determines the frequency resolution of the measurement.
- Smaller RBW provides finer frequency detail but increases measurement time.
- Larger RBW sacrifices detail but speeds up measurements.
- Compliance test standards specify the RBW to use over specific frequency bands.
- Video Bandwidth (VBW)
- VBW affects the display refresh rate.
- Smaller VBW provides better resolution but slower updates.
- Larger VBW gives faster updates but less detail.
Types of Spectrum Analyzers
There are three different and distinct types of spectrum analyzers: the Fourier or FFT analyzer, the heterodyne analyzer, and the hybrid spectrum analyzer, which uses elements from the first two.
Preventing Damage to A Spectrum Analyzer
The most susceptible point for damage to a spectrum analyzer is the mixer, which is often positioned behind a variable input attenuator. The front of the spectrum analyzer contains a bright yellow sticker that says how much signal input is acceptable in dBm. The warning often includes a statement about how much DC voltage is acceptable (it is often 0 VDC from microwave spectrum analyzers!).
Pro Tip: Carefully read the bright yellow sticker and understand its meaning. Do not apply more signal to the front-end of a spectrum analyzer than it will accept. If in doubt, use extra attenuation to the front-end to prevent damage and do not apply DC voltage by ensuring the signal is AC coupled by placing a coaxial DC block in front of the input connector.
Summary
Spectrum analyzers are powerful tools for understanding signal behavior across different frequencies. Whether you are analyzing wireless communication signals, troubleshooting interference, running compliance tests, or characterizing electronic devices, a spectrum analyzer is your trusted companion!
References and Further Reading
- “Understanding Basic Spectrum Analyzer Operation,” Rohde & Schwarz.
- “Spectrum Analyzer How-To Guide: What They Are, What They Measure, & How to Use Them,” Tektronix.
- “How Does a Spectrum Analyzer Work,” Keysight.
