A Spectrum Analyzer measures the amplitude versus the frequency of an input signal. On a spectrum analyzer display, the amplitude is plotted on the y-axis and the frequency on the x-axis. From spectrum analysis dominant frequency, power, distortion, harmonics, and bandwidth of an input signal can be measured.
Spectrum analyzers operate in four different manners:
- FFT (fast Fourier Transform)
- Hybrid superheterodyne-FFT
- Realtime FFT
Each of these has unique advantages or disadvantages for different applications. Spectrum analyzers also come in three different forms depending on area of use: benchtop, portable, and handheld. Despite the similarities in appearance, a spectrum analyzer is different from an oscilloscope: an oscilloscope plots voltage (amplitude) on the y-axis and time on the x-axis. Some units can function as both oscilloscopes and spectrum analyzers.
Download a Primer
on Spectrum Analyzers and an RFID Application Note
Key features to consider
Types of Spectrum Analyzer Operation
- Frequency Range. This should be a primary consideration. Choose a unit that can measure frequencies in the your operating range
- Noise Floor. Noise floor is the lowest noise level when no signal is put through the unit. This represents the lowest signal level that a spectrum analyzer can measure. This typically is dependent on resolution bandwidth (RBW)
- Dynamic Range. Dynamic Range is the difference between the noise floor level and the maximum measurable level. This affects the ability to accurately measure amplitude
- Harmonics. A spectrum analyzer will generate its own harmonics from an input signal. If these self-generated harmonics exceed the input signal harmonics, an error will occur
- Spurious Noise. When no signal is put through a spectrum analyzer, interference that looks like an actual signal with a specific frequency can occur due to spurious noise from the unit itself
- Phase Noise. Phase noise is the purity of an input signal. It is measured as dBc with a frequency offset; a lower offset translates to a more pure signal
A Swept-Tuned spectrum analyzer down-converts the input signal to match the center frequency of a band-pass filter. This allows use of the unit’s full frequency range. In this method of operation, the unit’s resolution bandwidth (RBW) is determined by the bandwidth of the band-pass filter. Generally the smaller the resolution bandwidth the greater the frequency resolution; a high frequency resolution allows the unit to distinguish between frequencies that are close together. Unfortunately a low RBW/high frequency resolution results in more acquisition and computation time.
- FFT (Fast Fourier Transform)
FFT is an algorithm used to convert time to frequency. A FFT-based spectrum analyzer uses FFT algorithms to produce a frequency spectrum of a given input signal. However this method is extremely demanding to unit processing power and the required analog-to-digital converter, resulting in limited frequency ranges for FFT-based units.
- Hybrid superheterodyne-FFT
This method addresses the main issues with FFT-based and swept-tuned spectrum analyzers by combining the two technologies into one unit. In this method the input signal is down-converted first, then digitized, and finally FFT techniques are used to acquire the spectrum. This results in significantly improved acquisition and computation time, although there is still processing time required to sample the spectrum and calculate the FFT. This processing time results in "blind time", meaning that while calculation of the spectrum is being performed, the instrument shows gaps and misses information. This “blind time” occurs with hybrid, FFT, and swept-tuned units.
A real-time spectrum analyzer eliminates “blind time” completely. These units convert signals via FFT, however they can process FFT algorithms in parallel, resulting in gapless and overlapped RF spectrums. Tektronix invented this technology over 20 years ago and incorporates it into all of their spectrum analyzers.
Spectrum Analyzer Forms
A benchtop spectrum analyzer typically weighs about 30 pounds and requires connection to wired AC power (some benchtop units do offer rechargeable battery packs). Because of this, this type of unit would typically be found in a laboratory or manufacturing environment. Benchtop units typically offer better performance and specifications than portable or handheld units. They also are typically well-ventilated with multiple fans to dissipate the resulting heat from operation.
This type of spectrum analyzer is useful when measurements need to be taken outside or when the unit needs to be carried while in use. These units typically weigh less than 15 pounds and incorporate battery-powered operation.
This type of spectrum analyzer is the lightest and smallest of the three. These units are battery-powered and consume very little power. They are also quite small in size and usually weigh less than two pounds.
Spectrum Analyzers for EMI/EMC Testing
What is EMI/EMC?
Almost every electronic product sold in the world must pass EMC tests. Prices have come down considerably to permit in house EMC diagnostics.
Electromagnetic Compatibility (EMC) and Electromagnetic Immunity (EMI) testing involves measuring electromagnetic fields (Radiofrequency) and comparing those measurements to limits set forth by the country in which the end product is going to be sold.
In the US, these limits are set forth, investigated, and enforced by the FCC. In Europe, it is the EU and the corresponding CE certification that determine the test limits. The specifications are also different depending on the intended end use of the product and the marketplace.
Compliance is self-determined. That is to say, companies are responsible for proving that their product is compliant only if there is a complaint filed against the product. This may seem easy, but, failure to comply could result in a cease shipment/recall of all products in the field, fines, and imprisonment for management. End result – customers rely on special test houses (Compliance Labs) to perform the required testing.
Test include radiated emissions (how much RF is zinging off of a product and attached cables) and conducted emissions (how much radiation is being conducted through the power line).
Driving Growth Factors for EMI/EMC Testing
- Living in a Digital World: The addition of digital communications and control has created a host of new EMC issues, even with traditionally low emission analog systems. Old designs with new digital circuits need to be retested.
- Compliance Testing. Less than a fine, but not by much. Compliance testing can cost $2-3k dollars PER DAY to test. Typically, the labs will just give results but not solve the issue. They will consult on the solution for additional fees. This can get very expensive and inconvenient. Compliance testing is usually at a lab far away. Adds time to the development cycle.
Instead, make pre-compliance testing in-house. Design changes can be checked instantly.
Download an Application Note
on EMI Testing from Instek
Tracking Generator Option for Spectrum Analyzers
Tracking generators (TG) are useful for qualification and functional testing of cables, adapters, and antennas used throughout
RF Field Strength Meters
RF Field Strength Meters measure RF levels and electric field strength. These units are designed to perform basic signal tests without the need for an expensive, fully capable spectrum analyzer with a range of additional features. RF field strength meters are typically small, handheld units that operate via battery power. Some units also incorporate frequency counters.
Key features to consider in selecting RF Field Strength Meters
- Frequency Range. This should be a first consideration. Choose a unit that can measure frequencies in the your operating range
- Noise Floor. Noise floor represents the lowest signal level that the unit can measure
- Sweep Mode. Sweep Mode is how the unit will search for and pick up frequencies. Methods include single run, free run, and squelch run selectable
- Internal Attenuation. Some units offer internal attenuation features that can extend the input frequency range