What is the best budget spectrum analyzer?

Digital circuitry produces a wide spectrum of harmonic frequencies, and spectrum analyzers are essential tools for displaying these RF frequency spectrums (power or voltage) versus frequency. This makes them indispensable for EMC troubleshooting. Historically, these instruments are among the most expensive, with lab-quality, portable benchtop spectrum analyzers priced between $10k and $20k. For those considering secondhand options, prices range from $1k to $5k, albeit with the trade-off of weight and initial cost.

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However, many smaller or mid-sized companies may not have the budget for a high-end, lab-quality analyzer. Fortunately, several manufacturers now offer more affordable, high-quality instruments suitable for troubleshooting and pre-compliance work. I’ve compiled a list of several such instruments for your consideration.

Figure 1. A typical spectrum analyzer display showing narrow band harmonics from an Ethernet clock and broadband emissions from a DC-DC converter. Both extend beyond 1.5 GHz! The white circle indicates the U.S. LTE cellular and close to the GPS band at 1575 MHz.

Choosing a Spectrum Analyzer

When selecting a spectrum analyzer, there are several key factors to consider: cost, frequency range, allowable frequency span, form factor, and acquisition type.

Cost – Prices can range from as low as $50 to as high as $5,000. Reliable benchtop analyzers are available around the $2,000 mark. Portable analyzers vary between $100 and $2,500, contingent on quality and features.

Frequency Range – Although many analyzers cover up to 1 GHz, which is the minimum for general troubleshooting, certain EMC tests necessitate measurements down to 10 kHz (for military/aerospace) or 150 kHz (for commercial/industrial). Depending on the application, upper frequency ranges can extend to 6 GHz (commercial/industrial) or even 18 GHz (military/aerospace).

Allowable Frequency Span – Most commercial spectrum analyzers won't present issues in this area, but lower-cost software-defined radios (SDRs) used as spectrum analyzers may have limited frequency spans. This can restrict what frequencies you can monitor simultaneously, affecting troubleshooting efficiency.

Form Factor – The ideal analyzer is usually benchtop-designed with a small footprint, leaving more room for other equipment. For fieldwork or frequent travel, portable analyzers are worth considering. USB-controlled analyzers, reliant on a PC for display and control, also offer a unique solution.

Acquisition Type – There are two primary measurement acquisition types: swept and real-time. Swept analyzers liken to traditional oscilloscopes, sweeping frequencies between start and stop points and then analyzing and displaying data in intervals. This could miss pulsating or intermittent frequencies. Real-time analyzers, due to their fast A/D conversion and powerful processors, display frequency information almost instantaneously, making them ideal for capturing intermittent interference or digital modulations.

Budget/Hobby Analyzers

The following instruments may not be suitable for serious EMC troubleshooting but can display both broadband and narrowband harmonic energy, making them useful for hobbyists or gross emission checks.

RF Explorer – For hobbyists or firms with a tight budget, the RF Explorer, costing $130, is a good option. It offers a frequency range of 240 to 960 MHz, with an optional $60 Wi-Fi module. Although its dynamic range is -90 dBm, it is sensitive enough for near-field probe observations.

Figure 2. The RF Explorer model WSUB1G tunes from 240 to 960 MHz and costs $130.

For a broader range, the improved WSUB3G model tunes from 15 MHz to 2.7 GHz, priced at $260. Both models are available from Seeed Studio, a hobbyist electronics store based in Shenzhen, China.

The AirSpy – As an SDR-based spectrum analyzer, AirSpy is extremely low-cost and well-suited for general EMI troubleshooting. It tunes from 24 MHz to 1.8 GHz, with sensitivity down to -130 dBm. A Spyverter companion extends this range down to DC to 60 MHz. Both units connect via USB and are controlled by a PC running Windows.

A significant effort has gone into providing shielding and connector transient protection. The AirSpy costs $199, and the Spyverter an additional $59, both available on AirSpy’s website.

Figure 3. The AirSpy software-defined radio (left) and the Spyverter low-frequency converter (right).

Driven by the free "Spectrum Spy" software, AirSpy can display an accurate spectral plot, featuring a "waterfall" display that tracks frequency versus time, useful for identifying intermittent EMI pulses. The SpectrumSpy software also records emissions to the PC’s disk drive.

Figure 4. The AirSpy measuring emissions from an embedded processor board. The 270 MHz processor clock is clearly visible.

The AirSpy website offers links to various useful software applications. The commonly-used “SDR#” (SDR Sharp) is available but limited to a 10 MHz span. For more similar controls to a traditional spectrum analyzer, I recommend the SpectrumSpy application. Although it lacks some advanced features like measurement in dBuV, max hold, and adjustable resolution bandwidths, it supports general measurements and broad frequency spans, making it a practical tool for EMI troubleshooting.

Tiny Spectrum Analyzer – The "TinySA" has become very popular within the hobbyist community. Available for under $100 through Amazon and other sources, it performs as a swept spectrum analyzer. Official information on obtaining genuine units is available at TinySA's official wiki.

Figure 5. The TinySA tunes from 100 kHz to 350 MHz and 240 to 960 MHz.

General specifications include:

• Spectrum analyzer with two inputs, high quality MF/HF/VHF input for 0.1MHZ to 350MHz, lesser quality UHF input for 240MHz to 960MHz.
• Signal Generator with sine output for 0.1MHz to 350MHz and square wave output for 240MHz to 960MHz when not used as spectrum analyzer.
• Switchable resolution bandpass filters between 2.6kHz and 640kHz.
• Color display showing 290 scan points covering up to the full low or high frequency range.
• Input step attenuator from 0dB to 31dB for the MF/HF/VHF input.
• Built-in calibration signal generator for automatic self-test and low input calibration.
• USB connectivity for PC control (spectrum analyzer or signal generator).
• Rechargeable battery supporting at least 2 hours of portable use.
• Max input level +10dBm.

However, some limitations are inherent in this compact and affordable design:

• Internal phase noise sets a lower limit for phase noise measurements.
• The minimum resolution bandwidth of 2.4kHz limits spectral detail visibility.
• The high input (240MHz to 960MHz) has limited image suppression and a single built-in attenuator level.
• The high input attenuator is frequency-dependent, varying between 25dB and 40dB.
• Measurement time per point increases below 30kHz resolution bandwidth due to filter implementation.
• Performance limitations in shielding and filters may lead to visible images and spurs, though spur suppression and switching to below IF may help.
• Sensitivity decreases below 0.1MHz.
• For frequencies below 1MHz, disabling the automation gain control (AGC) and enabling the low noise amplifier (LNA) is recommended for better measurement quality.
• Radiation from the TinySA MCU at 48MHz and its harmonics may affect measurements when using the supplied telescopic antenna or low RBW.

Despite these limitations, for general emissions troubleshooting, the TinySA may suffice. The display includes markers and a waterfall display, and it can be controlled via USB using free software.