Buying spectrum analyzers – a brief guide for non-RF engineers

Wireless technology is everywhere.

The ability to communicate over long distances has been necessary since the dawn of society. However, nearly all forms of communication (smoke signals and heliographs excluded) predating the late 18th century were either spoken or written. Telecommunication was not a popular idea until the first telegraphs appeared. The telegraph enjoyed some popularity from the mid 1800’s through the early 20th century, but it still required wired infrastructure which limited its success. It was not until the work of renowned classical physicists such as Maxwell and Hertz that the theory behind transmitting electromagnetic waves was proven. Several decades later, Italian inventor Guglielmo Marconi and German inventor Karl Ferdinand Braun were able to realize and demonstrate practical radio frequency communications over long distances. These discoveries quickly enabled quantum leaps in societal and technological progress by making wireless communication a reality for the masses.

In order to optimize the application of RF communication in any given device, it is necessary to analyze components of the transmitted signal. One of the most common devices for doing this is a Spectrum Analyzer.

Spectrum analyzers are used to view radio frequency signals in the frequency domain; that is, with respect to frequency instead of time. The most basic type of measurement is the spectral density of the signal; amplitude versus frequency.

spec_1
Basic measurement of a 50 MHz signal across a 2 MHz span; that is, within a 49 MHz to 51 MHz window

Even for a basic spectrum measurement such as this, there are a number of performance metrics inherent to the instrument itself that can affect the result. Within the realm of spectrum analyzer types, there are different hardware architectures which are optimized for certain applications (after all, engineering is the art of compromise).

Without diving down the rabbit hole of Spectrum Analyzer theory, I would like to refer those who are interested to the following recommended reading:

Now, we’ll discuss some of the basic specs which you should use when comparing different spectrum analyzers.

  • Frequency range: Quite literally, the frequency range within which the instrument can measure. For most IoT work (with an emphasis on 2.4 GHz ISM band), up to 3 GHz will cover the basics. The minimum frequency can also matter, but usually down to 9-10 KHz is typical for brand name entry level SAs.
  • DANL (Displayed Average Noise Level): This is the noise floor of the instrument with no input signal present. This spec is usually the main determinant of the instrument’s overall dynamic range, where dynamic range spans from the DANL to the maximum input level (typically between +20 to +30 dBm).
    • Some spectrum analyzers have internal preamplifiers which can further extend the noise floor. If you plan on doing E/H near field probing (for EMI emission testing) or simply measuring low-power signals, you should aim to get a unit with the preamp option included/enabled.
  • Phase noise: With respect to RF engineering, phase noise describes the spectral “impurity” of any real oscillator. A pure, ideal sine wave is one frequency, and one frequency only. This would appear as a single vertical line on an amplitude vs frequency graph. In reality, frequencies produced by real oscillators contain signals that have notable power outside of the principal frequency (aka “carrier”). This results in the curvature seen in the graph of basic spectrum measurements. It is therefore desirable for the spectrum analyzer itself to have the lowest possible phase noise.
phase_noise_diagram
Visual depiction of phase noise – courtesy of electronics-notes.com

There are other specifications which may be of interest if you are really invested in splitting hairs. I would suggest the following reading:

Now that we have established (in my opinion) the top three selection criteria for comparing different SAs, it is time to take a look at the range of used and new equipment available within your budget.

There are lots of older, used spectrum analyzers on eBay and other second hand markets. A majority of these units will be Hewlett Packard/Agilent or Rhode and Schwarz brand. Even by today’s standards, some of these units still provide respectable performance. If you need to make measurements up into the K band (26.5 GHz), a new brand name spectrum analyzer in this range could easily exceed $50,000. However, you can get models such as the venerable HP 8563E or Agilent E4407B for a few thousand in working condition.

In my search for a “well rounded” budget spectrum analyzer, I would not necessarily go for maximum frequency. At that price, you are sacrificing other performance metrics such as better noise floor and phase noise characteristics offered by newer models. Additionally, newer spectrum analyzers come with software suites that can perform demodulation and other types of useful computational measurements. Plus, newer models are not nearly as massive as these older units which predate the use of surface mount components.

So, with a absolute maximum budget of $4000, I set out to find the best bang-for-buck new-ish spectrum analyzer I could find. I ended up doing a comparison of several models, both USB and full-fledged standalone units. Although I’m on a budget, I personally prefer to buy from brand name firms based in the US (and Germany… because German engineering).

Comparison_image

If you do not exercise the same personal preferences as I do, there are also several competitive models from up and coming Chinese firms such as Siglent and Rigol, both of whom are big players in the budget test equipment market.

So which spectrum analyzer did I end up getting? That would be the Rhode and Schwarz FPC1000. I was able to locate an awesome deal (at TEquipment.com – no sponsorship or affiliation whatsoever), where they were bundling the FPC1000 with $2900 added value in software (3 GHz upgrade, preamp unlocked in software, advanced measurements, EMI receiver mode, and modulation analysis). If you have an extra $1500+ burning a hole in your pocket, R&S also offers the FPC1500 which is the same spectrum analyzer architecture but with a built in tracking generator that can also function as a one-port VNA.

I hope this guide has been useful for anyone looking to make their first spectrum analyzer purchase. There is a lot to become familiar with, and there are countless resources I was able to leverage to familiarize myself with this subject. One of the best information sources I learned from was “The Signal Path” – a YouTube channel focused on RF experiments and equipment teardowns, from entry level gear like the units discussed here to $1.3 million dollar, 110 GHz oscilloscopes and everything in-between!

Be sure to view follow the discussion about this article on the EEVBlog forums, where you can find many useful tips: Click Here

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