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Can a $129 spectrum analyzer be any good?

-March 01, 2012

Every EMC or design engineer should own a spectrum analyzer. Most new ones, however, are expensive, and used ones are large and heavy. Until recently, these instruments have been priced too high for electronics hobbyists or engineers on a budget. Imagine my surprise when I found the RF Explorer, a small handheld spectrum analyzer priced from as little as $99. Could it possibly be any good?

There are five RF Explorer models. Four are single-band units, each covering the most-used ISM (Industrial Scientific Medical) bands--433 MHz, 868 MHz, 915 MHz, and 2.4 GHz--and one model that encompasses all bands, except 2.4 GHz. The 2.4 GHz band may be retrofitted into the all-band model for an extra $55. The RF Explorer uses the Silicon Labs Si4431 receiver chip (240 MHz to 960 MHz). I purchased the $129 WSUB1G. It tunes from 240 MHz to 960 MHz. (See below for a list of features and specifications.)

To test the RF Explorer, I attached an H-field loop probe from Beehive Electronics and started probing a crystal
 RF Explorer handheld spectrum analyzer

Probing a crystal oscillator demo board with the RF Explorer displays usable harmonics. Click on image to enlarge.

oscillator demo board used in my EMC seminars (figure). The sensitivity was sufficient to display usable harmonics.

The analyzer is relatively easy to configure for frequency and span (or high and low limits) and reference level. Once the span is set, pressing the left and right keys causes the frequency to change in half-span steps. Pressing the up/down keys changes the vertical range in steps according to the defined vertical scale. The RBW (resolution bandwidth) is automatically set and displayed at the bottom by pressing Return during a measurement. This displays the center frequency, span and RBW. Pressing Return once again displays the start, center, and stop frequencies.

I like the fact the unit includes modes for normal, max, averaging, and max hold. Normal mode displays the signals with no calculations. Max takes the last (1 through 28 user-defined) sweeps and displays the peak amplitudes. Averaging takes the last (1 through 28 user-defined) sweeps and calculates the average. Max hold changes to a persistent display, recording the highest-level amplitudes detected.

The instrument firmware is open source, so the designer, Arial Rocholl (from Spain), and an active worldwide user group are always improving the performance and adding additional features. Units can be easily upgraded with the latest code. Another nice touch is that free client software is available for both the PC and Mac platforms for remote programming, display, and waveform capture through the USB port.

While the RF Explorer is certainly usable as a limited EMC troubleshooting tool and for general spectrum measurements, it does have a few drawbacks. The most obvious is the limited frequency range of 240 MHz to 960 MHz. I'd like to see the unit could tune down to at least 10 MHz or even 1MHz, and go up to 1.2 GHz. Unfortunately, the Silicon Labs Si4431 receiver IC limits the bandwidth. Rocholl is currently looking into alternatives.

The RF Explorer has an input-power limit of +5 dBm. Rocholl is very careful to bring this to the attention of the user, but this is another limitation of the receiver IC. Use a power limiter or attenuator should first prior to measuring an unknown signal.

While the frequency appears to be within the specified tolerance, I did notice that the unit seems to measure about 4 dB to 5 dB low at the frequencies I tried. Rocholl acknowledges this is an issue with some frequencies and is working on a user-based calibration procedure.

Use of a spectrum analyzer is vital for the EMC troubleshooting process and general RF design. While this model lacks a completely adequate frequency range, the price/performance is spectacular and may be just the ticket for cash-strapped companies who just need a minimal instrument to beat down their emissions problems.

The RF Explorer series is available through www.seeedstudio.com/depot/ (under "Hacking and Measurement") as well as a few other hobby-oriented sources. The online version of this article contains a link to a YouTube video that demonstrates the basic operation.



RF Explorer published specifications

Frequency band
240 MHz to 960 MHz

Frequency span

112 kHz to 100 MHz

Backlit LCD display

128x64 pixels

Connector

Standard SMA, 50-Ω

Antenna

Wideband Nagoya NA-773 telescopic antenna included

Dynamic range

-115 dBm to 0 dBm

Absolute Max input power

+5 dBm

Average noise level (typical)

-110 dBm

Resolution bandwidth (RBW)

automatic 2.6 kHz to 600 kHz

Modes

normal, averaging, max, max hold, and overwrite

Size

113 mm x 70 mm x 25mm

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