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Allan Linton-Smith reviews an $80 ebay “bargain” tinySA: a 0.1MHz to 960MHz Spectrum Analyser I bought this “tinySA” spectrum analyser/signal generator on ebay for just $80 including delivery! It is a standalone device which can be connected to a computer for recharging and reprogramming. W hile oscilloscopes are used to measure and view signal amplitude (voltage) vs time, a spectrum analyser is used to measure and view a signal amplitude vs frequency. Like oscilloscopes, over time, cheaper and smaller spectrum analysers are becoming available. When I spotted the tinySA for sale, I had to get one as I use spectrum analysers often, and I wanted to know if a device this cheap was any good. It is a standalone device and is connected to a computer or USB charger. It can be programmed using tinySA software from www.tinysa.org/wiki/ It arrived neatly packed in a cardboard box with a lid and included two SMA cables, an SMA female-female converter, a small 10-30cm telescopic antenna and a USB Type-C charging cable. It comes in a nice little pocket-sized black enclosure and has two SMA connectors; one is the high-frequency input 86 Silicon Chip or output (260-960MHz), while the other is the input or output for lower frequency signals, down to 100kHz. It does not have a tracking generator; it is merely switched between analysis mode or generator mode. However, it can be used for plotting RF frequency response using the “max hold” setting and an external sweep generator. It worked straight out of the box. It’s remarkably accurate too, and we didn’t even have to charge it straight away. RF Spectrum Analysers are usually very expensive devices, often costing thousands of dollars (even old preloved ones). Australia’s electronics magazine So for $80, this seems like an excellent deal. And while some cheap modules we’ve tried either didn’t work at all or instantly self-destructed, this one gave useful readings immediately. Using it If you have ever used a “real” benchtop spectrum analyser, you will know that they may need a significant warm-up time and a lot of setting up. But this one required almost no adjustment. The resolution bandwidth (RBW) and reference level were set automatically, and the instrument discovered a signal immediately! Spectrum analysers definitely require a bit more ‘tuning’ than an Oscilloscope, but this little device makes life easy. Except for RF enthusiasts, most of us don’t really need an RF spectrum analyser all that often. But when you need one, you need it. So it makes sense to not spend heaps on a benchtop unit which will just be gathering dust for 99.9% of the time. siliconchip.com.au So if you are an experienced spectrum analyser operator, you will be able to use this device straight away. If you are a beginner, we will get you started with a few easy examples and practical applications. It sounds simple to use a spectrum analyser, but you need to set up some basic settings such as the frequency range you wish to examine and the resolution you require to analyse signals which may be close together. For example, if you wish to view an AM signal modulated with a 10kHz signal, you must use an RBW (resolution bandwidth) less than 10kHz. Otherwise, you will only see the carrier frequency and not the sidebands. Many readers will be familiar with modern oscilloscopes which can automatically set and display a trace. But with spectrum analysers, you have to tell it which signal or band (amongst many) you wish to examine. If you know your signal is around 10MHz, then you just set the centre frequency at 10MHz and the span for say 2MHz. Features & specifications • Low-cost, compact device. • Spectrum analyser and signal generator modes (cannot be used at the same time). • Two spectrum analyser inputs: MF/HF/VHF (0.1MHZ-350MHz) and UHF (240-960MHz). • Selectable resolution bandwidth (RBW) for both inputs, 2.6-640kHz. • Colour display showing 290 scan points up to the full low or high frequency range. • Input step attenuator of 0-31dB for the MF/HF/VHF input. • Two signal generator outputs: MF/HF/VHF sinewave output 100kHz to 350MHz; UHF square wave output 240-960MHz. • Automatic self-test and low-frequency input calibration. • USB socket allows it to act as a PC-controlled spectrum analyser. • Rechargeable battery lasts at least two hours. A marker will appear, telling you the exact frequency of the strongest detected signal in that range, and its amplitude. This particular instrument has a specified range of 100kHz to 960MHz, which will meet most hobbyists’ needs. But it has some limitations that you need to be aware of. Limitations For a start, you must be careful what you hook up to its inputs. The signal level cannot exceed 10dBm or 700mV AC (10mW into 50Ω), or you could damage it. Importantly, you also need to avoid applying any DC voltage to the inputs. Spectrum analysers are really sensitive devices, so it’s a good idea to always use it with an external attenuator until you are sure the signal is safe for a direct connection. SMA 20dB attenuators are available Here the “Tiny SA” is measuring a -30 dBm signal from an RF generator at 300.1MHz. The centre frequency was set at 300MHz and the resolution bandwidth set at 362KHz. The scan took 132 milliseconds to complete. siliconchip.com.au Australia’s electronics magazine September 2021  87 The tinySA’s display when fed with a 25MHz -30dBm carrier with 10kHz, 50% amplitude modulation. RBW was automatically set to 3.1kHz, its best resolution. The delta reading is 10.047kHz, and the carrier is shown as -28.7dBm, which is pretty good accuracy. We used this low-cost, low-noise RF preamplifier in combination with the tinySA analyser to detect signals down to -125dbm. That is about the minimum signal which expensive benchtop analysers can detect. for around $20 on ebay and similar sites. The displayed average noise level (DANL) is -105dBm, and that is the lowest detectable signal level. This changes depending on the resolution bandwidth. A lower bandwidth setting gives a lower noise level. More expensive spectrum analysers can often digitise a broad frequency range at once using an FFT (fast Fourier transform) technique. But the tinySA uses a resolution filter which is swept across the desired frequency range (just like tuning a radio). The oscillator that does the sweeping, together with the power detector that measures the signal power, require some settling time, and the scanning speed of the tinySA is limited. The narrower the filter, the more time it needs to settle. The fastest scanning speed occurs with RBW set to 300kHz or wider, and is about two scans per second. But with increased frequency span and/or decreased RBW, the scanning speed decreases. For example, a scan from 0-350MHz with RBW set to 10kHz takes about two minutes. Also, due to the low cost and very small form factor, you will find that the analyser sometimes develops spurious peaks called ‘spurs’ which can be attenuated by various settings, such as “spur reduction”. Other limitations are the lack of resolution bandwidth settings below 3.1kHz, and that signals in close proximity are impossible to resolve. The same signal as above, but fed to a more expensive spectrum analyser with ten times better resolution. The result is smoother and more accurate. This instrument weights 28kg, though, so it isn’t easy to hold in one hand! The tinySA’s start and stop frequencies were set to 88MHz and 108MHz, and the supplied 30cm aerial connected to capture FM radio stations in Sydney. In waterfall mode, each peak is recorded, and you can see the regular intervals between stations. The marker sits on the most powerful radio signal, 104.12MHz (2DAY FM). 88 Silicon Chip Conclusion While the tinySA is a handy little instrument, it is a bit limited compared to a bigger, more expensive spectrum analyser. As they say, there ain’t no such thing as a free lunch! Still, if you don’t have a spectrum analyser and don’t want to spend lots on buying one, it would be a great choice to start in the field of frequency domain analysis. SC Australia’s electronics magazine siliconchip.com.au The tinySA was fed with a 1MHz signal from a function generator, and a THD (total harmonic distortion) measurement was made. This shows the THD for the oscillator as 0.2%. This measurement was made by averaging 16 traces. The display is difficult to read because the other measurements partly obscure it, but it is handy all the same. Silicon Chip Binders REA VALU L E AT $19.50 * PLUS P &P Are your copies of SILICON CHIP getting damaged or dog-eared just lying around in a cupboard or on a shelf? Can you quickly find a particular issue that you need to refer to? A 1MHz signal from a function generator was analysed using the “harmonic” measurement setting, rather than the THD setting, and the results are a bit more revealing. The first harmonic is shown as -60.5dB and the second harmonic as -57dB below the fundamental. Keep your copies safe, secure and always available with these handy binders These binders will protect your copies of SILICON CHIP. They feature heavy-board covers, hold 12 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover The tinySA’s signal generator was set to 10MHz -15dBm and fed to a benchtop spectrum analyser, and here is the resulting plot. The first harmonic is -56.2dB, and the second harmonic is -46dB relative to the fundamental. That’s almost as good as our benchtop function generator! siliconchip.com.au Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Order online from www. siliconchip.com.au/Shop/4 or call (02) 9939 3295 and quote your credit card number. *See website for delivery prices. Australia’s electronics magazine September 2021  89