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A 900MHz Touchscreen Vector Network Analyser for less than $60.00? It wasn’t long ago that a Vector Network Analyser (VNA) would cost as much as a car, or more. But now you can buy one for peanuts: this one was under $AU55 delivered! In case you don’t know, a VNA can be used to test and analyse antennas, transmission lines, filter networks and other RF-related passive networks. So it’s a very useful instrument to have if you are doing any RF work at all. T his little device was only released recently, but it already has countless fans, umpteen discussion groups and hours of YouTube videos showing how to use it – by people from novices to super experts. The NanoVNA is available from many sellers on ebay and AliExpress, so as long as you are willing to wait a little while for it to arrive from China, it isn’t hard to purchase your very own VNA. By the way, VNAs aren’t just useful for radio engineers. High-speed digital buses can have very high edge rates that translate into frequency components in the multi-GHz range. So a good VNA can be used to characterise such buses, assuming you know how to use it! As the name suggests, the NanoVNA is small, measuring just 85 x 54 x 16mm and weighing 73.5g. It’s powered by an internal 400mAh lithium-polymer battery that’s recharged from a 5V USB source, and has a colour touchscreen interface and two SMA connectors for interfacing to the outside world. The only other adornments are an on/off slide switch and left/right ‘joy- stick’ pushbutton for control. Ours came with three SMA terminators: open, closed and 50Ω, plus a USB Type-C cable for charging the internal battery and for connecting to a computer. The SMA terminators are required to calibrate it, and this there were quite a few holdouts still using type-B connectors until recently, and plenty of random devices still use the B types. Is this a harbinger that type-C connectors are becoming more standard now? Anyway, for some handy Joe Smith tips regarding the physical handling, calibration and connecting to USB software, see this video: https://youtu.be/ mKi6s3WvBAM What is a VNA? should be done regularly. Some sellers also include a short SMA cable, but ours didn’t come with one. As an aside, this is one of the first ‘el cheapo’ devices we’ve seen with a USB type-C micro socket on it. This has been the ‘new standard’ for smartphones for some time now, but Vector Network Analysers are one of the predominant lab/field instruments used for RF and microwave design purposes. They are ideally used to test the response of DUTs (devices under test) as a function of frequency. Fig.1 shows the basic arrangement of a VNA. It applies a swept frequency signal source to one end of the DUT, and measures the amplitude and phase of the signals at both ends of the DUT relative to a separate fixed reference signal source (the “local oscillator”). These measurements are often made by mixing the local oscillator and test signals to get a sum and difference Review By Allan Linton-Smith 34 Silicon Chip Australia’s electronics magazine siliconchip.com.au signal, then feeding this through a low-pass filter to isolate the difference signal. The resulting signal (which is much lower in frequency) then goes to an analog-to-digital converter. By using three such receivers, and digital signal processing, the VNA can measure the amplitude and phase of the original, transmitted and reflected signals and thus fully characterise the DUT. The DUT can be a passive or active device. Examples of passive devices that can be tested by a VNA are cables, filters, splitters, connectors, couplers and antennas. Active devices for testing this way can be RF amplifiers, RF filters and semiconductors. The NanoVNA is basically a sweep generator which can measure the reflected signal and calculate the amplitude, phase, standing wave ratio (SWR), impedance, capacitance and inductance all at the same time! The primary signal from the internal sweep generator output is fed to the DUT, and the reflected signal is compared to the transmitted signal. The power ratios (actually, their square roots) vs frequency are then processed. Much information can be obtained from the results, including: • losses (such as cable and antenna losses) • standing wave ratios • impedance (at very high frequencies) siliconchip.com.au A OR B SIGNALS FROM DIRECTIONAL DEVICES MIXER DIGITAL SIGNAL PROCESSOR LOW-PASS FILTER ANALOG TO DIGITAL CONVERTER LOCAL OSCILLATOR SC GENERIC VNA RECEIVER BLOCK DIAGRAM SIGNAL SOURCE 2020 DIRECTIONAL COUPLER DIRECTIONAL COUPLER TEST PORT 1 TEST 1 REFERENCE MIXER MIXER ADC IF AMP ADC IF AMP LOCAL OSCILLATOR IF AMP TEST 2 ADC TEST PORT 2 MIXER TRANSMISSION/REFLECTION VNA BLOCK DIAGRAM SC 2020 Fig.1: an overview of how a typical VNA works. The receiver block at top is repeated three times in the diagram below (dashed red outlines), to measure the test signal and the signals at either end of the DUT relative to a common reference signal (local oscillator). A digital signal processor (DSP) crunches the numbers from these three receivers to generate useful plots which describe the RF behaviour of the DUT. Australia’s electronics magazine April 2020  35 Fig.2: this plot shows out the signal generator built into the NanoVNA cannot deliver anywhere near as much amplitude over the 300-900MHz range as compared to the 54-300MHz range. So measurements made above 300MHz will likely contain a lot more noise than those at lower frequencies. • capacitance • inductance • phase information This is all highly useful to designers of RF circuits, antennas and HF or microwave devices. The low cost of this particular unit finally makes such tests easily accessible to amateurs and experimenters. VNAs can also be useful test instruments for tracking down faults and, as we discovered, it can also double as an accurate and convenient RF frequency generator. The NanoVNA manufacturer claims that it makes these measurements at up to 900MHz, although it really is only fully effective to 300MHz, as we shall demonstrate. One of the disadvantages of the VNA is that it makes all measurements in the frequency domain, unlike an oscilloscope, which measures in the time domain. So the information gleaned from the VNA must often be translated into the time domain to be useful. signal level is still high enough to give useful qualitative information up to 900MHz. This plot was obtained by feeding the NanoVNA’s output into a spectrum analyser which was set to “maximum hold”, thus memorising a succession of all the maximum points. The roughness of the graph from 300-897MHz is merely an artefact where the analyser sweep has not coincided with the generator sweep, because the analyser sweep is much slower (66ms). Due to the number of points and the sweep time, this measurement took several hours to make! You can use the NanoVNA as a reasonable accurate frequency generator. Fig.3 shows a spectrum analysis of the unit’s output when set to 250MHz; we measured a peak noise reading of -115dBm at an offset of 100kHz offset from 250MHz fundamental. This noise level is quite acceptable, being around 100dB below the signal level. To set it up for a fixed frequency output like this, you merely set identical start and stop frequencies, or select a single frequency from the menu. As shown in Fig.4, we detected signals up to around 1.2GHz, which are the harmonics of lower frequencies when the NanoVNA was set to sweep over its full range. -4.91dBm at 1.2GHz is 127mV into 50Ω. Some sellers are charging upwards of 5x the price for Nanos which have supposedly been extended to 1GHz, so look out! Conclusion While the NanoVNA has some limitations compared to a multi-thousanddollar instrument, it is nonetheless a Tests Fig.2 shows our measurement of the output signal level from the NanoVNA generator over the range of 54897MHz. The output is not linear and drops significantly, by about 9-11dB, above 300MHz. We believe that the 36 Silicon Chip Fig.3: a spectrum analysis of the test signal fixed at 250MHz. This indicates that the test signal is very clean, with noise levels around 100dB below the signal itself. So it could be quite useful just as an RF signal generator. Australia’s electronics magazine siliconchip.com.au very useful device. Anyone working with RF circuitry or antennas will likely find it well worthwhile, especially considering the price. It helps to be aware of its limitations to make full use of it; you will likely also have to do a fair bit of reading on the operation of VNAs to understand which modes to use and how to interpret the rather esoteric information and graphs displayed! Even if you only need a VNA occasionally, for little more than the price of a nice dinner, it’s hard to argue that the NanoVNA is not good value. You might as well get one ‘just in case’ you never need it... You may find the following links useful. * Beginners’ guide: siliconchip.com. au/link/ab0f * A video that would be useful to amateur enthusiasts: https://youtu. be/8kx9SWbEcXI * A complete guide to and mathematical explanations of VNA operation: siliconchip.com.au/link/ab0g (or purchase the complete book, “The VNA Applications Handbook”). SC Fig.4: this plot shows spurious signals in the 900-1200MHz range, generated during a sweep across its normal 54-900MHz test range. These are presumably from test signal harmonics. So the device may not be very useful above 900MHz, even if it could generate test signals that high. AUSTRALIA’S OWN M I CR O M I T E TOUCHSCREEN Since its introduction in February 2016, Geoff Graham’s mighty Micromite BackPack has proved to be one of the most versatile, most economical and easiest-to-use systems available – not only here in Australia but around the world! 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Voltage/Current Reference (Oct16 – siliconchip.com.au/Series/305) Energy Meter (Aug16 – siliconchip.com.au/Series/302) Super Clock (Jul16 – siliconchip.com.au/Article/9887) Micromite Boat Computer (Apr16 – siliconchip.com.au/Article/9977) V 3 BackPack: Ultrasonic Parking Assistant (Mar16 – siliconchip.com.au/Article/9848) * JUST $7500 See August 2019 (Article 11764) P&P: Flat $10 PER ORDER (within Australia) *P Price is for the Micromite BackPack only; not for the projects listed. siliconchip.com.au Australia’s electronics magazine April 2020  37