LiteVNA Review

LiteVNA64

The LiteVNA is a portable vector network analyser, it offers reasonable performance for a very low price, making it an ideal tool for electronics hobbyists and amateur radio operators, it can be had for under £100 from China.

For those who are not familiar with a VNA, it measures the amplitude and phase response of a circuit or device over a wide range of frequencies, this is often used for the characterisation of filters, antennas, components, resonant circuits, coaxial cables, amplifiers and much more, if you do any kind of work with RF this is a must have tool.

The LiteVNA is an evolution of the NanoVNA which was cloned and enhanced by various companies producing a number of different versions, rather than confuse you with a long list I will just say the current best options are the LiteVNA and the more professional LibreVNA, buying anything else is essentially a waste of money.

Design & Build

In terms of design there is only so much I can say as it’s closed source to prevent a flood of bad quality clones, and to be honest I’m not entirely qualified to go into the details, suffice to say it’s a two port T/R only VNA that can operate between 50 kHz and 6.3 GHz, with even higher possible using harmonics, the performance is rather average, but considering a good mainstream VNA will set you back at least £1500 it’s impossible to find a fault with it.

The LiteVNA is of a surprisingly high build quality, it has a sturdy plastic case and feels like a solid piece of test equipment, it comes nicely packaged in a box that includes a SMA calibration kit, two SS405 coaxial cables (SMA male), SMA female to female adapter, USB-C cable and a carry strap.

For input it has a color LCD touchscreen and a jog switch, the screen can be flipped in software and it uses a simple yet effective user interface, the USB-C is used for charging as well as communication with software available for Windows, Linux and Mac OSX, a MicroSD card can also be used to optionally save screenshots and data (provided it’s under 32GB in size).

It’s available in two different models, the 62 and 64, aside from screen size they are practically identical, but I’d suggest going for the 64 as it isn’t really that much more expensive and the bigger screen helps.

Practical Examples

Here are some examples that highlight how useful the LiteVNA can be and why you should really have one.

The first is a measurement of an adjustable monopole antenna that has been extended to about 1.7 meters, as you can see there is a fairly decent match at 42MHz, which as expected is roughly 1/4 the length of the antenna.

The next two are cable loss measurements up to 6 GHz of two different cables, the first is a very cheap cable, whilst the second is the better quality SS405 that came with it.

I could go on but you get the idea, the LiteVNA has many useful applications.

Conclusion

If you don’t already have a VNA then this is a must have addition to any electronics lab or radio shack, for the price it’s quite hard to find a reason not to have one, of course if you do own a higher quality one this is probably useless, although being so portable and compact it can be a nice addition.

Also make sure you get it from a reliable seller, I highly recommend the following two sellers:
Aliexpress
BangGood

Make sure you update the firmware to the latest once you get it.

Crystal Radio Improvements

Previously I made a basic crystal radio, since then I’ve made a number of design improvements.

Crystal Radio Schematic

Multiple Bands

There is only a few medium wave stations where I live so I decided to add shortwave as well, to achieve this I used a rotary switch and a 7uH inductor, this is wound on a T50-2 iron oxide toroid, this ensures a high Q factor that would be difficult to achieve with an air core, Unfortunately it’s nearly impossible to tune as it covers near enough the entire shortwave band, so a bit of a failure on my part.

Antenna Coupling

Originally I used a fixed 10pF coupling capacitor, I have since replaced that with a 35pF variable trimmer capacitor, I find around 25-30pF to be optimal, although it’ll depend upon your antenna and how selective / sensitive you want it.

Audio Amplifier

To make listening more comfortable and improve reception of weak signals I made an audio amplifier, this can be switched in with a DPDT toggle switch and is powered from four AA batteries, this is capable of driving a 8 ohm loud speaker at a modest volume.

The circuit is nothing special but works quite well, care needs to be taken with the input wiring to avoid hum, shielded or coaxial cable is best.

Case

This time I decided to make something half decent looking, I glued together some scrap MDF sheet to make a base and front panel, I then applied walnut wood veneer to the front and varnished it, on the front I mounted two 3.5mm jacks, one for crystal earphones and the other the amplifier output, the tuning capacitor, DPDT toggle switch, 1 pole 12 way rotary switch and the volume and gain pots.

On a piece of brass sheet I have a BNC connector for the antenna input, I made a bit of a mess of the veneer during construction but oh well, lesson learned, don’t drill wood veneer.

Results

Adding an audio amplifier really boosts what can be received, another thing I have considered is adding a LNA (Low Noise Amplifier) as the RF front-end, however at this point it would make more sense to use a different radio technology.

Overall I’m quite pleased with it but there is definitely room for improvement, but for now I’m done with crystal radios, my next attempt will likely be a regenerative or perhaps superheterodyne radio.

Making a Crystal Radio

Crystal Radio Schematic
Click for full size

A crystal radio is a radio designed for receiving audio (voice) broadcasts, it was invented around the beginning of the 20th century and became extremely popular in the 1920’s and 1930’s, allowing millions of people to access radio broadcasts that otherwise would have been prohibitively expensive, the majority would have been self constructed sets rather than commercial devices, this sparked a huge interest in radio and electronics and ultimately was the catalyst for the rise of commercial radio.

The reduced cost of vacuum tubes and increased reliability led to the downfall of the crystal radio, however it continued to see sporadic usage and has had numerous revivals to the present day, the ease of construction, low cost and no need for a power source means it still sees usage in poor countries and is a popular project for electronics and radio hobbyists.

Crystal radios are primarily designed to receive Amplitude Modulated (AM) broadcasts, although examples have been made that can receive Frequency Modulated (FM) broadcasts, the most common band used is the medium wave broadcast band which roughly spans 530 to 1,700 kHz.

Theory of Operation

While there are many variations in design all crystal radios consists of four main blocks, the antenna, tuner, detector and speaker.

The antenna is chosen to receive radio waves as efficiently as possible, for medium wave AM band a monopole, loop or ferrite rod antenna is a common choice, there are two options for connecting the antenna to the tuner, magnetic or capacitive coupling, I opted for the latter which is the purpose of C2.

The tuner in the majority of sets consists of a fixed inductor (L1) and a variable tuning capacitor(C1), the tuning capacitor needs a value of at least 500pF to cover the entire medium wave band, for inductance a value between 200uH and 250uH is often used, this forms an LC tank circuit which resonates at a specific frequency, given by the equation:

$$ f_0 = \frac {1}{2\pi\sqrt{LC}} $$

Solving for inductance gives the equation:

$$ L = \frac {1}{4\pi^2Cf^2} $$

It's preferable to use the lowest inductance possible as longer coils tend to have greater resistance which reduces the Q (quality) factor of the coil, however since Q is tied to bandwidth too much can also be an issue.

In my case for the medium wave band I decided on 220uH inductor and a 720pF tuning capacitor I had in my parts bin, you will most likely want to wind your own inductor as commercial inductors of this value normally use ferrite cores which can introduce losses, it may work fine but I haven’t tested it, in any case using an air core coil is traditional.

The detector (D1 & C3) is used for demodulating (extracting) the audio from the RF carrier, back in the day this was an actual lump of crystal such as galena or iron pyrite, the ‘cat’s whisker’ would be adjusted over the surface until a sensitive spot was found, various alternatives have also been used, for simplicity I went with a 1N34A germanium diode (alternative 1N270), the diode choice is very important, germanium starts to conduct as low as 0.1V, I tried a common silicon 1N4148 which gave very poor sensitivity, a schottky diode may be a viable alternative here, the value of the capacitor is not critical, 1nF worked for me.

Finally for the speaker I used a high impedance piezoelectric earpiece, these have an extremely high impedance, so much so that a 100k resistor (R1) is required to provide a suitable discharge path, traditionally a more conventional high impedance speaker would be used giving an impedance around 3k to 10k, these are still available from places like ebay albeit more expensive.

Assembly

A crude but functional crystal radio

For designing the inductor I used the excellent free program Coil32 which I highly recommend you check out, this gave me the number of turns required.

Winding the coil is a bit of an art in itself and can only be learned through experience, I typically use a wax coated cardboard tube, wind the coil as neatly as possible, then coat it with more wax to secure it, this has the advantage over varnish that it sets hard right away, in any case the value is not super critical so there is no need to worry if you do not have an LCR meter handy, go with the calculated number of turns and add a few more for luck.

I mounted the coil and tuning capacitor on a piece of scrap wood, since this is so simple I decided against using a PCB and simply wired it point to point, I made provision for swapping the detector parts using female machine pin headers so I could experiment.

I plan on remaking it much nicer at some point, there is many good examples made by Dave Schmarder.

Testing

Having a strong AM station nearby is pretty much required, you will have trouble receiving weaker stations, although with a good antenna it should be possible, I had no problems receiving a 2kW station located about 3km away even inside the house with a cheap monopole, some European countries no longer broadcast medium wave so you may have more difficulty there.

For a test signal I used my TinySA with 1kHz AM modulation, a signal inserted at -7dBm was clearly audible and was detectable as low as -23dBm, although I noticed a decrease in sensitivity at the low end of the band likely because Q is frequency dependent.

Improvements

The nice thing about crystal radios is there is a lot of things you can do to improve them, such as adding an audio or RF amplifiers, different bands, different detectors, different coil winding methods, audio transformers and so on, so making one is definitely worth the time.

TinySA Spectrum Analyser Review

The TinySA is a low cost portable spectrum analyser that also functions as a signal generator, this is a true spectrum analyser that has a wide dynamic range and can measure a signal up to 960 MHz, it is developed by Erik Kaashoek and has open source firmware but not hardware.

It features a 2.8″ LCD color touchscreen in a 90×58 mm plastic case, it includes a rechargeable internal 650mAh battery for portable use but can also be powered and recharged by the included USB C cable, it has two SMA female ports for input and output and a jog switch for additional control.

Hardware

The TinySA is primarily designed to operate between 0.1 to 350 MHz, input and output for this range uses the low SMA port, this includes a 0-31dB attenuator (1dB steps), the minimum RBW (Resolution BandWidth) is 3 kHz giving it a reasonably good signal resolution, it has a maximum of 290 measurement points when not connected to a PC and overall RF performance is decent, keeping in mind this is portable and low cost.

The high port operates between 240 to 960 MHz and is lower quality than the low port, but still quite functional for many usage cases, this also includes the calibration signal generator fed from a 30MHz TCXO which is used to calibrate the level of the low port, a single level attenuator is included which varies from 22.5dB to 40dB depending on frequency, image suppression of this port is poor so it should be considered as a free extra rather than the main purpose of the TinySA.

Overall the hardware is exceptionally good for the price, but you should not expect similar performance to a modern spectrum analyser, it is however more than good enough for most applications that do not require high precision measurements.

Included with the package is two SMA male cables and a SMA female to female adapter, as well as a small SMA extendable antenna, although mine broke pretty quickly.

Signal Generator

The TinySA also functions as an excellent signal generator, note that this cannot be used at the same time as the spectrum analyser function, the low port can put out a 0.1 to 350 MHz sine signal between -76 and -7dBm, it can also perform a frequency and level sweep as well as AM, Narrowband FM and Wideband FM modulation making it extremely versatile.

The high port can also put out a signal between 240 to 960 MHz square wave, with a level between -38 and +16dBm, as well as frequency sweep and narrow and wide FM modulation, it’s important to note that being a square wave there is a very high harmonic content which easily exceeds 2GHz making this capable of producing a signal in the many GHz range, as such it should never be used to drive a power amplifier and antenna.

Harmonic output at 1.75GHz with FM modulation

Software

The software is extremely well made and easy to use, and for the most part reliable although I have had the occasional freeze, even with the small touchscreen it’s quite usable by those possessing fat fingers, the firmware is easy to update and impossible to brick due to mistakes making it very user friendly.

Software for PC use is also available for Windows and limited use with Linux, this can extend the number of measurement points to many thousands giving even more resolution.

High port connected to an antenna and PC, 3000 points

Overall

I’m very impressed by the value for money offered by the TinySA, normally for a spectrum analyser you’d need to pay several hundred pounds, to get a reasonably similar alternative for $50 is huge, even though it’s more limited, the functionality is perfectly good for many usage applications such as verifying the output of a radio, tuning filters, RFI and EMI testing and much more, as such any electronics lab should have one of these as a must buy item.

Beware that there are some poor clones being sold out there that may perform much worse, an official list of sellers is available on the wiki.