AB9IL.net: Malachite Mini SDR

Written and curated by Philip Collier / AB9IL
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The Malachite Mini SDR represents current state of the art in quadrature sampling (QSD) type software defined radios. It is very small, consumes little electrical power, and has a nice balance of functions. In days of yore (the year 2000), broadcast or utility listeners would need a rack of equipment to do what the Malachite SDR does in a cigarette-pack form factor. Being so small induces its own problems; we'll review that later. I am writing this in 2021 to say that this SDR has had some good revisions since its 2019 introduction and warrants consideration by anyone looking for a small and very capable radio.

What Architecture is the Malachite SDR?

The Malachite SDR uses a quadrature sampling detector (QSD) circuit. A small tuner, the MSI001, generates the RF oscillator signal. It is not a pure zero IF / direct conversion radio: it mixes the input from the antenna to an intermediate frequency, where filtering and gain control are applied. This IF signal is what is converted to baseband by the QSD. A 16 bit audio analog to digital converter (ADC) converts the quadrature signal into the digital domain, presenting the user with a 160 kHz tunable passband. Filtering and demodulation are handled digitally, and the result is passed to amplifiers and audio outputs.

A 16 bit audio ADC provides plenty of resolution, and signals sound as clear as on any top notch receiver. Such great sound is conditional, though: the front end RF circuitry must not be overloaded. What I mean is that the user must take care to not overload the radio by coupling it to a large antenna with no filtering or attenuation. That first mixer stage in the MSI001, is something I would designate as a weak point which prevents the radio from having the stellar dynamic range expected of pure Softrrock SDRs. In most instances, there won't be a problem, but there are very few radios which will not overload when subjected to super strong local signals. Another element of clear and beautiful reception is making sure the radio has a noise-free DC power supply. Noisy power causes noisy RF, and noisy audio. Yes, the internal battery provides the cleanest power; a well filtered linear supply of 5 volt USB power would be great for this radio. The number three, and most common, source of power for most users will be a USB connection to a computer. Most modern PCs actually provide pretty good power. I say "most" because there are a few machines which provide rather noisy DC: your experience may vary.

The radio has an effective RF gain control, enough to prevent overload in most environments - and enough gain to bring in weak signals when necessary. Users have reported the presence of electrical noise from the display backlight. It seems to be present mostly from longwave through about 10 MHz, where it is mostly below the atmospheric background noise.

With a weak signal sensitivity of 0.3 microvolts through 1 GHz and a dynamic range of about 85 dB, the receiver is sensitive enough for casual and demanding listeners alike.

Nicely blending superheterodyne and QSD circuitry, and a very broad tuning range, the Malachite SDR is well suited for monitoring favorite broadcasts or maritime / aircraft / ham bands. Actually, with CAT control and quadrature output, it can do just about any sort of monitoring duty. This little radio would make a fine weather satellite receiver, for example, fully capable of tracking doppler shifted downlinks.

How is the Malachite SDR software?

The radio has typical functions one expects in an SDR. There is a touch screen where users select modes and frequency bands. Signals appear on a waterfall display with a familiar color palette. For more granular control, there are a number of menus where it is possible to adjust numerous aspects of Malachite SDR operation. For example, the clock PPM error may be stored as are gain settings.

Specific features implemented in the software will vary according to the version of firmware installed in the radio. Out of the box, the features are basic; improvements and some nice bleeding edge goodies are available after installing the paid upgrades from RX9CIM (malahit_sdr@rambler.ru). Important new features in the upgrades include not only a synchronous AM demodulator but also selectable upper or lower sideband synchronous AM demod modes. Give the software coders enough time and we will see open-sourced versions of the firmware, as good as anything produced by the OEM manufacterer in Russia.

The radio is controllable with an external computer through its built-in CAT support, so a whole world awaits the properly equipped: automatic tuning for satellites, unlimited memories, and so on. Quadrature baseband signals or demodulated audio may also be transferred out of the radio via the USB interface. Doing that, any number of external decoders or DSP processes mau be applied to signals brought in by the Malachite SDR.

What is the build quality?

Build quality is a matter of who manufactured the unit. The original Russian units are of high quality and have the latest stock firmware installed. There appear to be two versions built in China; one with an excellent case and internal build quality. The other Chinese variant has a different case and seems to not share the same high quality standards. I am convinced that more versions will emerge, with better and better build qualities.

The radios have either rotary knobs or thumbwheels on the side for tuning and volume control. These have been a problem across all builds, mostly because they are delicate and tend to function erratically. For example, a smooth roll of the tuner thumbwheel does not produce a smooth movement in frequency. I would argue that the encoders are noisy because they have insufficient means of signal conditioning to prevent skippy action. Being such a small radio, I doubt the designers were willing to cram in more parts in an effort to obtain smoother encoder operation.

Buy a Malachite SDR instead of a KiwiSDR or RSP1A?

That is not a fair comparison, as the other receivers serve different kinds of listeners. The malachite is for users who wan a portable radio to carry and operate anywhere, with some advanced programmable features. The KiwiSDR, on the other hand, is small but far less portable and intended to operate in conjunction with a networked computer. The Malachite radio is more comparable the non-SDR portable radios, where it sits at the top of its class for portability and performance. As to the RSP1A, it is also in a different category, being a PC - centric radio, not a portable self-contained unit.

In my own situation, the nearest competitor to the Malachite SDR is my Android smart phone. I use mobile data or wifi to connect to any of hundreds of WebSDR and KiwiSDR internet connected radios. I pick a server near the desired transmitter and listen to signals on my bluetooth earbuds. It is a totally different kind of listening than on a local radio on a local antenna. I actually would consider a Malachite SDR for receiving satellites and aeronautical monitoring. It receives so nicely on air band that it surpasses the capability of my RTL-SDR.

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