A MMDVM is usually a computing device running multiple radios, each of which is used for a separate channel with it's own filters and power amplifier hardware. Each channel can run a separate protocol if desired.
However in order to save on radio hardware, Adrian wanted to use his LimeSDR as the radio hardware in his MMDVM system. The LimeSDR is a transceiver which has enough bandwidth to implement several channels just by itself. To do this Adrian uses his MMDVM-SDR software.
His implementation runs multiple instances of MMDVM-SDR, one instance for each channel. Then a GNU Radio flowgraph with LimeSDR block connects to each of these instances, transferring data between GNU Radio and MMDVM-SDR via ZeroMQ or TCP sockets. The bulk of Adrian's post explains the architecture in detail. Adrian writes:
The setup can transmit 7 digital carriers in 200 kHz occupied spectrum, and each radio channel can be assigned to a different mode or digital voice network as configured in MMDVMHost.
This is based on the work of Jonathan Naylor G4KLX and Rakesh Peter (r4d10n).
Adrian also notes that this is still a work in progress and there are still several limitations including high latency and issues with filtering, overload and poor channel rejection.
One talk by Alex Pettit describes how to build a radio telescope from a an umbrella and some "Faraday fabric" which is copper cloth. The results show more than adequate performance for the cost, making this an affordable and easy entry to radio astronomy.
Alex Pettit - Umbrella Antennas
Another video presented by Dr. Wolfgang describes building small to medium sized radio telescopes. He explains how small radio telescopes less than 3 meters in size can work well for receiving the 21cm Hydrogen line, and how SDRs are the best choice of receiver for them. Many examples of small dish installations are shown.
Dr. Wolfgang Herrmann: Building Small/Medium Size Radio Telescopes
Over on the saveitforparts YouTube channel the creator has uploaded a video showing how he was able to image geosynchronous satellites with his modified motorized RV satellite dish. The idea is to scan the sky using the motorized dish, taking Ku-band RF power readings at each point in the sky. The result forms a heatmap image of satellite transmissions in the sky. For the most part, the satellites detected are TV satellites and they are at known positions in the sky.
However, in one of his recent scans saveitforparts appears to have detected an unknown satellite just outside of the geostationary plane. He goes on to discuss what it could have been, noting that it is most likely to be the AMSC 1 telecommunications satellite.
Recently I spotted a strange "UFO" with my homemade radio telescope / microwave imager. I've used this imager before to spot television satellites in geostationary orbit, but this unknown object was something new to me.
Spoiler Alert: I was able to determine that I'm probably seeing a geosynchronous (but not geostationary) satellite in an inclined / elliptical orbit. Specifically, I think this is the AMSC-1 telecom satellite, which is in a type of orbit designed to cover high latitudes like Northern Canada.
These types of satellites don't seem to show up too often on my telescope / imager setup, since they're not as common and aren't usually aimed directly at my location. This is the first time I've managed to spot one (if that's what I'm seeing), so it seems kind of rare to catch it with this particular equipment!
Folks might also ask if this "UFO" could be the sun or moon producing microwave signals, but those were both off to the left of the scan, not where the mystery signal showed up. It's also probably not a reflection / side lobe / "lens flare", I do get those, but they show up as rings around the main signals, and in fact this mystery signal has its own faint ring around it. Since my dish takes 3-4hrs to do a full scan, this also isn't something fast like a plane or low-orbit satellite as those don't show up on my imager (I'm essentially taking a very long time exposure).
I'm still planning to upgrade / rebuild this mini radiotelescope device in the future, hopefully with more flexibility to pick up different frequencies. That should let me see even more satellites (and maybe other space stuff!).
Mysterious Space Object Detected With DIY Radio Telescope
In his Hackaday.io post (and a post on the main Hackaday blog), Tom Farnell explains how he used two 10-meter tape measures combined with an RTL-SDR Blog V3 software defined radio to receive numbers stations in the HF bands. We want to add that this antenna isn't restricted to just numbers stations, and could receive many different types of shortwave and amateur stations on HF.
In his post Tom explains what numbers stations are and why they are interesting. In brief, a numbers station is a radio broadcast of a voice saying a bunch of numbers continuously. These stations are known to be espionage related, containing some sort of coded message for international spies to decode.
Tom goes on to show how the antenna is constructed. As HF antennas need to be long to get the best reception, Tom uses the long metal tape measure and attached it to the included dipole assembly that comes with the RTL-SDR to increase them to an appropriate length.
Intercepting Spy Radio Messages With A Tape Measure
Over on his TechMinds YouTube channel Matt has uploaded a new video showing him testing out the new RTL-SDR Blog V4 dongle that we released a couple of weeks ago. In the video Matt explains the differences between the RTL-SDR Blog V3 and V4 dongles, and then goes on to show the Blog V4 dongle in action. He finishes by comparing reception between the V3 and V4, noting the reduced interference on the HF bands due to the lack of Nyquist folding from direct sampling.
We note that the first batch of the RTL-SDR Blog has currently sold out, but a new larger batch will be ready to go on sale around the end of September. So please keep an eye on the blog's main page and store if you are interested in picking one up.
RTL SDR V4 - Now with Built-In HF Upconverter + More Features
Over on his YouTube channel Aaron who created and maintains the DragonOS SDR Linux distribution, has uploaded a video demonstrating how to use an RTL-SDR and SoftEOT/PyEOT to decode North American wireless train telemetry.
HOT (Head of Train), EOT (End of Train) and DPU (Distributed Power Unit) telemetry is sent from various parts of a train and contains information about things like voltages, brake line pressure and to monitor for accidental separation of the train.
In his video Aaron uses his DragonOS Linux distribution, SDR++ with an RTL-SDR Blog V4 dongle and the SoftEOT and SoftDPU decoders. SoftEOT and SoftDPU are both Windows programs, however Aaron shows how to use WINE to run them in Windows. Later he shows how to use an alterative decoder called PyEOT which is based on GNU Radio.
GNU Radio conference talks are generally about cutting edge radio research topics and applications that involve the use of GNU Radio, a popular DSP framework for SDRs. If you are interested, previous years talks can be found on the GNU Radio YouTube channel.
The talks at GRCon23 will be livestreamed on YouTube for free, and we have pasted the links to each days live stream link below. We recommend activating YouTube notifications on each video so you won't miss the start.
NOTE: We have found more stock of R828D chips, so the V4 should be in production hopefully until the end of 2025.
We're happy to announce the first release of our new RTL-SDR Blog V4 dongle which is based on the R828D tuner chip. The pricing is US$39.95 for the V4 dongle with antenna set, and US$29.95 for the dongle only, including free shipping to most countries.
Currently we are only shipping this model from our warehouse in China and the initial production batch is small and so we are limited in stock. However, now that we have confirmed that production of the first small batch of V4 has gone very well, we will be ramping up production, and stocking Amazon USA within 1-2 months as well.
The V4 comes with several improvements and changes that are listed below.
Improved HF Reception. Now uses a built in upconverter instead of using a direct sampling circuit. This means no more Nyquist folding of signals around 14.4 MHz, improved sensitivity, and adjustable gain on HF. Like the V3, the lower tuning range remains at 500 kHz and very strong reception may still require front end attenuation/filtering.
Improved filtering. The V4 makes use of the R828D tuner chip, which has three inputs. We triplex the SMA input into three bands, HF, VHF and UHF. This provides some isolation between the three bands, meaning out of band interference from strong broadcast stations is less likely to cause desensitization or imaging.
Improved Filtering x2. In addition to the triplexing, we are also making use of the open drain pin on the R828D, which allows us to add simple notch filters for common interference bands such as broadcast AM, broadcast FM and the DAB bands. These only attenuate by a few dB, but may still help.
Improved phase noise on strong signals. Due to an improved power supply design, phase noise from power supply noise has been significantly reduced.
Less heat. Due to the improved power supply design the V4 uses slightly less current and generates slightly less heat compared to the V3.
Cheaper price! The price of the R860 chip which is used in the V3 and most other RTL-SDR brands increased significantly at the beginning of 2023 which is part of the reason as to why RTL-SDR dongles have been increasing in price recently. For the V4 we are making use of an existing stockpile of R828D chips which are now priced cheaper than new productions of the R860. In a time when high inflation keeps pushing prices up this is incredibly welcome.
There are some other minor changes including a new bias tee LED and a small cutout hole in the enclosure so it's easy to tell when the bias tee is on.
Of course the same innovations that we brought in with the V3 are still implemented such as the sleek conductive black metal enclosure which works as a shield and doubles as a heatsink, a thermal pad to sink heat away from the PCB, 1PPM TCXO, SMA connector, USB noise choking and improved ESD protection.
The V4 however does come with some disadvantages compared to the V3 that need to be noted:
Due to the increased filtering there can be an average of 2-3 dB less sensitivity on some bands. Please see the MDS measurement graph below for the full picture.
The V4 requires the use of our RTL-SDR Blog drivers. Our RTL-SDR blog drivers are on GitHub. Please be sure to follow the installation instructions on the quickstart guide carefully. In most cases using our drivers simply means running our install-rtlsdr-blog.bat file, or replacing a dll file. (UPDATE: The default Osmocom branch now supports the RTL-SDR Blog V4, as does most other software)
The V4 is a Limited Edition Design. The R828D tuner chip is completely out of production now and the number of units we can produce is limited by the number of chips held by our contract manufacturer in China. They have indicated that there should be enough stockpile for about a years worth of production. (UPDATE: We have sourced more chips and should be able to continue production at least well into 2025)
Because of these tradeoffs we will continue selling the V3 alongside the V4.
More About the V4 Design
The R828D
The core change on the RTL-SDR Blog V4 design is the change from the R860 tuner chip to the R828D tuner chip. The R828D was previously a more expensive chip, however with the huge price increases on the R860 which came in effect at the beginning of the year we have decided to make use of existing R828D stock which is now cheaper than the R860.
The R828D is very similar to the R820/R860 and shares much of the same circuitry. However, instead of just one input, it comes with three switchable inputs. We have used these three inputs together with a triplexer to create a dongle with some extra input filtering. In the past there have been some R828D based dongles on the market, but all designs are based on TV receiver circuits. Because our design is different, you will need to use our RTL-SDR Blog driver branch which has added compatibility for our R828D design.
Also please note that because the R828D chip stock is limited, and it is no longer in production, the V4 design is also a limited design which we expect to be able to sell for about a year.
HF Design
The HF design consists of a SA612 double-balanced mixer circuit with front end filtering, which is connected to the 28.8 MHz oscillator that is also used for the tuner and RTL2832U chip. This means that HF frequencies are upconverted by 28.8 MHz. Our drivers handle this upconversion seamlessly, so you just need to tune to 0 - 28.8 MHz in order to receive HF. There is no need to set any offset.
An upconverter design also means that unlike direct sampling full gain control is available, and also there is no folding of signals across 14.4 MHz due to Nyquist.
Adding Basic Input Filtering
One of the main problems with RTL-SDR dongles is overload from strong broadcast stations such as broadcast FM, broadcast AM and DAB. By using a triplexer circuit we can make use of the three inputs on the R828D tuner chip to provide some filtering. The triplexer splits the input signal into HF (0 - 28 MHz), VHF (28 MHz - 250 MHz), and UHF+ (250 MHz - 1.766 GHz). This means that interference from something like strong broadcast FM at 88-108 MHz is more isolated when we are tuned to the HF and UHF bands.
We've also made use of the open drain pin on the R828D (which does not exist on the R860) to implement a simple switchable notch filter for the main problem broadcast bands. These notch filters cover broadcast AM, broadcast FM and DAB, and reduce them about an additional 5-10 dB. By default the notch turns ON when tuned out of these bands, and is turned OFF when tuned within them.
In terms of sensitivity, the disadvantage of adding more filtering is that it can reduce sensitivity in some bands. However, sensitivity of the RTL-SDR is usually not a problem in most situations, as we're usually limited by desensitization from strong out of band signals as mentioned above. If sensitivity is a priority an LNA such as our wideband LNA should be used anyway, for any RTL-SDR brand or model. Any front end LNA will totally dominate the sensitivity figures, making any sensitivity measurements of the RTL-SDR itself irrelevant.
Revised Power Design
The revised power design makes use of a more modern LDO with significantly better power supply noise rejection which results in much lower phase noise seen on strong narrow signals. There are also some PCB tweaks to reduce internally produced noise. The LDO improvement also has the effect of reducing power usage and lowering heat.
Other Changes
We've also added an LED to the bias tee, so it's easier to tell if it has been activated in software.
MDS Measurements
The minimum discernable signal (MDS) is a test we can do to determine what is the minimum power level that a receiver can detect.
The results show that the MDS has significantly improved on the HF bands thanks to the upconverter design. However, there is some minor degradation in the VHF and UHF band.
MDS Measurements (Low values are better)MDS Comparison (Higher means better sensitivity for the V4)
Two Tone Isolation & Desensitization Test
Strong out of band signals can cause an SDR to desensitize on other bands. For example, very strong broadcast FM (which is common), can cause signals being received on other frequencies to be received with a lower signal to noise ratio.
In this test we injected an "interference" tone (Tone A) at 95 MHz, and injected a second tone (Tone B) at another frequency. We then slowly increased the power on Tone A. When we noticed a 3 dB drop in the signal strength of Tone B we recorded the power level of Tone A that this occurred at.
This gives us a way to see the effect of the triplexer filters and notch filters when compared against the Blog V3 which has no filtering. A higher recorded value means that a stronger signal is required to desensitize the receiver, meaning that the strong signal handling capability is improved.
From the difference graph we can see that isolation results within the same triplexer band are improved by about 8 dB thanks to the notch, and then out of band isolation is improved by 28 - 43 dB thanks to a combination of the triplexer filters and notch.
We note that between 305 - 1405 our measurements were limited by the max power out from our signal generator, and we believe the true results are roughly 5dB better than what was recorded at these frequencies.
Two Tone Sensitivity Drop TestIsolation Improvement in the RTL-SDR Blog V4
Should I upgrade if I have an RTL-SDR Blog V3?
If you are happy with the RTL-SDR Blog V3's performance, then there is absolutely no need to upgrade as you will likely see similar performance. However, if you are purchasing a new dongle it may be wise to consider the V4 model as we believe the V4 will be a receiver that is more suitable in many situations.
Thanks
We wanted to extend some thanks to Erlend S. Ervik/LB6MI, Jack T. and everyone over the years who has given some input to RTL-SDR design.
