Software Defined Radio (SDR) category

Software-defined radio (SDR) is a radio communication system where components that have been traditionally implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on a personal computer or embedded system.

Experimenting with software defined radio used to be expensive, but now it is cheap. Nowadays it is very cheap to start experimenting with SDR. Most receivers use a variable-frequency oscillator, mixer, and filter to tune the desired signal to a common intermediate frequency or baseband, where it is then sampled by the analog-to-digital converter. Cheapest wide receiving range well working device is to use suitable DVB-T receiver stick (10-20 Euros/Dollars) and suitable software (very many alternatives, for example SDRsharp and Gnu Radio).

My article Software defined radio with USB DVB-T stick started the long list of SDR related postings. The newest postings now are Filter measurements with RF noise source and Antenna measurements with RF noise source.


  1. Tomi Engdahl says:

    HF SDR Transceiver RS-918 mcHF

    Here we take a look at the RS-918 HF SDR Transceiver, a typical clone of the mcHF.

    mcHF QRP transceiver

  2. Tomi Engdahl says:

    How To Run A First-Generation Cell Phone Network

    Retro tech is cool. Retro tech that works is even cooler. When we can see technology working, hold it in our hand, and use it as though we’ve been transported back in time; that’s when we feel truly connected to history. To help others create small time anomalies of their own, [Dmitrii Eliuseev] put together a quick how-to for creating your own Advanced Mobile Phone System (AMPS) network which can bring some of the classic cellular heroes of yesterday back to life.

    Few readers will be surprised to learn that this project is built on software defined radio (SDR) and the Osmocom-Analog project, which we’ve seen before used to create a more modern GSM network at EMF Camp. Past projects were based on LimeSDR, but here we see that USRP is just as easily supported.

    HowTo: Running the 1G Analog Phone from 1997

  3. Tomi Engdahl says:

    Building a crystal radio set
    Updated Friday, 30th August 2019

    For many of the original teenagers, home-built crystal radios gave them their first contact with the world beyond their bedrooms. Here, Fraser Robertson describes how to make your own.

  4. Tomi Engdahl says:

    Raspberry Pi Hat Adds SDR With High Speed Memory Access

    An SDR add-on for the Raspberry Pi isn’t a new idea, but the open source cariboulite project looks like a great entry into the field. Even if you aren’t interested in radio, you might find the project’s use of a special high-bandwidth memory interface to the Pi interesting.

    CaribouLite is an affordable, educational, open-source SDR platform that is also a HAT for the Raspberry-Pi family of boards (40-pin versions only). It is built for makers, hackers, and researchers and was designed to complement the SDR current eco-systems offering with a scalable, standalone dual-channel software-defined radio.

    Unlike many other HAT projects, CaribouLite utilizes the SMI (Secondary Memory Interface) present on all the 40-pin RPI versions. This interface is not thoroughly documented by both Raspberry-Pi documentation and Broadcomm’s reference manuals. An amazing work done by Lean2 (code in git repo) in hacking this interface contributes to CaribouLite’s technical feasibility. A deeper overview of the interface is provided by G.J. Van Loo, 2017 Secondary_Memory_Interface.pdf. The SMI interface allows exchanging up to ~500Mbit/s between the RPI and the HAT, and yet, the results vary between the different versions of RPI. The results further depend on the specific RPI version’s DMA speeds.

    In our application, each ADC sample contains 13 bit (I) and 13 bit (Q), that are streamed with a maximal sample rate of 4 MSPS from the AT86RF215 IC.

    RF Channels:

    Sub-1GHz: 389.5-510 MHz / 779-1020 MHz
    Wide tuning channel: 30 MHz – 6 GHz (excluding 2398.5-2400 MHz and 2483.5-2485 MHz)

  5. Tomi Engdahl says:

    Listen To The RF Around You

    These days, we are spoiled for choice with regard to SDRs for RF analysis, but sometimes we’re more interested in the source of RF than the contents of the transmission. For this role, [Maker_Wolf] created the RFListener, a wideband directional RF receiver that converts electromagnetic signal to audio.

    The RF Listener is built around a AD8318 demodulator breakout board, which receives signals using a directional broadband (900 Mhz – 12 Ghz) PCB antenna, and outputs an analog signal. This signal is fed through a series of amplifiers and filters to create audio that can be fed to the onboard speaker. Everything is housed in a vaguely handgun shaped enclosure, with some switches on the back and a LED amplitude indicator.

  6. Tomi Engdahl says:

    The Curious Case Of The Radio Amateur And The Insulin Pump

    A substantial part of gaining and holding an amateur radio licence relates to the prevention of radio interference. In days past this meant interference to analogue television broadcasts, but with ever more complex devices becoming commonplace in homes it applies to much more. This has hit the news in Marion County Florida, where a radio amateur in a senior’s community has shut down his radio station after a potential link emerged between it and another resident’s insulin pump. There is a legal challenge ongoing that relates to the complex’s rules over transmitting antennas.

    It’s obviously a serious occurrence for an insulin pump to be affected by anything, and it sounds as though the radio amateur concerned has done the right thing. But it’s clear that something has gone badly wrong in this case whether it’s due to the amateur radio transmissions or not, because for a manufacturer to produce a medical device so easily affected by RF fields should be of concern to everyone.

    Woman fights to have ham radio operations banned after potential interference with insulin pump

    A Marion County woman is taking on her neighborhood association, in a matter she said puts her health at risk.

    Michelle Smith, a Type 1 Diabetic, and a consultant determined that her neighbor’s ham radio hobby might have interfered with the doses of insulin being pushed out from her pump.

    The 55+ community where she lives hired that consultant and told the neighbor to shut down his amateur radio station.

    But a copy of the community’s rules shows a change was put in place that could pave the way for other similar antennas to be installed.

    In the manicured subdivision of Indigo East near Ocala, managed by On Top Of The World, two neighbors say they’ve thought of moving away from the development’s amenities because of the ongoing dispute.

    Smith has been in the back and forth with the community’s association for more than a year after noticing the insulin pump she uses to manage her Type 1 Diabetes was suddenly giving the wrong amount of the medicine that keeps her alive.

    “So I switched pumps, bought another one, switched reservoirs, threw insulin away, did everything I knew of to troubleshoot,” she said.

    After doing some research, she suspected the problem might be a few doors down in equipment that is now unplugged and collecting dust.

    David Birge was told to shut down his ham radio operation after On Top Of The World hired an independent consultant to investigate Smith’s complaints.

    That engineer determined the “amateur radio operator could have produced” radio frequency levels that exceeded those Smith’s insulin pump is intended to operate in.

    “I’ve lost a hobby I’ve enjoyed more than half of my lifetime, and the equipment sitting in my office is not plugged in,” Birge said.

    Though his operation was shut down for now, the community’s board of directors changed the wording in its rules and regulations to potentially allow more of these amateur radios in the future — changing the definition of antenna allowed after approval from “a device used to receive” to one that could also “transmit” radio frequency signals.

  7. Tomi Engdahl says:

    Cable Modem Turned Spectrum Analyzer

    Hopefully by now most of us know better than to rent a modem from an internet service provider. Buying your own and using it is almost always an easy way to save some money, but even then these pieces of equipment won’t last forever. If you’re sitting on an older cable modem and thinking about tossing it in the garbage, there might be a way to repurpose it before it goes to the great workbench in the sky. [kc9umr] has a way of turning these devices into capable spectrum analyzers.

    While it’s somewhat down to luck as to whether or not any given modem will grant access to this feature, for the ones that do it seems like a powerful and cheap tool. It’s agnostic to platform, so any computer on the network can access it easily, and compared to an RTL-SDR it has a wider range. There are some limitations, but for the price it can’t be beat which will cost under $50 in parts unless you happen to need two inputs like this analyzer .

    Cable Modem Spectrum Analyzer

    Finally, I dd manage to acquire an Arris SB6183. Score. As a “user-procured” device, the cable companies couldn’t (or didn’t) push a fix into it. I connected the Ethernet cable to my Toughbook, logged in to the web interface (, and was instantly granted access to the spectrum analyzer function. I keyed up a nearby handheld transmitter in the VHF band (154 MHz), and saw the spectrum respond accordingly! Obviously, with no antenna connected, I expected very little in terms of performance.

    Once I got home, I added couple RF adaptors, then had an SMA magnet mount scanner antenna connected to the F type connector.

    Now – lets not pretend there are no Limitations. There is never a completely free lunch. The analyzer is capable, but has some limitations. Here’s the basic feature rundown that mine has-

    Amplitude reporting units: dBm
    Noise floor: Approximately -100 dBm
    Maximum Span: 1 – 1000 MHz
    Minimum Span: 6 MHz
    Clear Write / Max Hold modes, with video averaging available
    Max input level unknown, I would guess 0 dBm would be a safe maximum.
    Input impedance: 75 ohms
    Automatic marker will follow a mouse pointer over the trace
    mouse click-to-drag to change center frequency on the fly

    Regarding the amplitude capability: The noise floor appears to be around -100 dBm (+7 dBuV) in the 75 ohm system. This is not too bad, actually – many commercial spectrum analyzers have higher noise floors than this (though they usually have narrower RBWs, better selectivity, and are more accurate). Definitely good enough to locate a transmitter.

    I would tend to think the accuracy of the cable modem would be a bit better than an RTL-SDR, since the Broadcom chipset is responsible for making sure that latency, errors, etc., are minimized when this device is acting as a transceiver for getting the user’s high bandwidth internet connection synchronized with the server on the other end. As I said before, the RTL-SDR often suffers from some offset, which varies from unit to unit. The cursors are somewhat limited, and it would seem that the channelized nature of the spectrum analyzer on the Broadcom chips has some degree of error – though it is quite repeatable, where I have seen some RTL-SDRs drift.

    As I mentioned above, this is set up for 75 ohm CATV systems, NOT the 50 ohm typical impedance used in test equipment or radio gear… Generally, this mismatch loss is not too bad, so we have to assume there’s ~1.2 dB of mismatch loss.


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