The SDRplay RSPdx SDR Receiver Features Frequency Range of 1 kHz up to 2 GHz (Review)

    • by Sebastian Westerhold
    • Reading time: 6 min

    The SDRplay RSPdx is a 14-bit single-tuner receiver with continuous coverage from 1 kHz up to 2GHz. Three input connectors, an ample array of software selectable front-end filters and an external reference clock input make it stand out compared to other SDR receivers on the market.

    Overview of the SDRplay RSPdx

    The three software selectable inputs of the RSPdx provide flexibility for multi-antenna set-ups. Input Port 1 and 2 are SMA connectors with continuous coverage from 1 kHz up to 2 GHz. Port C is a BNC type connector and provides coverage from 1 kHz to 200 MHz.>7p>

    SDRplay RSPdx
    The SDRplay RSPdx in all its splendour.

    Various input filters, available on all input ports, can be used to suppress unwanted signals. The FM, MW and DAB broadcast bands can be blanked out by three, individually selectable notch filters. A 500 kHz low-pass filter can be used for increased VLF signal performance. A more complete overview of all selectable filter options can be found in the table below.

    Table1 filters
    Software-selectable filter options.

    Antenna Bias Voltage

    An integrated Bias-T (only available on Port B) can provide 4.7 V with a maximum current of 100 mA. The internal 0.5 ppm TCXO provides a stable reference clock source. If a higher frequency stability is required, an external 24 MHz reference signal can be connected to the RSPdx through a female MCX connector.

    SDRplay RSPdx Software

    As with previous SDRplay products, plenty of software options are available, and you don’t have to give up your favorite third-party software.

    SDRuno is a free software package offered by the manufacturer for use with all SDRplay receivers. It offers an intuitive and effective way to control the software selectable features of the RSPdx (and other SDRPlay receivers).

    Screenshot of the SDRuno software.

    Spectrum Analyzer

    RSP Spectrum Analyzer is also available from the manufacturer’s website and turns the RSPdx into a powerful spectrum analyzer.

    If you prefer using well-established software packages, like SDR-Console, HDSDR, CubicSDR or even GNU Radio, you are in luck. The manufacturer offers API packages that allow the RSPdx to play well with third-party software.

    Build Quality

    The build quality of the RSPdx appears to be quite rugged. While I wouldn’t recommend dropping it on the floor intentionally, I wouldn’t be too concerned either if it happened accidentally. With a weight of just 315 g at a size of 113 mm × 94 mm × 35 mm, it could almost be called pocket-size. It is small enough to be a great companion when traveling by plane and capable of making a boring stopover a lot more interesting.

    Getting Started with the SDRplay RSPdx

    After following the Start Here procedure of the manufacturer and installing the necessary drivers and SDRuno, I was amazed by the extensive number of options the software provides. It took a while to explore and try out all possible settings. Various plugins are available to further extend the range of options. For instance, a DAB plugin enables reception of digital broadcast stations.

    Fast Scan Functions

    The scan function is incredibly fast. The user can choose from preconfigured search ranges, such as the VHF air or marine bands. Custom search ranges can, of course, also be set up. Active channels that have been found are automatically put into a list and can either be named and stored or be locked out from the scanning process.

    The SDRuno scan function in progress.

    Besides a frequency search scan, a memory search scan capability is, of course, also included. For each memory channel, the corresponding antenna port, modulation type and – if needed – an individual threshold level can be set. This feature enables the user to quickly scan across a broad range of frequencies and radio services using multiple antennas for different bands.

    SDRplay RSPdx with mini whip
    The SDRplay RSPdx with a Mini Whip antenna for VLF/HF reception experiments.

    Outstanding sensitivity

    To test the HF and VLF performance, I used an inexpensive Mini Whip antenna. To my surprise, the Mini Whip also worked quite well using just the 4.7 V from the internal Bias-T on the ANT B port. Even in my office, inside a concrete building, I was able to listen to various amateur radio conversations on the 80 m and 40 m band. Such outstanding sensitivity is a great feature indeed. While sensitivity levels are not provided by the manufacturer, Minimum Discernible Signal (MDS) levels have been demonstrated to be between approx. −120 dBm and better than −140 dBm for frequencies above 144 MHz in an ARRL laboratory test.

    VLF Performance

    I was also very curious about the device’s VLF performance. With the same indoor setup, using my Mini Whip antenna, I was able to receive various services in the VLF range. Navtex weather reports, DCF77 and a nearby non-directional beacon (NDB) could be received without difficulty. Activating the integrated 500 kHz low-pass filter had an audible impact on the noise floor.

    Capturing a Filter Response

    To test the usability of the RSP Spectrum Analyzer software, I decided to measure the filter response curve of a 1090 MHz ADS-B SAW filter using a noise source. A broadband noise source was fed into the filter, and the output of the filter was connected to the ANT A port of the RSPdx. The center frequency was set to 1090 MHz with a span of 200 MHz. An average and a peak trace were set up to be displayed simultaneously. As expected, the pass-band characteristic of the SAW filter quickly appeared on the display.

    Test setup to measure the band-pass response curve of a 1090 MHz SAW filter.

    Broadcast Filters

    The software-selectable broadcast filters proved themselves to be quite effective. Especially in the VHF air band and upper VHF range, noise reduction was rather good. The broadcast notch filters proved just how effective they are when I forgot to disable them while browsing the FM broadcast band. I was getting quite frustrated because I couldn’t tune in to any of my favorite radio stations until I noticed that these filters were still enabled. After deactivating them, music started flowing out of my speakers.


    The SDRplay RSPdx is the perfect tool for receiver setups with multiple antennas. It also appears to be the only SDR on the market optimized for VLF reception. My only criticism concerns the rather low voltage supplied by the internal Bias-T. A possible workaround, if used in a remote setup, would be to use the 4.7 V to activate an external relay or FET to control a higher bias voltage source.

    Its extended frequency range, the three software-selectable antenna ports, the external reference input and the software-selectable filters are a real plus over other SDR receivers and strongly speak in favor of the RSPdx. Overall, I am quite impressed by the performance of this little SDR receiver and the wide range of applications it enables.



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