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Receiving equipment
Using the SDRPlay "RSP" receivers on a WebSDR

The need for more bandwidth:

In the beginning, the Northern Utah WebSDR used only two different types of receivers:
Figure 1:
The RSP2pro and RSP1a receivers
Click on the image for a larger version.
The SDRPlay RSP1a and RSP2pro receivers

Newer options

Since the Northern Utah WebSDR went online, a number of newer devices capable of providing wider bandwidth have become available for reasonable price - namely from suppliers like AirSpy, SDRPlay, and the FunCube Dongle folks.  For the most part, the capability of these devices are more or less equal in terms of performance as they all use similar techniques to capture a segment of the HF spectrum - and they do it in the same way that the older "Softrock" receivers did:  Use a local oscillator to convert a slice of the spectrum to "audio" baseband (but this "audio" could go up to several megahertz!) and then digitize it.

Unlike lower-cost hardware like the RTL-SDR dongle, all of these devices include various types of filtering, from that applied at the antenna port to limit the impinging signals to the general frequency range of interest (e.g. passing only a portion of the HF spectrum, blocking AM/FM/DAB signals, etc.) to filters applied to the baseband after the frequency conversion to limit that applied to the analog-digital converter.  Additionally, they all have means by which the amount of signal applied to the analog-to-digital converter can be controlled - usually via adjustment of gain and/or attenuation.  Typically, these devices can also capture much more than the 192 kHz of an audio sound card, and some can also exceed the 2-ish MHz bandwidth of an RTL-SDR dongle while doing so with more than 8 bits of resolution.

While the higher bandwidth is nice, the most important aspects to us were better filtering, being able to adjust signal levels and higher analog-to-digital resolution.  With none of the options mentioned above are capable of directly digitizing 16 bits like the "Softrock + Sound card" method, having batter filtering the ability to adjust signal levels allows the constant adaptation to current band conditions to make the most of the existing capabilities.

Taking a commercial transceiver - the Icom IC-7300 - as an example, it has "only" 14 bits of analog-to-digital conversion, but "on the air" it performs admirably - far better in most of the ways that matter to the typical operator than even the "best" radios of the 80s and 90s.  The reason that it can do this is that it has a large number of filters preceding its analog-to-digital converter - and it constantly adjusts the gain and attenuation along the signal path between the antenna and the converter to prevent overload from strong signals as much as practical, but also to keep signal levels as high as they can be maintained without causing overload.  Even though the IC-7300 has different architecture than the receivers being discussed (e.g. it is "Direct-sampling" whereas those that we are discussing use frequency conversion before digitization) the needs are the same:  Filtering and managing of signal dynamics.

The SDRplay

As readers of this page will have observed, we chose the SDRPlay RSP1a - mostly for reasons of price and availability - the latter being a concern at the time we purchased them (late 2021).  As mentioned before, we considered the others (notably AirSpy and Funcube) but with similar architecture and overall performance, they didn't hold a huge advantage.  Additionally, the SDRPlay has the availability of an API (Application Programming Interface) that allowed us to write our own programs to "talk" to it.  While it is unfortunate that this API is not open source (and as such, there are issues that we cannot directly address ourselves) it has been "good enough" for our needs.

While some of the other options have better RF signal path filtering than the SDRPlay RSP1a, that was irrelevant to us as we already have in place band-pass filtering specific to each amateur band - both to protect the original "Softrock" receivers from signal from elsewhere in the HF spectrum, but also because they provide a significant degree of lightning protection since they greatly limit the amount of energy that could pass from a nearby strike:  Because the WebSDR is remote, we simply cannot run up there, unscrew the coax cable and "throw it out the window" every time a black cloud rolls by.

Interfacing with the PA3FWM WebSDR

The PA3FWM WebSDR software has only two usable "input" types:
Because the use of the 8 bit RTL-TCP was a "non-starter", we concentrated on the sound card interface.  Testing indicated that the PA3FWM WebSDR software operated properly at 768 kHz (and likely 1536 and 3072 kHz - but the kernel limitations apply here) so we developed our own set of drivers and utilities to make this happen:
With the use of the above utilities, the PA3FWM may be operated to at least 768 kHz, allowing single-receiver-per-band coverage of every amateur band below 10 meters in their entirety using the 16 bit signal path.

Other tricks

Once you have a receiver interfaces as above using the sound card interfaces, there exists the possibility of doing other things as well:

The SDRPlay receiver can be used to provide reasonable performance in terms of covered bandwidth and the ability to handle the signals present within the covered range on a WebSDR system.  As with any SDR, one should pay close attention to the signal dynamics (e.g. gain/amplification settings) as well as appropriate filtering of signal on the RF input to maximize performance under a wide variety of conditions.

Additionally, one can leverage a single receiver to provide multiple outputs which can include anything from being able to cover more than one amateur band at the same time or providing the raw I/Q data from the receiver to other applications, eliminating the need for having another (redundant) receiver to cover the same bit of spectrum.

Pages about other receive gear at the Northern Utah WebSDR: Go to the main "RX Equipment page.

Additional information:
Back to the Northern Utah WebSDR landing page