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Building the SoftRock SDR

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This is the first of two posts which take a look at the relatively simple and easy to construct SoftRock software-defined radio (SDR) that is designed for HF amateur radio use.

SDR is a particularly exciting technology and in previous posts I've focused on complex software like GNU Radio and mainstream applications such as GSM. However, an awful lot of great work with SDR is being done in the amateur radio community, with a focus mainly on signals under 30MHz, narrower bandwidths and often an elegant simplicity.

The first of two posts, this takes a look at construction of a SoftRock receiver for the 40 meter band (7-7.3MHz).

What about rtl-sdr?

You might wonder why anyone would construct a single band, narrow bandwidth receiver when for around the same cost you could purchase rtl-sdr hardware with far greater coverage and bandwidth. However, this needs to be modified to bypass the tuner in order to cover HF and only provides 8-bit samples, which results in much reduced dynamic range when compared with a SoftRock receiver connected to a 16-bit PC soundcard. With band specific filtering also contributing to performance, and the ability to pick out weak signals being a key consideration in amateur radio.

There is also something to be said for, and fun to be had in constructing equipment from components!

Construction

The kit came supplied with a single page schematic and a link to detailed build documentation online. With only a slight variation in the bill-of-materials this design can be constructed for 20, 30, 40, 80 or 160M bands.

The first stage of construction is concerned with the power supply and this consists of simply a diode, 5v regulator and a few capacitors. Should you wish you could omit the regulator and power the receiver from a USB port, and some versions of the kit include additional hardware for this.

With the PSU assembled 12v was supplied to the board and a quick check made to confirm operation of the 5v rail.

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Next was the local oscillator (LO), a Colpitts crystal-controlled design that in the case of a 40M band receiver is at 28.224MHz. The online documentation instructs you to solder the crystal in place, but the kit I purchased included a second crystal and a socket that is to be fitted instead.

Once this section of the receiver had been populated, operation of the LO was confirmed with a shortwave radio.

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The receiver works by downconverting and quadrature sampling a 48kHz chunk of spectrum that is then fed into a PC sound card and processed by software. In order to do this a quadrature sampling detector (QSD) is used and this needs to be supplied with the signal from the antenna along with clocks at 14.112MHz (LO/2) and 7.056MHz (LO/4). Dividers are used to derive these from the LO and this part of the PCB was populated next.

Once again an adjacent shortwave radio was used for testing.

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Operational amplifiers are used to boost the in-phase (I) and quadrature (Q) signals output by the QSD before they are fed into the PC soundcard, and the components for these were added next. With correct assembly confirmed by measuring the voltages present at test points.

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Hand soldering the surface mount components was easy enough and the trickiest part — if like me it's some time since you've wound small coils — came next. As a toroidal inductor and a transformer with bifilar secondaries needed winding for the band pass filter.

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With the coils wound it was then time to add the components for the mixer.

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The coils were then soldered in place, along with an audio lead and a wire antenna for testing.

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A simple kit for cutting edge technology

One of the great things about the SoftRock design is that it enables you to create a high performance — for it's given application — receiver for very little cost and from component parts, using nothing more than a sound card input for interfacing with a PC running the SDR. In the process of doing so learning more about such systems than you might if you bought off-the-shelf hardware.

The SoftRock hardware may be simple but don't let that fool you, as when it's combined with a PC it forms a fully-fledged SDR!

In the follow-up post we'll take a look at software that can be used with the SoftRock receivers.

Andrew Back

Open source (hardware and software!) advocate, Treasurer and Director of the Free and Open Source Silicon Foundation, organiser of Wuthering Bytes technology festival and founder of the Open Source Hardware User Group.

20 Dec 2012, 9:37

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