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Software Defined Radio - A Project

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Category: Equipment

Published on Friday, 29 April 2011 06:58

Written by Rusty - kc0vcu

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Article initially appeared in the November 2010 edition of the Quarterwave. You can read the article in the QW and more articles by downloading the original QW at http://qw.tcfmc.org/QW-2010-November.pdf

The project has been put in the background as a result of my moving in with my fiancé, and I'll be following up as the project progresses, but to get things started:

If you haven’t heard of Software Defined Radio (SDR), I won’t ask what rock you’ve been living under. We each have our own set of interests in any activity, and Amateur Radio covers a wide variety of different interests, of which SDR is only a small part.

That said, I’m not going to spend a lot of time describing SDR. First of all because there are a lot of people out there who have done a far better job than I can at this point. Second of all, I don’t believe in just writing ‘filler’ content when I don’t have to.

A brief summary of what SDR is however is probably in order. Many modern radios have SDR involved at some level. It may be a DSP filter that can look at the general characteristics of the band near the frequency that you are trying to listen to, and filter out noise that may be changing over time in frequency or type. It may be that new rig that the manufacturer has built to look like a classic rig with all sorts of buttons and knobs, dials and readouts, yet they tell you that down the road the radio will get even more features and all you have to do is download a firmware update to the radio.

For some people the only SDR that they are interested in is little more than an appliance that looks like a large field radio that you ‘control’ with a PC or Mac, and that just happens to do all the audio processing from the mixer stage in software on the PC.

A fair definition of what SDR is is a radio that passes the down converted received radio frequencies into a computer without passing it through filters or mixers, and the software on the computer performs the functions that you would expect of those components in a radio transceiver.

There are SDR platforms that start actually processing the RF signal directly and extract from it the signals in the frequencies that you are looking for, as well as platforms that take unfiltered audio from a standard receiver and do the processing on that signal. Whether you consider one to be purer SDR than another is likely to be a debate that will run for as long as the question of whether linking repeaters across the Internet rather than through a radio link is legitimately part of Amateur radio.

I’m not going to pass judgment myself. From my perspective each poses an interesting challenge to work with, and I have my own reason for getting into SDR.

What I’m going to do over a few articles is build and see if I can get on the air with a kit based SDR rig. I’ve actually bought a couple of different kits, because I’m looking to do more than just get on the air. I’m also interested in building my building skills. For many people over the years, building their own radios has been a challenge that they have met with varying levels of success.

One of the classic images of ham radio has been the amateur who is pouring over the documentation that came with the Heath-Kit radio parts and the hours spent actually building different parts of the radio, testing the audio amplifiers, the receiver circuits, the transmitter and PA. Getting everything put together in a working rig is a challenge that in it’s own right is part of amateur radio, and can lead to a far better understanding of the principles of electronics and radio equipment.

However for most amateurs today, the Heath-Kit idea is a thing of the past. The company itself is relegated to history, but on top of that, home built kits have gone in two directions. Most have gone to the very expensive high end radios. And while the parts and hardware that you build yourself will cost less than if you were to buy an equivalent capability radio off the shelf, there are a lot of lesser radios that many people are just as happy with.

The other direction has mostly been small kits of accessories. PIC based frequency counters. Home made tuners for QRP radios. APRS Trackers. Computer headset interfaces for your rig so you didn’t have to buy the commercial interfaces. It got to the point that a couple of years ago the ARRL put out a challenge for a couple of different radios. The result was a pair of radios that were based on mostly the same hardware. Both rigs were under $100 in parts.

SDR in most cases is not a kit based platform. That said, the Softrock platform is a kit based SDR radio, and in general is vary affordable. You can start with SDR receivers in one of several bands, including a kit that will allow you to build for any of 4 bands. There are also kits that will allow you to tune the radio frequency being received by tuning it with a computer over a USB interface. And they have a couple of transceiver kits.

I’ve ordered a couple of kits. Two are SDR receivers, the other is an SDR transceiver.

The SDR receivers I’ve bought are the lite + usb xtall v9.0 receiver, and the Upgraded 20m Lite II receiver. The transceiver is the rxtx + xtall v 6.3 radio. I’ve also discovered that the software needed for testing the radios while they are being built is now available for Linux, so I will be using that as well.

I decided to try both the lite + usb xtall v9.0 and the upgraded 20m Lite II radios, in part because they were both available, and also because I wanted to give different control options a try. The rxtx radio, as it will be built, is a qrp rig. That said down the road I may be able to put together an amplifier to bring it up to a 50-100 watt transmitter, though that may not be necessary.

This can easily get to be a 9 part (or more) series, however I’m not planning on taking 2 years to build these radios, and I don’t think you want to wait through that many publications to read the full story. So here’s the plan. I’ve given you a brief overview of what the concept of SDR is here. The next story will be my experience building the hardware, and the following story will cover the software that I end up using, and wrapping up the series. I’ll talk about specific radios as they differ from the other radios as I am building them, and in how they work for me. I may very well end up with only one working radio. If that’s the case, then that’s OK. That’s what I end up with. But I’m hoping to have at least a receiver and a transceiver, and in the bet of possible worlds I’m hoping that I will end up with three working radios. Why three? Well, I’m hoping to build the 20 meter radio for monitoring that band, and the xtall + USB radio to listen on another band at the same time. If I can I’m hoping to build the transceiver to be able to switch between bands, so that I can work wherever I’m encountering operators.

For control of the radios, and to actually convert the received output to something usable I’m going to be trying several different computers. My primary desktop may be the most capable, but it’s not very portable, and I’m thinking that the best use I will have, especially for the rxtx at default power, will be in a portable station, I’m going to be trying a couple of different laptops and netbooks. Since none of these have a stereo line in interface, I’m going to have to get an external sound card device of some sort to capture stereo and bring it into the computer. I haven’t decided what that will be yet.

So that’s the basic logic behind the decisions I’ve made so far, and the general direction I’m hoping to take this series. Hang in there for the ride, and I hope you find the experience to be interesting, and if it helps you to take on a project that you have been wondering about, that’s a big part of what I would like to see happen.

Check back here in a few months. I’m pretty sure that there will be things to talk about, and perhaps even a few pictures to review.