ARLA/CLUSTER: Um SDR feito em casa

[João Gonçalves Costa]

[http://i2.ytimg.com/vi/1ShYefsbnAE/default.jpg][http://www.vf.utwente.nl/~ptdeboer/ham/sdr/waterfall05.png]
Homebrew software defined radio

VK3YE has made available a video showing a demonstration of his homebrew software defined radio.

The YouTube description reads:

Testing a very basic software defined radio on the 80 and 40 metre amateur bands.

Uses the easy to use SDRadio software developed by I2PHD (see http://www.sdradio.org/) and a simple hardware interface which is a direct conversion receiver without audio filter or amplifier stages.

Apart from an RF amplifier (not really necessary) all the receiver's gain comes from the computer (microphone in used, not Line in).

This is a very 'bare bones' implementation for use with a low-end laptop with a mono soundcard input. Unlike a 'proper' SDR there is no image rejection. Hence signals will appear twice on the spectrum display (one USB and the other LSB) and some juggling may be required to dodge interference. Also signal to noise ratio will never be as good as a single-signal receiver.

The hardware unit comprises of (i) transistor VFO (using ceramic resonators for 3.58 MHz (80m) and 7.2 MHz (40m), (ii) buffer stage, (iii) single balanced diode product detector and (iv) transistor RF amplifier (optional). Your favourite direct conversion receiver circuit without audio filtering or amplification would work fine.

This unit is USB powered but a 9 volt battery would also be OK if you want to keep it independent from the computer (this also helps reduce interference radiated from the computer).

Performance isn't as good as a home station rig but in some aspects is better than a basic direct conversion receiver. The AGC is a bit slow for my liking and it would be good to have an option to switch it off.

There is a slight delay between when you adjust a setting and when the resultant audio is produced. A weak signal is generally clearer and less noisy on the commercial rig as well. And tuning can be fiddly when trying to null out an interfering signal on the opposite image (a full SDR with stereo sound card would fix this).

Nevertheless for its simplicity this is an amazing receiver and would be a good beginners project.

Watch Homebrew software defined radio - a demonstration : http://www.youtube.com/watch?v=mfdgxZPc2BI

Soft Radio Yahoo Group
http://groups.yahoo.com/group/soft_radio/

Soft Rock 40 Yahoo Group
http://groups.yahoo.com/group/softrock40/
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