Wednesday, December 8, 2010

Software Defined Radio - What Is It

Over the past year or so, I have made mention of the SDR (Software Defined Radio) in some facebook posts.  I get questions from time to time, so I thought I would write a little bit about how this works and what it is, exactly.

I have 2 different SDR's actually.  One is a receiver only, the other is a transmitter / receiver.  Both of my radios cover the HF (shortwave) bands, from 1.6 - 30 MHz.  The radios are kits, yea, you have to build them.  They are a PC board which you have to populate the board with the parts.  You supply the case for it, but the newest versions supplies all of the connectors.

The finished radios - Softrocks have no knobs, no volume control, no tuning dial, nothing.  It is a black box with an antenna connector, a USB port, and 1 or 2 audio connectors, depending on if your board is receive only or is a transceiver and a power connector.

The USB port is used to tune the radio.  The software which you use with this board sends tuning code to the board via the USB.  It also  automatically selects the band filters for the frequency youre dialed to.

The audio connector on the board sends a stereo audio frequency signal to the sound card.  This signal is audio, but there is a slight phase difference between channels that is derived by the mixer on the board.  Actually, there are 2 mixers on the board, with a common local oscillator , however, one mixer is fed a phase delayed signal from the local oscillator (90 degrees).  The outputs of the 2 mixers therefore has a slight phase shift in the mixed down signals.

In transmitting, the transmitter basically reverses the above process, amplified and transmitted to about 1 watt, which I feed into a small linear amplifier outboard.

Because of this special stereo audio, the DSP chip in your sound card and the associated software is able to distinguish between upper sideband, lower sideband, AM, narrow band FM, double sideband (with or without carrier), DRM...anything you might want to receive.

The software (there are several different programs available) displays the entire spectrum that the soundcard sample rate allows.  For instance, if your sound card is capable of a 96 Kbps sample rate, you are able to see 96KHz of spectrum at one time on your display.  Lets say you are looking at the 49 meter shortwave band at night, you will see a spectrum analyzer display that is just under 100 KHz wide, and you will see every station within that bandwidth on the display.  The software is calibrated in dBm, and you can see which signals are stronger or weaker and what frequency theyre on.  A simple mouse click will allow you to select any signal that you can see.  The demodulated output appears at your line output of your sound card.

One of the biggest features of this technology is the filtering.  By "grabbing" and dragging the audio passband window in the software, you can continuously vary the bandpass and center frequency of the filter so you can eliminate annoying heterodynes and other interference from the sighal you want to actually listen to.  There are preset filters, which vary from 2.4 KHz to 16 KHz in the AM mode (there are other filters for SSB).  One can select one of these filters and modify them any way you want to enhance the intelligibility if the signal you want to hear.

Synchronous detection on AM is available in some programs that allow you to listen to shortwave stations in the presents of "selective fading"  Selective fading is a phenomenon where the carrier fades, but the sidebands arent faded.  This causes severe distortion during the fade, but the synchronous detector eliminates that problem by injecting a phase locked BFO to the carrier frequency, simulating the original carrier, and since the BFO does not fade, you will never know there was a selective fade going on (unless you look at the display).

All of these features are in the software.  The hardware part is just a mixer and oscillator that produces an audio baseband signal for which the software can do its work.  The hardware end of it is extremely simple.  There are receivers available for as little as $20.  The all band  receivers are about $50 or so.  When these are used with a good quiet sound card, the performance is quite unbelieveable.

I hope this answers a lot of questions as to what this technology is about.  This is a basic, non-technical explanation.  There are more in-depth websites that describe the technology in far greater detail.
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