Here's my software definable radio, and here's the source code.
The Digital Crystal Set is not a general purpose Software Defined Radio (SDR). For an SDR that's got features I recommend:
Here's a sample I'll use in this manual. The wav file has 2 weak medium wave stations to tune to.
You'll need the "Microsoft .NET Framework Version 2.0 Redistributable Package (x86)". Try this link.
Download the Digital Crystal Set and run it.
There's no configuration. You will need a sound card and speakers.
Click Open and select the mwam2.wav. Click Play. You will see the PLL
struggle then lock, as you listen to "DJ-V
Bloodshot" on the album
Fused Rocking Beats.
Tune away from 1000Hz to -7950Hz. (Yes there's a minus. Don't panic - it just
means we're below the local oscillator frequency.) Now you can listen to Sunrise
Radio.
Click "Pass through" to listen to the raw IQ. "SDR" reverts to the signal processing path.
Adds a constant bias / gain to the input stream. Useful if the sound card / SDR hardware is not balanced. Errors there will introduce the image signal on top of the wanted signal.
With the sample file on 1000Hz, changing any of the settings produces a heterodyne. (There's a carrier/ sproggy around -1000Hz) The signal on -7950Hz is not so fussy - there's no interfering carrier around 7950Hz.
Use the Mixer Frequency to tune either side of the local oscillator. Useful tuning limits are defined by the sampling rate and the quality of the sound card used in the recording.
This switches in a 3kHz Butterworth 5 pole infinite-impulse-response (IIR) low pass filter. This sits before the AM detector.
This switches in the Phase Locked Loop. The PLL error frequency is displayed here
Each division represents one Hz.
The PLL settings can be changed by clicking on "PLL Settings..." Increasing the bandwidth makes the lock more snappy.
Audio gain sets the sound volume.
This show the IQ phase of the signal before the AM detector.
Here's the core of the radio:
The complete source code can be downloaded here.
The mixer down converts the signal to 0 Hz.
There are 2 lowpass filters, each for the I and Q signals.
The AM detector take the amplitude of the real signal component, removing the DC component.
"Why the Phase Locked Loop? - ordinary broadcast radios don't have them."
The PLL is there to prevent the heterodyne between the station's carrier and the locally generated carrier.
When you tune in a station on an ordinary AM radio there is no heterodyne. There is no locally generated carrier in an ordinary AM radio. (The radio's first local oscillator is used to convert to the IF frequency.)
When recovering audio with SSB, we don't care about the carrier phase. You can resolve SSB on a AM radio by beating with a carrier introduced from a signal generator.
With two sidebands present, the sum of both sidebands are used. Each sideband must not interfere with the other.
The locally generated carrier has to be phase locked to the broadcast carrier, hence the PLL.
www.codeproject.com/cs/media/cswavrec.asp
www.thecodeproject.com/cs/media/cswavplay.asp
Comments to nick@anotherurl.com
Page last updated July 9th 2006