Software Defined Radio
Update - go to this page to get the latest version!
Digital Crystal Set
Here's my software definable radio, and here's the source code. (Take a look at Youssef's update, Summer 2011, below...)
What the Digital Crystal Set is...
- Demonstrates that real time signal processing is possible under the Common language Runtime.
- C# only! No nasty DLL's with C++ or, dare I say it, C.
- AM only.
- Only plays wav files.
- All processing is in the time domain. I'll leave FFT's for a later version.
- A sampler that just might motivate others to play with SDR using C#.
What the Digital Crystal Set is not...
The Digital Crystal Set is not a general purpose Software Defined Radio (SDR). For an SDR that's got features I recommend:
The Manual
Obtain a IQ wav file
Here's a sample I'll use in this manual. The wav file has 2 weak medium wave stations to tune to.
Install the .NET 2.0 runtime.
You'll need the "Microsoft .NET Framework Version 2.0 Redistributable Package (x86)".
Download
Download the Digital Crystal Set and run it.
Configuration
There's no configuration. You will need a sound card and speakers.
Playing...
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.
Features
Pass through / SDR
Click "Pass through" to listen to the raw IQ. "SDR" reverts to the signal processing path.
Bias and Gain settings
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.
Mixer Frequency
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.
Low Pass
This switches in a 3kHz Butterworth 5 pole infinite-impulse-response (IIR) low pass filter. This sits before the AM detector.
PLL
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
Audio gain sets the sound volume.
IQ phase plot
This show the IQ phase of the signal before the AM detector.
Source Code
Here's the core of the radio:
The complete original source code can be downloaded here. (Take a look at Youssef's update, Summer 2011, below...)
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 is the PLL required?
"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.
Update
Summer 2011, and Youssef Touil contacts me. He's added a simple AM detector and USB and LSB detectors.
The source to his version is here.
Thanks Youssef!
Links
Comments?
Comments to nick@anotherurl.com
Page last updated 14th August 2011
