CarrierTracking

The CarrierTracking object implements two functions :

• Implements a Phase Locked Loop (PLL) which locks to the carrier of the input frequency and outputs the input signal mixed with that carrier, downconverted to DC.
• Implements an optional post-mix low-pass filter (LPF).

Internally a IData queue of 10 elements is allocated for input and ouput.

Typical use case :

Digital modems (see the Modems Introduction section ) need an oversampled input stream. When receiving digital transmission subject to Doppler, the CarrierTracking will perform :

• Tracking the center of the data transmission and compensate for Doppler,
• If post-mixer LPF enabled, will reduce undesired signals.

It can be used after a DDC to track a satellite downlink and follow Doppler shift without having to recenter manually on the carrier.

  class CarrierTracking {
        constructor CarrierTracking();
        bool configure( { "filter_enable" : bool, "cutoff" : number , "lock_wait" : boolean } );
        bool write( IQData );
        IQData read( bool wait);

        bool isLocked();
        number getFrequencyError();
  }

.configure

Enables or disables the post-mixer Low-Pass filter.

bool configure( { 'filter_enable' : bool, 'cutoff' : number } );

Parameters :

• 'filter_enable' : boolean value, turns on (enables) or off (disables) the post-mixer filter,
• 'cutoff' : float, sets the frequency cutoff of the low-pass-filter
• 'lock_wait' : if true, no data is sent to the output if the PLL is not locked

.write

Push IQ data block for processing.

    var boolOk = write( IQDatain );

Returns false is the internal queue is full.

.isLocked

Returns the internal PLL lock indicator.

.getFrequencyError

Returns the offset in Hz of the internal local oscillator

.read

Extracts a previously processed block from the internal queue.
If the queue was empty :
the returned block length will be 0 if the wait* boolean is false * the call will return only when data is available otherwise.

    var IQDataout = resampler.read( bool wait );

Example

In this example, we generate a complex tone centered at 12 kHz. The tracking filter will recenter the signal.
The incoming sample rate is set at 48 kHz, and with given parameters, the post-mixer LPF will filter at 4.8 kHz.

var IQ = DSP.tone(12000, 8000, 48e3);

var tracker = new CarrierTracking();
tracker.configure( { 'filter_enable' : true, 'cutoff': 0.1 }) ;
// push IQ to the Tracking
tracker.write(IQ);

// read
var out = tracker.read();
out.dump(); 
out.saveToFile('/tmp/out.cf32');