GNU Radio 3.6.2 C++ API
digital_fll_band_edge_cc Class Reference

Frequency Lock Loop using band-edge filters. More...

#include <digital_fll_band_edge_cc.h>

Inheritance diagram for digital_fll_band_edge_cc:

Public Member Functions

 ~digital_fll_band_edge_cc ()
void set_samples_per_symbol (float sps)
 Set the number of samples per symbol.
void set_rolloff (float rolloff)
 Set the rolloff factor of the shaping filter.
void set_filter_size (int filter_size)
 Set the number of taps in the filter.
float get_samples_per_symbol () const
 Returns the number of sampler per symbol used for the filter.
float get_rolloff () const
 Returns the rolloff factor used for the filter.
int get_filter_size () const
 Returns the number of taps of the filter.
void print_taps ()
int work (int noutput_items, gr_vector_const_void_star &input_items, gr_vector_void_star &output_items)
 just like gr_block::general_work, only this arranges to call consume_each for you
- Public Member Functions inherited from gr_sync_block
void forecast (int noutput_items, gr_vector_int &ninput_items_required)
 Estimate input requirements given output request.
int general_work (int noutput_items, gr_vector_int &ninput_items, gr_vector_const_void_star &input_items, gr_vector_void_star &output_items)
 compute output items from input items
int fixed_rate_ninput_to_noutput (int ninput)
 Given ninput samples, return number of output samples that will be produced. N.B. this is only defined if fixed_rate returns true. Generally speaking, you don't need to override this.
int fixed_rate_noutput_to_ninput (int noutput)
 Given noutput samples, return number of input samples required to produce noutput. N.B. this is only defined if fixed_rate returns true. Generally speaking, you don't need to override this.
- Public Member Functions inherited from gr_block
virtual ~gr_block ()
unsigned history () const
void set_history (unsigned history)
bool fixed_rate () const
 Return true if this block has a fixed input to output rate.
virtual bool start ()
 Called to enable drivers, etc for i/o devices.
virtual bool stop ()
 Called to disable drivers, etc for i/o devices.
void set_output_multiple (int multiple)
 Constrain the noutput_items argument passed to forecast and general_work.
int output_multiple () const
bool output_multiple_set () const
void set_alignment (int multiple)
 Constrains buffers to work on a set item alignment (for SIMD)
int alignment () const
void set_unaligned (int na)
int unaligned () const
void set_is_unaligned (bool u)
bool is_unaligned () const
void consume (int which_input, int how_many_items)
 Tell the scheduler how_many_items of input stream which_input were consumed.
void consume_each (int how_many_items)
 Tell the scheduler how_many_items were consumed on each input stream.
void produce (int which_output, int how_many_items)
 Tell the scheduler how_many_items were produced on output stream which_output.
void set_relative_rate (double relative_rate)
 Set the approximate output rate / input rate.
double relative_rate () const
 return the approximate output rate / input rate
uint64_t nitems_read (unsigned int which_input)
 Return the number of items read on input stream which_input.
uint64_t nitems_written (unsigned int which_output)
 Return the number of items written on output stream which_output.
tag_propagation_policy_t tag_propagation_policy ()
 Asks for the policy used by the scheduler to moved tags downstream.
void set_tag_propagation_policy (tag_propagation_policy_t p)
 Set the policy by the scheduler to determine how tags are moved downstream.
gr_block_detail_sptr detail () const
void set_detail (gr_block_detail_sptr detail)
- Public Member Functions inherited from gr_basic_block
virtual ~gr_basic_block ()
long unique_id () const
std::string name () const
gr_io_signature_sptr input_signature () const
gr_io_signature_sptr output_signature () const
gr_basic_block_sptr to_basic_block ()
virtual bool check_topology (int ninputs, int noutputs)
 Confirm that ninputs and noutputs is an acceptable combination.
template<typename T >
void set_msg_handler (T msg_handler)
 Set the callback that is fired when messages are available.
- Public Member Functions inherited from gr_msg_accepter
 gr_msg_accepter ()
 ~gr_msg_accepter ()
void post (pmt::pmt_t msg)
 send msg to msg_accepter
- Public Member Functions inherited from gruel::msg_accepter
 msg_accepter ()
virtual ~msg_accepter ()
- Public Member Functions inherited from gri_control_loop
 gri_control_loop (float loop_bw, float max_freq, float min_freq)
virtual ~gri_control_loop ()
void update_gains ()
 update the system gains from the loop bandwidth and damping factor
void advance_loop (float error)
 update the system gains from the loop bandwidth and damping factor
void phase_wrap ()
 Keep the phase between -2pi and 2pi.
void frequency_limit ()
 Keep the frequency between d_min_freq and d_max_freq.
void set_loop_bandwidth (float bw)
 Set the loop bandwidth.
void set_damping_factor (float df)
 Set the loop damping factor.
void set_alpha (float alpha)
 Set the loop gain alpha.
void set_beta (float beta)
 Set the loop gain beta.
void set_frequency (float freq)
 Set the Costas loop's frequency.
void set_phase (float phase)
 Set the Costas loop's phase.
float get_loop_bandwidth () const
 Returns the loop bandwidth.
float get_damping_factor () const
 Returns the loop damping factor.
float get_alpha () const
 Returns the loop gain alpha.
float get_beta () const
 Returns the loop gain beta.
float get_frequency () const
 Get the Costas loop's frequency estimate.
float get_phase () const
 Get the Costas loop's phase estimate.

Friends

DIGITAL_API
digital_fll_band_edge_cc_sptr 
digital_make_fll_band_edge_cc (float samps_per_sym, float rolloff, int filter_size, float bandwidth)

Additional Inherited Members

- Public Types inherited from gr_block
enum  { WORK_CALLED_PRODUCE = -2, WORK_DONE = -1 }
 Magic return values from general_work. More...
enum  tag_propagation_policy_t { TPP_DONT = 0, TPP_ALL_TO_ALL = 1, TPP_ONE_TO_ONE = 2 }
- Protected Types inherited from gr_basic_block
enum  vcolor { WHITE, GREY, BLACK }
- Protected Member Functions inherited from gr_sync_block
 gr_sync_block (void)
 gr_sync_block (const std::string &name, gr_io_signature_sptr input_signature, gr_io_signature_sptr output_signature)
- Protected Attributes inherited from gr_basic_block
std::string d_name
gr_io_signature_sptr d_input_signature
gr_io_signature_sptr d_output_signature
long d_unique_id
vcolor d_color
- Protected Attributes inherited from gri_control_loop
float d_phase
float d_freq
float d_max_freq
float d_min_freq
float d_damping
float d_loop_bw
float d_alpha
float d_beta

Detailed Description

Frequency Lock Loop using band-edge filters.

The frequency lock loop derives a band-edge filter that covers the upper and lower bandwidths of a digitally-modulated signal. The bandwidth range is determined by the excess bandwidth (e.g., rolloff factor) of the modulated signal. The placement in frequency of the band-edges is determined by the oversampling ratio (number of samples per symbol) and the excess bandwidth. The size of the filters should be fairly large so as to average over a number of symbols.

The FLL works by filtering the upper and lower band edges into x_u(t) and x_l(t), respectively. These are combined to form cc(t) = x_u(t) + x_l(t) and ss(t) = x_u(t) - x_l(t). Combining these to form the signal e(t) = Re{cc(t) \times ss(t)^*} (where ^* is the complex conjugate) provides an error signal at the DC term that is directly proportional to the carrier frequency. We then make a second-order loop using the error signal that is the running average of e(t).

In practice, the above equation can be simplified by just comparing the absolute value squared of the output of both filters: abs(x_l(t))^2 - abs(x_u(t))^2 = norm(x_l(t)) - norm(x_u(t)).

In theory, the band-edge filter is the derivative of the matched filter in frequency, (H_be(f) = frac{H(f)}{df}). In practice, this comes down to a quarter sine wave at the point of the matched filter's rolloff (if it's a raised-cosine, the derivative of a cosine is a sine). Extend this sine by another quarter wave to make a half wave around the band-edges is equivalent in time to the sum of two sinc functions. The baseband filter fot the band edges is therefore derived from this sum of sincs. The band edge filters are then just the baseband signal modulated to the correct place in frequency. All of these calculations are done in the 'design_filter' function.

Note: We use FIR filters here because the filters have to have a flat phase response over the entire frequency range to allow their comparisons to be valid.

It is very important that the band edge filters be the derivatives of the pulse shaping filter, and that they be linear phase. Otherwise, the variance of the error will be very large.

Constructor & Destructor Documentation

digital_fll_band_edge_cc::~digital_fll_band_edge_cc ( )

Member Function Documentation

int digital_fll_band_edge_cc::get_filter_size ( ) const

Returns the number of taps of the filter.

float digital_fll_band_edge_cc::get_rolloff ( ) const

Returns the rolloff factor used for the filter.

float digital_fll_band_edge_cc::get_samples_per_symbol ( ) const

Returns the number of sampler per symbol used for the filter.

void digital_fll_band_edge_cc::print_taps ( )

Print the taps to screen.

void digital_fll_band_edge_cc::set_filter_size ( int  filter_size)

Set the number of taps in the filter.

This sets the number of taps in the band-edge filters. Setting this will force a recalculation of the filter taps.

This should be about the same number of taps used in the transmitter's shaping filter and also not very large. A large number of taps will result in a large delay between input and frequency estimation, and so will not be as accurate. Between 30 and 70 taps is usual.

Parameters
filter_size(float) number of taps in the filters
void digital_fll_band_edge_cc::set_rolloff ( float  rolloff)

Set the rolloff factor of the shaping filter.

This sets the rolloff factor that is used in the pulse shaping filter and is used to calculate the filter taps. Changing this will force a recalculation of the filter taps.

This should be the same value that is used in the transmitter's pulse shaping filter. It must be between 0 and 1 and is usually between 0.2 and 0.5 (where 0.22 and 0.35 are commonly used values).

Parameters
rolloff(float) new shaping filter rolloff factor [0,1]
void digital_fll_band_edge_cc::set_samples_per_symbol ( float  sps)

Set the number of samples per symbol.

Set's the number of samples per symbol the system should use. This value is uesd to calculate the filter taps and will force a recalculation.

Parameters
sps(float) new samples per symbol
int digital_fll_band_edge_cc::work ( int  noutput_items,
gr_vector_const_void_star input_items,
gr_vector_void_star output_items 
)
virtual

just like gr_block::general_work, only this arranges to call consume_each for you

The user must override work to define the signal processing code

Implements gr_sync_block.

Friends And Related Function Documentation

DIGITAL_API digital_fll_band_edge_cc_sptr digital_make_fll_band_edge_cc ( float  samps_per_sym,
float  rolloff,
int  filter_size,
float  bandwidth 
)
friend

Build the FLL

Parameters
samps_per_sym(float) Number of samples per symbol of signal
rolloff(float) Rolloff factor of signal
filter_size(int) Size (in taps) of the filter
bandwidth(float) Loop bandwidth

The documentation for this class was generated from the following file: