I have been experimenting with message passing in the Signal Source block in GNU Radio companion. I can see from its source code that we can pass messages to change the frequency, amplitude, offset and phase of the source. For example, the following message PMT sent from a message strobe can change the amplitude of the signal to 0.5.
pmt.dict_add(pmt.make_dict(), pmt.intern("ampl"), pmt.from_double(0.5))
But when I viewed the code of UHD USRP Sink, I couldn't get a clear idea as to what commands can be sent to this block or that which parameters can be changed. I have read at some places in the documentation that frequency, gain, LO offset, timestamp, center frequency and other transceiver related settings of the USRP Sink can be manipulated through command messages.
What commands can be sent to the USRP Sink block from a message strobe (in the pmt format) and which parameters (and their keys) can be modified?
This is officially documented:
https://www.gnuradio.org/doc/doxygen/page_uhd.html#uhd_command_syntax
| Command name | Value Type | Description |
|---|---|---|
| chan | int | Specifies a channel. If this is not given, either all channels are chosen, or channel 0, depending on the action. A value of -1 forces 'all channels', where possible. |
| gain | double | Sets the Tx or Rx gain (in dB). Defaults to all channels. |
| power_dbm | double | Sets the Tx or Rx power reference level (in dBm). Defaults to all channels. Works for certain devices only, and only if calibration data is available. |
| freq | double | Sets the Tx or Rx frequency. Defaults to all channels. If specified without lo_offset, it will set the LO offset to zero. |
| lo_offset | double | Sets an LO offset. Defaults to all channels. Note this does not affect the effective center frequency. |
| tune | tune_request | Like freq, but sets a full tune request (i.e. center frequency and DSP offset). Defaults to all channels. |
| mtune | tune_request_t | Like tune, but supports a full manual tune request as uhd::tune_request_t. Defaults to all channels. |
| lo_freq | double | For fully manual tuning: Set the LO frequency (RF frequency). Conflicts with freq, lo_offset, and tune. |
| dsp_freq | double | For fully manual tuning: Set the DSP frequency (CORDIC frequency). Conflicts with freq, lo_offset, and tune. |
| direction | string | Used for timed transceiver tuning to ensure tuning order is maintained. Values other than 'TX' or 'RX' will be ignored. |
| rate | double | See usrp_block::set_samp_rate(). Always affects all channels. |
| bandwidth | double | See usrp_block::set_bandwidth(). Defaults to all channels. |
| time | timestamp | Sets a command time. See usrp_block::set_command_time(). A value of PMT_NIL will clear the command time. |
| mboard | int | Specify mboard index, where applicable. |
| antenna | string | See usrp_block::set_antenna(). Defaults to all channels. |
| gpio | gpio | PMT dictionary including bank, attr, value, mask for GPIO. See notes. |