我一直在尝试在 GNU Radio 伴侣的 Signal Source 块中传递消息。我可以从它的源代码中看到，我们可以传递消息来改变源的频率、幅度、偏移和相位。例如，从消息选通发送的以下消息 PMT 可以将信号的幅度更改为 0.5。
pmt.dict_add(pmt.make_dict(), pmt.intern("ampl"), pmt.from_double(0.5))
但是当我查看 UHD USRP Sink 的代码时，我无法清楚地知道可以向该块发送哪些命令或可以更改哪些参数。我在文档中的某些地方读到，USRP Sink 的频率、增益、LO 偏移、时间戳、中心频率和其他收发器相关设置可以通过命令消息进行操作。
可以从消息选通（以 pmt 格式）向 USRP Sink 块发送哪些命令以及可以修改哪些参数（及其密钥）？
【问题讨论】：
标签： gnuradio message-passing gnuradio-companion uhd
这是官方记录的：
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. |
【讨论】：
gpio 命令以字典中的元素作为参数调用set_gpio_attr。可以选择包含mboard 以指定主板。默认为0（第一个主板）“下面