GNU Radio Manual and C++ API Reference
The Free & Open Software Radio Ecosystem
UHD Interface


This is the GNU Radio UHD package. It is the interface to the UHD library to connect to and send and receive data between the Ettus Research, LLC product line. To use the UHD blocks, the Python namespaces is in gnuradio.uhd, which would be normally imported as:

from gnuradio import uhd
Definition: sptr_magic.h:26

The relevant blocks are listed in the UHD Interface group. The most important components are the gr::uhd::usrp_source and gr::uhd::usrp_sink blocks, which act as receivers/transmitters. Both are derived from gr::uhd::usrp_block, which defines many of the shared functions between those blocks.

External Documentation

Ettus Research maintains the comprehensive documentation to the underlying UHD driver, which can be found at:

The list of classes in the UHD Doxygen is located at:

Command Syntax

The UHD sink and source can be controlled by a message port. These message ports take commands, which are PMTs formatted as described in Using messages as commands.

In general, every command consists of one or more key/value pairs (either stored as a PMT pair, or a dictionary). A full list of keys is listed below.


pmt::pmt_t command = pmt::cons( // Make a pair
pmt::mp("freq"), // Key is 'freq' => sets the frequency
pmt::mp(1.1e9) // Set the frequency to 1.1 GHz
// Now pass 'command' into the USRP block's command port
PMT_API pmt_t cons(const pmt_t &x, const pmt_t &y)
Return a newly allocated pair whose car is x and whose cdr is y.
static pmt_t mp(const std::string &s)
Make pmt symbol.
Definition: pmt_sugar.h:24
std::shared_ptr< pmt_base > pmt_t
typedef for shared pointer (transparent reference counting).
Definition: pmt.h:83

This PMT would set the frequency to 1.1 GHz on all channels. We make use of the pmt::mp() function which automatically sets the PMT types. Assume we only want to set the frequency on channel 1 (i.e. the second channel). In this case, we must construct a dictionary:

command = pmt::dict_add(command, pmt::mp("freq"), pmt::mp(1.1e9)); // Specify frequency
command = pmt::dict_add(command, pmt::mp("chan"), pmt::mp(1)); // Specify channel
// Now pass 'command' into the USRP block's command port
PMT_API pmt_t dict_add(const pmt_t &dict, const pmt_t &key, const pmt_t &value)
Return a new dictionary with key associated with value.
PMT_API pmt_t make_dict()
Make an empty dictionary.

This command structure becomes more intuitive when thinking of sending the command PMT as a function call, where the key/value pairs are argument names and values, respectively. In the above example, the behaviour is the same as if calling

usrp_source.set_center_freq(freq=1.1e9, chan=1)

The main difference is that we can add more properties to the same command PMT, e.g. as such:

// 'command' is the same PMT as in the previous example
command = pmt::dict_add(command, pmt::mp("gain"), pmt::mp(23.0)); // Specify gain
command = pmt::dict_add(command, pmt::mp("antenna"), pmt::mp("TX/RX")); // Switch antenna
// Now pass 'command' into the USRP block's command port

When the USRP block interprets this command PMT, all properties will be set.

Common command keys

The following command keys are understood by both UHD Source and Sink:

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.
stream_cmd dict (stream command) PMT dictionary mirroring UHD's stream_cmd_t. Supported by RFNoC RX Radio. See below for details.

Special types:

  • tune_request: Like a uhd::tune_request_t, but always uses POLICY_AUTO. This is a pair, composed of (target_frequency, lo_offset)
  • tune_request_t: Exact copy of uhd::tune_request_t, allowing full control. See details below. It supports fully customized tunings with all policies and integer-N tuning. The policies are strings A, N, M for automatic, none and manual tuning.
  • timestamp: A pair composed of (long full_secs, double frac_secs). Similar to uhd::time_spec_t
  • gpio: This is a PMT dictionary with four key/value pairs: bank (string), attr (string), value (double) and mask (double). The gpio command calls set_gpio_attr with the elements from the dictionary as arguments. Can optionally contain mboard to specify the mainboard. Defaults to 0 (first mboard).
  • Stream command dictionaryA dictionary with keys stream_mode, num_samps, time_spec, and stream_now.

Further notes on command mtune:

The object is of type pmt::dict and has the same fields as uhd::tune_request_t:

  • dsp_freq: the dsp frequency as pmt double type.
  • dsp_freq_policy: policy for DSP tuning. Should be a string "A" (automatic), "M" (manual) or "N" (none). If not set, defaults to "A".
  • rf_freq: the rf frequency as pmt double type.
  • rf_freq_policy: policy for RF tuning. Should be a string "A" (automatic), "M" (manual) or "N" (none). If not set, defaults to "A".
  • target_freq: target frequency (for automatic running) as pmt double type
  • args: string containing additional arguments, for example for integer N tuning.

Further notes on command stream_cmd:

The object is of type pmt::dict and has the same fields as uhd::stream_cmd_t:

  • stream_mode: This is a string (not an enum or int) and takes the same values as the UHD Python API uses for stream modes (start_cont, stop_cont, num_done, and num_more).
  • stream_now: Boolean
  • time_spec: When stream_now is false, this is the absolute time when the stream command is executed.
  • num_samps: When stream_mode is num_done, this is the number of samples that will be received.


tune_rx = pmt.make_dict()
tune_rx = pmt.dict_add(tune_rx, pmt.to_pmt('rf_freq'), pmt.to_pmt(rf_freq))
tune_rx = pmt.dict_add(tune_rx, pmt.to_pmt('rf_freq_policy'), pmt.to_pmt('M'))
tune_rx = pmt.dict_add(tune_rx, pmt.to_pmt('dsp_freq_policy'), pmt.to_pmt('N'))
tune_rx = pmt.dict_add(tune_rx, pmt.to_pmt('args'), pmt.to_pmt('mode_n=integer,int_n_step=1000e3'))
Definition: pmt.h:38

Note: Not all commands are affected by time. See the UHD manual for details on timed commands.

Dictionaries vs pairs

Given the choices, it may be unclear if it's preferable to send multiple commands to the USRP block with a single key/value pair each, or send a single dict with all the values.

In general, the dictionary should be preferred. It has some distinct advantages:

  • If it carries a timestamp, this timestamp is valid for all key/value pairs it may be applied to.
  • All settings will be applied at once. With multiple messages, other blocks might be sending interfering messages while the messages are being processed.

Configuring a UHD object

A typical option parser setup for a UHD device looks like

parser = OptionParser(option_class=eng_option)
parser.add_option("-a", "--args", type="string", default="",
help="UHD device address args , [default=%default]")
parser.add_option("", "--spec", type="string", default=None,
help="Subdevice of UHD device where appropriate")
parser.add_option("-A", "--antenna", type="string", default=None,
help="select Rx Antenna where appropriate")
parser.add_option("-s", "--samp-rate", type="eng_float", default=1e6,
help="set sample rate (bandwidth) [default=%default]")
parser.add_option("-f", "--freq", type="eng_float", default=None,
help="set frequency to FREQ", metavar="FREQ")
parser.add_option("-g", "--gain", type="eng_float", default=None,
help="set gain in dB (default is midpoint)")

To use these options to create a UHD source object:

stream_args = uhd.stream_args()
self.u = uhd.usrp_source(device_addr=options.args,
# if no gain was specified, use the mid-point in dB
if options.gain is None:
g = self.u.get_gain_range()
options.gain = float(g.start()+g.stop())/2
self.u.set_gain(options.gain, 0)
# Set the center frequency
self.u.set_center_freq(options.freq, 0)
# Set the subdevice spec
self.u.set_subdev_spec(options.spec, 0)
# Set the antenna
self.u.set_antenna(options.antenna, 0)

Frequently, your application may need a sample rate that is not supported by the UHD device. If you have extra CPU power to spare, you can easily set the sample rate you want, then ask the device what the actual sample rate set was. Then, you can easily create an arbitrary resampler to take care of the difference.

desired_rate = options.samp_rate
actual_rate = self.u.get_samp_rate()
resample = desired_rate / actual_rate
# Use the filter.pfb version and pass only the resample factor.
# This block builds a half-band filter for you
self.resampler = filter.pfb.arb_resampler_ccf(resample)


gr-uhd will emit log messages when unexpected events occur.

For under- and overflows, as well as command time errors, since these tend to happen in high-rate scenarios, such errors are counted for logging intervals. The first error after a time longer than the logging interval has passed is logged immediately, but subsequent log entries are accumulated.

The logging interval can be configured using the GNU Radio preferences framework (i.e. gnuradio.conf file), as follows:


It is an integer that defaults to 750, representing the minimum number of milliseconds between logged under-, overflow events or command time errors.