UHD Interface

Introduction

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

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:

http://files.ettus.com/manual/

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

http://files.ettus.com/manual/annotated.html

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.

There is a legacy format, which will be deprecated in the future, where commands may be tuples, formatted as:

(command, value, [channel])

See older versions of this manual for documentation on this deprecated command format.

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.

Example:

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

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:

pmt::pmt_t command = pmt::make_dict();
pmt::dict_add(command, pmt::mp("freq"), pmt::mp(1.1e9)); // Specify frequency
pmt::dict_add(command, pmt::mp("chan"), pmt::mp(1)); // Specify channel
// Now pass 'command' into the USRP block's command port

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
pmt::dict_add(command, pmt::mp("gain"), pmt::mp(23.0)); // Specify gain
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.
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.
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.
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.

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)
• timestamp: A pair composed of (long full_secs, double frac_secs). Similar to uhd::time_spec_t

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:

self.u = uhd.usrp_source(device_addr=options.args,
                         io_type=uhd.io_type.COMPLEX_FLOAT32,
                         num_channels=1)

self.u.set_samp_rate(options.samp_rate)

# 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
if(options.spec):
    self.u.set_subdev_spec(options.spec, 0)

 # Set the antenna
 if(options.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.

self.u.set_samp_rate(options.samp_rate)

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)