Self-Calibration

UHD software comes with several self-calibration utilities for minimizing IQ imbalance and DC offset. These utilities perform calibration sweeps using transmit leakage into the receive path (special equipment is not required). The results from a calibration are written to a file in the user's home directory. UHD software will automatically apply corrections at runtime when the user re-tunes the daughterboard LO. Calibration results are specific to an individual RF board.

Note: When a calibration table is present, and the user wishes to override the calibration settings through the API: the user should re-apply the desired setting every time the LO is re-tuned.

UHD software comes with the following calibration utilities:

uhd_cal_rx_iq_balance: - minimizes RX IQ imbalance vs. LO frequency
uhd_cal_tx_dc_offset: - minimizes TX DC offset vs. LO frequency
uhd_cal_tx_iq_balance: - minimizes TX IQ imbalance vs. LO frequency

The following RF frontends are supported by the self-calibration utilities:

RFX Series transceiver boards
WBX Series transceiver boards
SBX Series transceiver boards
CBX Series transceiver boards
UBX Series transceiver boards
USRP N320

Calibration Utilities

UHD software installs the calibration utilities into <install-path>/bin. Disconnect any external hardware from the RF antenna ports, and run the following from the command line. Each utility will take several minutes to complete:

uhd_cal_rx_iq_balance --verbose --args=<optional device args>
uhd_cal_tx_iq_balance --verbose --args=<optional device args>
uhd_cal_tx_dc_offset --verbose --args=<optional device args>

See the output given by --help for more advanced options, such as manually choosing the frequency range and step size for the sweeps.

Note: Your daughterboard needs a serial number to run a calibration utility. Some older daughterboards may not have a serial number. If this is the case, run the following command to burn a serial number into the daughterboard's EEPROM:

<install dir>/lib/uhd/utils/usrp_burn_db_eeprom --ser=<desired serial> --args=<optional device args>

Calibration Data

By default, calibration files are stored in the user's home/application directory ($XDG_DATA_HOME):

Linux: ${HOME}/.local/share/uhd/cal/
Windows: LOCALAPPDATA%\uhd\cal\

Calibration files are binary files with a .cal file extension.

If you would like to specify a custom directory, you can do so with the $UHD_CAL_DATA_PATH environment variable.

Calibration files can easily be moved from one machine to another by copying the "cal" directory, or individual files therein. Re-running a calibration utility will replace the existing calibration file. The old calibration file will be renamed so it may be recovered by the user.

Modify Calibration Data

There might be reasons to analyse or modify the calibration data outside UHD's calibration process. Because the data is stored using FlatBuffers this can be done without relying on UHD. UHD provides all FlatBuffers schema files in <install-path>/share/uhd/cal.

First, install FlatBuffers. The package can be obtained from https://google.github.io/flatbuffers/. Once installed, .cal files can be converted to JSON using

    flatc --strict-json -t <install dir>/share/uhd/cal/<foo>_cal.fbs -- <data>.cal

where <foo>_cal.fbs is the scheme file used for the data, e.g. pwr_cal.fbs for power calibration. data.cal is a calibration file in your working directory. This will generate a <data>.json in the same directory.

The JSON data can be converted back to binary using

    flatc -b <install dir>/include/uhd/cal/<foo>_cal.fbs <data>.json

This generates a <data>.cal that can be read by the calibration routines of UHD.

Converting UHD 3.x calibration data to UHD 4

Older versions of UHD used a CSV-based format for storing calbration data for IQ imbalance and DC offset correction on some devices (e.g., X300, N200 motherboards and WBX/SBX/CBX/UBX daughterboards).

Going forward, all calibration data is stored as binary, to facilitate storing it on device's flash memory, among other reasons. Running the uhd_cal_* utilities will automatically generate the calibration data in the new format.

To convert existing calbration data to the new format, use the convert_cal_data.py utility. By default, it will convert all existing data. Use convert_cal_data.py --help to get a full list of command line options. The tool is installed with the other utilities, for example into /usr/share/lib/uhd/utils, depending on your OS and CMake settings.

Ignoring Calibration Files

At runtime, the user can choose to ignore a daughterboard's calibration file by adding "ignore-cal-file" to the arguments. With the UHD API, it can be done as follows:

auto usrp = uhd::usrp::multi_usrp::make("type=x300,ignore-cal-file=1");

uhd::usrp::multi_usrp::make

static sptr make(const device_addr_t &dev_addr)

Using tx_waveforms as an example, the user can apply this argument as follows:

tx_waveforms --args="addr=192.168.10.2,ignore-cal-file=1" --freq=100e6 --rate=1e6