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Device Calibration and Frontend Correction

# 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
- OBX 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\\endiskip

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/OBX 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

# Frontend Corrections

The calibrations for IQ imbalance and DC offset compensation rely on frontend correction logic that is located in the FPGA.

Note that USRP E310, E320, N300, N310 and B200-Series use a dedicated RFIC which does its own calibration. For those, any calibrations are very device-specific and are not covered in this section.

## Single-Tap DC Offset Compensation

A DC offset is a fixed voltage that is permanently present on the signal of interest. It is an additive error, meaning if the signal of interest is x(t), the actual, observed signal y(t) includes a constant offset:

\[ y(t) = x(t) + D \]

If the constant value D is known, it can simply be subtracted again. For this reason, the DC offset compensation consists of a simple adder. The calibration tool uhd_cal_tx_dc_offset will estimate the value D' which minimizes the DC offset for various frequencies (i.e., D' is approximately equal to -D).

To program the adder, set the DC offset correction value using these API calls:

- uhd::usrp::multi_usrp::set_tx_dc_offset()
- uhd::usrp::multi_usrp::set_rx_dc_offset()
- uhd::rfnoc::radio_control::set_tx_dc_offset()
- uhd::rfnoc::radio_control::set_rx_dc_offset()

For RX DC offset compensation, the estimation of D can be difficult. Therefore, the RX DC offset compensation also includes an automatic mode, which acts as a notch filter around DC. It is implemented as a single-tap IIR filter with the following difference equation:

\[ y[k] = x[k] - \alpha * y[k-1] \]

The values for \(\alpha\) are device-dependent, but the default value is \(2^{-20}\).

To enable the automatic DC offset correction, call one of these API calls with an argument of 'true':

- uhd::usrp::multi_usrp::set_rx_dc_offset()
- uhd::rfnoc::radio_control::set_rx_dc_offset()

DC offset is often caused by LO leakage. It is therefore advised to offset-tune the radio to avoid having the LO in the band of interest. The DC offset compensation will not only remove LO spurs, but also affect the signal of interest if the LO is within the band of interest.

Superheterodyne receivers such as the TwinRX do usually not require DC offset correction values.

## Single-Point IQ offset compensation

In order to correct IQ imbalance, the user can specify two real values A and B. In the FPGA, the complex signal is modified according to the following equation:

\[\begin{pmatrix} I' \\ Q' \end{pmatrix} = \begin{pmatrix} A/64+1 & 0 \\ B/64 & 1 \\ \end{pmatrix} \begin{pmatrix} I \\ Q \end{pmatrix} \]

This is a normalized version of the usual correction matrix for IQ imbalance which is easier to implement in the FPGA. Because the top-left value of this matrix depends on the magnitude error of the IQ imbalance, and the bottom row depends on the phase error, the value "A" is often referred to as the "magnitude correction value", and the value "B" is referred to as the "phase correction value". These values are however not identical to the actual phase and magnitude error caused by IQ imbalance.

The uhd_cal_tx_iq_balance and uhd_cal_rx_iq_balance tools will simply search for A and B values which minimize the error caused by IQ imbalance.

Notes:

- The API calls uhd::usrp::multi_usrp::set_rx_iq_balance() and uhd::usrp::multi_usrp::set_tx_iq_balance() take complex numbers as a correction value, which is generated by std::complex<double>(A, B). The complex math is not necessary, given the correction algorithm, but the argument was chosen due to its symmetry to the DC offset correction APIs, and also because treating the correction value as a complex number lets it be handled and stored easily and exactly as with the DC offset correction.
- This correction algorithm is suboptimal for high bandwidths, albeit better than nothing.

IQ imbalance is caused by the inphase and quadrature paths of a direct-conversion receiver being slightly different in amplitude as well as their phase not being exactly 90 degrees apart (due to analog components). It is therefore not required for superheterodyne architectures like the TwinRX.