QSG175: Silicon Labs Direction-Finding Solution Quick-Start Guide
Real-Time Location (RTL) Library
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4.4.1 Description of Operation
The IQ sample quality analysis tool may be used to further investigate what could be the source of observed problems in the AoA/AoD
angle values. If the calculated angles seem to be good and the position values calculated from these are within the expected range,
investigating the quality analysis figures may not be of use.
The IQ sample quality analysis measures a few statistical key figures from the samples and presents them per antenna element level.
These include:
• Antenna’s signal level (in dB), average over snapshots in the same radio packet
• Antenna’s signal to noise ratio (SNR), noise measured as a level variation between antenna’s samples in the packet
• Phase value: antenna’s unrotated average phase value, in radians (relative to the delay between antenna elements in the array)
• Phase jitter: antenna’s unrotated phase variation, in radians
A few common, or not antenna related, values are included for a packet:
• The radio channel the packet was sent on
• Apparent supplemental tone frequency, in Hz
• Reference period signal to noise and distortion ratio (SNDR)
If enabled, the IQ sample quality analysis code detects some predefined conditions and raises a warning flag for each in case the limits
were exceeded. These limits are defined inside the RTL library and are subject to change without further notice. The limits are chosen
such that warning flags should not be raised in normal conditions but only occasionally. However, since every environment is different
the actual number of warnings may vary from system to system. These flags are shown to the user as bits in a bitmask output of the
overall status query function. Further information can be then queried using the detail query functions. The API allows querying details
for the latest packet received, or for the latest packet using a specified radio channel.
The following figure shows an example phase signal of a 1 x 4 antenna array. The signal is otherwise ideal, but some exaggerated error
is seen in the detected phase (=delay) values for the second antenna in the array during the snapshots 2 and 3. The amplitude graph
also resembles a step function, but unlike the phase signal, the amplitude value does not change the same way along the angle the signal
is arriving on. In real life the change from an antenna to next does not take place immediately but there is a transfer period during which
the signal settles. This is affected by the radio system and filtering.
4.4.2 Some Interpretation Examples
Here are some practical examples of what can be learned from the quality figures in different situations:
• The figures seem mostly good, but some channels have a high amount of noise.
These radio channels may be occupied by some other system.
• Some antennas show much lower amplitude levels. The amount of reduction, and also the antennas on which this is seen,
changes along the position of the followed tag.
Radio signal reflects from the surfaces of the surrounding space causing interference with the received signal (called multipath
propagation).
• Some antenna values show a lot of noise or the phase value is not changing relative to the tag movement as it does for other
antennas
The antenna board may be defective, preventing it from receiving the signal from one or more antennas.
• All the antennas’ phase values are the same
The tag is directly above the antenna board at a 90° elevation angle.
The antenna board may be defective and is not switching at all.