Oscillator - Infineon BGT60ATR24C Application Note

BGT60ATR24C ES shield
XENSIV™ 60 GHz radar system platform
Hardware description
power rails in the chip. High attenuation of voltage fluctuations in the MHz range is provided by ferrite beads.
For example, the SPI (which runs at up to 50 MHz) induces voltage fluctuations in the digital domain. These
fluctuations would then transfer into the analog domain if not for the decoupling filters incorporated. The
ferrite beads are chosen because they can handle the maximum current of the sensor (approximately 200 mA)
with a low DC resistance (below 0.25 Ω) and an elevated inductance. The high inductance figure will reduce the
cut-off frequency of the low-pass filter, thereby providing better decoupling at lower frequencies.
Figure 3 Schematics of the low-pass filters
3.3

Oscillator

Infineon's XENSIV™ BGT60ATR24C radar sensor requires an external 80 MHz oscillator with low phase jitter and
low phase noise to provide a stable system reference clock. Therefore, the BGT60ATR24C shield employs an
NDK NZ2520SHA quartz oscillator, as depicted in Figure 4. This oscillator source will output a stable 1.8 V digital
signal. The most important parameters when choosing an oscillator are phase jitter and phase noise. Other
oscillators should have similar phase jitter and phase noise to the NDK NZ2520SHA. The R6 series resistor
reduces the RF level at the sensor so that it is at the optimal range for the BGT60ATR24C. If a redesign of the
board contains a different signal source or a vastly different layout is implemented, the value of R1 (150 Ω) may
have to be adjusted. A higher resistance results in a lower signal at the radar sensor. If the signal level is too
low, the phase noise of the sensor will degrade. With a low resistance, the signal level at the sensor will be
heightened. Consequently, in the range-Doppler illustration of the radar data, a peak (or ghost target) will
appear for low distances.
Figure 4 The oscillator circuit on the BGT60ATR24C shield
For this reason, the phase noise needs to be measured as well as the radar data. This can be illustrated with a
range-Doppler plot to optimize the series resistance of the layout. The series resistance must be varied by
soldering different resistors into the circuit. An optimized series resistance will show ideal phase noise behavior
of the sensor, paired with a clean range-Doppler plot. If the phase noise behavior is non-ideal, the resistance
value must be lower. If a peak appears in the range-Doppler plot, the resistance must be higher.
Application note 6 Revision 1.10
2023-02-14