Announcements

Tip / Sign in to post questions, reply, level up, and achieve exciting badges. Know more

cross mob
nDonnelly
Level 3
Level 3
10 questions asked 25 sign-ins 10 replies posted

Hi,

I have developed a board using the BGT60LTR11AIP sensor, it is operating in pulsed SPI mode. I was wondering if there are any trade-offs in increasing the lock detection time window for the PLL (REG 6[15:13]), as I my circuit only operates correctly for a time window of 4.6ns (default is 1.5ns). I am using a 38.4MHz crystal as suggested., with 12pF and 15pF.

I understand that by increasing the time window I am increasing the accepted phase difference for the comparator, but I am interested in what is causing this larger phase difference, if you have any suggestions?  When I have a lock detection time window of anything less than 4.6ns, I am seeing a pulse at around 30kHz on DIV_O and when it is operating correctly (at 4.6ns) it is 2kHz on DIV_O. 

Thanks,

Nicole

0 Likes
1 Solution
Yaran_W
Moderator
Moderator
Moderator
250 replies posted 100 solutions authored 25 likes received

Hi nDonnelly,

It’s great that you’ve followed the recommended values for the crystal and its placement. Here are some suggestions to limit the factors that could be affecting your circuit:

  1. Power Supply Fluctuations: Make sure the power supply to the radar system is stable and clean. Power supply fluctuations can affect the performance of the radar system . You can use decoupling capacitors close to the power supply pins of your sensor to reduce noise. Also, ensure that your power supply can handle the maximum current required by your circuit .

  2. Temperature Variations: Temperature changes can cause frequency drift in the crystal oscillator. Consider using temperature compensation techniques or a temperature-compensated crystal oscillator (TCXO) if temperature variations are significant in your application.

  3. Noise: High-frequency noise can affect the performance of your circuit. You can use filtering techniques to reduce noise. For example, a common-mode choke can be used to deal with conducted common-node noise .

As for the crystal oscillator circuit, here are some common issues that you might want to check:

  1. Load Capacitance: The load capacitance, which includes the capacitive load of the oscillator circuit and the stray capacitance of the PCB traces, should match the specified load capacitance of the crystal .

  2. Drive Level: The drive level, or the amount of power dissipated in the crystal, should be within the specified range for the crystal. Excessive drive level can cause frequency shifts and potentially damage the crystal .

  3. Startup Time: The oscillator circuit should provide enough gain to start the oscillations within the specified startup time .

  4. Spurious Modes: Crystals can oscillate at overtone frequencies or spurious frequencies in addition to their fundamental frequency. Make sure your circuit is not causing the crystal to oscillate at an undesired frequency .

BR

Yaran

View solution in original post

0 Likes
3 Replies
Yaran_W
Moderator
Moderator
Moderator
250 replies posted 100 solutions authored 25 likes received

Hi nDonnelly,

The BGT60LTR11AIP sensor is a fully integrated millimeter wave Doppler motion sensor with antenna in package . The lock detection time window for the PLL (Phase-Locked Loop) is configurable and can be set to different values .

Increasing the lock detection time window means that the PLL will accept a larger phase difference for the comparator. This could potentially lead to a more stable lock, but it might also increase the chance of locking onto a harmonic or spurious signal. This is because the PLL is less selective and might lock onto a signal that is close to the desired frequency but not exactly at it.

The fact that you are seeing a pulse at around 30kHz on DIV_O when the lock detection time window is less than 4.6ns, and 2kHz on DIV_O when it is operating correctly (at 4.6ns), suggests that the PLL might be having trouble locking onto the correct frequency. This could be due to a number of factors, such as noise, temperature variations, power supply fluctuations, or issues with the crystal oscillator circuit.

You mentioned that you are using a 38.4MHz crystal with 12pF and 15pF. It’s worth checking if these values are within the recommended range for the crystal and the sensor. Also, make sure that the crystal is placed as close as possible to the sensor to minimize any parasitic capacitance or inductance that could affect the oscillator frequency.

BR

Yaran

0 Likes

Hi Yaran,

 

Thanks for your quick reply!

The 12pF and 15pF are recommend in the REF BGT60LTR11AIP MO development board, and the crystal is placed right beside the sensor so that is not an issue! For the power supply I have also followed the LDO suggestions in the shield schematic. Do you have any suggestions on how to limit the factors you mentioned, and what in particular could be wrong with the crystal oscillator circuit?  I have also included an image of the schematic and the link for the REF BGT60LTR11AIP application note (REF BGT60LTR11AIP M0 (infineon.com))

nDonnelly_0-1700018766800.png

 

0 Likes
Yaran_W
Moderator
Moderator
Moderator
250 replies posted 100 solutions authored 25 likes received

Hi nDonnelly,

It’s great that you’ve followed the recommended values for the crystal and its placement. Here are some suggestions to limit the factors that could be affecting your circuit:

  1. Power Supply Fluctuations: Make sure the power supply to the radar system is stable and clean. Power supply fluctuations can affect the performance of the radar system . You can use decoupling capacitors close to the power supply pins of your sensor to reduce noise. Also, ensure that your power supply can handle the maximum current required by your circuit .

  2. Temperature Variations: Temperature changes can cause frequency drift in the crystal oscillator. Consider using temperature compensation techniques or a temperature-compensated crystal oscillator (TCXO) if temperature variations are significant in your application.

  3. Noise: High-frequency noise can affect the performance of your circuit. You can use filtering techniques to reduce noise. For example, a common-mode choke can be used to deal with conducted common-node noise .

As for the crystal oscillator circuit, here are some common issues that you might want to check:

  1. Load Capacitance: The load capacitance, which includes the capacitive load of the oscillator circuit and the stray capacitance of the PCB traces, should match the specified load capacitance of the crystal .

  2. Drive Level: The drive level, or the amount of power dissipated in the crystal, should be within the specified range for the crystal. Excessive drive level can cause frequency shifts and potentially damage the crystal .

  3. Startup Time: The oscillator circuit should provide enough gain to start the oscillations within the specified startup time .

  4. Spurious Modes: Crystals can oscillate at overtone frequencies or spurious frequencies in addition to their fundamental frequency. Make sure your circuit is not causing the crystal to oscillate at an undesired frequency .

BR

Yaran

0 Likes