The KW41Z has support for an external 26 MHz or 32 MHz reference oscillator. This oscillator is used, among other things, as the clock for the RF operation. This means that the oscillator plays an important role in the RF operation and must be tuned properly to meet wireless protocol standards. The KW41Z has adjustable internal load capacitors to support crystals with different load capacitance needs. For proper oscillator function, it is important that these load capacitors be adjusted such that the oscillator frequency is as close to the center frequency of the connected crystal (either 26 MHz or 32 MHz in this case). The load capacitance is adjusted via the BB_XTAL_TRIM bit field in the ANA_TRIM register of the Radio block.
The KW41Z comes preprogrammed with a default load capacitance value. However, since there is variance in devices due to device tolerances, the correct load capacitance should be verified by verifying that the optimal central frequency is attained. You will need a spectrum analyzer to verify the central frequency. To find the most accurate value for the load capacitance, it is recommended to use the Connectivity Test demo application. This post is aimed at showing you just how to do that.
In this case, the Agilent Technologies N9020A MXA Signal Analyzer was used to measure, configured with the following parameters:
- FREQ (central frequency): 2405 MHz (test will be conducted on channel 11)
- SPAN (x-axis): 100 KHz
- AMPTD (amplitude, y-axis): 5 dBm
To perform the test, program the KW41Z with the Connectivity Test application. The project, for both IAR and KDS, for this demo application can be found in the following folder:
<KW41Z_connSw_1.0.2_install_dir>\boards\frdmkw41z\wireless_examples\smac\connectivity_test\FreeRTOS
NOTE: If you need help programming this application onto your board, consult your Getting Started material for the SMAC applications. For the FRDM-KW41Z, it is located here.
Once the device is programmed, make sure the device is connected to a terminal application in your PC. When you start the application, you're greeted by this screen:
- Press 'ENTER' to start the application.
- Press '1' to select the continuous tests mode.
- Press '4' to start a continuous unmodulated transmission. Once the test is running, you should be able to see the unmodulated signal in the spectrum analyzer.
- Press 'd' and 'f' to change the XTAL trim value, thus changing the central frequency.
Now, considering the test in this example is being performed in 802.15.4 channel 11, the central frequency should be centered exactly in 2.405 GHz, but on this board, it is slightly above (2.4050259 GHz) by default. In order to fix this, the XTAL trim value was adjusted to a value that moves the frequency to where it should be centered.
Once the adequate XTAL trim value is found, it can be programmed to be used by default. This other post explains how to do this process.
