Wireless communication systems require several different components or parts to achieve reliable systems. Components like the antenna, radio and XTAL are all key elements in wireless communication. Here however, the XTAL will be discussed.
In the Kinetis W series, for example, the XTAL used for wireless operation is usually the oscillator also used as a core clock. Now, while this external oscillator is connected to the MCU, it is also connected to an internal programmable capacitor bank. What is the purpose of these capacitor banks? To allow frequency trimming.
And why would you want to trim the frequency provided by this oscillator? Well, to properly adjust the central frequency to where it should be operating. This option exists because not every design is going to be the same: not the same PCB, not the same components, not the same manufacturing process. Thus, having the option to adjust the frequency provided by the external oscillator allows to any possible device to operate under the same conditions is essential.
Let’s say your design is using a 32 MHz external oscillator, but because of the conditions of your whole design, the operating frequency ends up being slightly different. Now, if this design transmits over the air through 802.15.4, there could be some consequences to this slight shift in frequency. This capture shows a transmission made without being centered in the desired channel.
This signal should be centered exactly on 2405 MHz, as specified by IEEE 802.15.4 channel 11.
As you may see, in this case the frequency is actually centered on 2405.0259 MHz.
Trimming these capacitors to change the frequency obtained from the oscillator can help to adjust error. In this case, the frequency was adjusted so that it was centered in the central frequency of the desired channel, to prevent any possible mistakes while transmitting to other devices.
Once the XTAL is trimmed, the signal is effectively centered on 802.15.4 channel 11's frequency, 2405 MHz.
Both transmit and receive are affected by incorrect frequency trim. Receiver performance is degraded when either (or both) of the transmitting or receiving stations have a frequency offset. And if both transmitting and receiving stations have frequency offsets in opposite directions the result is the receiver experiences the sum of the frequency offsets.
Now, when trimming the frequency of a design, there are two possibilities:
- That the board layout design, board manufacturing and component selection have repeatable values of resistance, capacitance and inductance, resulting in a stable XTAL trim – The components and manufacturing process of the board are reliable enough, allowing you to characterize the XTAL trim during the system development and then use it every board during production.
- That the design and component selection do not result in a stable XTAL trim – If there is considerable variation between different boards of the same design or components used in the board manufacturing, you would need to implement a XTAL trim procedure during the production process, and somehow program that trim value into the device's NVM.
For evaluation purposes, a manual adjustment could be done to a single device, modifying the corresponding XTAL trim register, and then including said adjustment in the evaluation application.