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CC3100 & CC3200 Frequency Tuning
Adjusting the center frequency error in SimpleLink WLAN
Why is it needed
WLAN standards for 802.11b/g specifies the maximum frequency error to be within 25ppm. Beyond this the device may have difficulty in interoperability with multiple access points. Hence it is important to restrict the frequency error to a small value and ensure that the average frequency error across multiple boards is centered around 0ppm.
The frequency at which a crystal oscillates, depends on the net capacitance load seen between the crystal pins. This capacitance includes the shunt capacitors added on the board , the PCB trace parasitic capacitance , the component pad capacitance, device pin capacitance etc. To achieve the specified frequency from the crystal (eg: 40Mhz) , the net capacitance load should exactly match the specified CL spec of the XTAL. For eg: the “Q24FA20H00396” XTAL speced for Cl of 8pF would resonate at 40Mhz accurately only when the net capacitance between the XTAL P/M pins is accurately 8pF. As the capacitance moves away from 8pF the frequency error changes (a typical plot is shown below indicates the ppm error with variation in Cl).
The CC31xx/CC32xx uses the 40Mhz XTAL to derive the RF frequency. Based on the 802.11 IEEE standard the max allowed ppm error in the channel frequency is 25 ppm (for 11b/g/n standards). This ppm error includes the offset frequency error in XTAL, the variation due to temperature and aging (all provided in the XTAL datasheet). Any frequency error caused due to capacitance load error is additional to the errors mentioned in the datasheet of the XTAL and should be minimized.
Adjusting the 40Mhz XTAL shunt caps in CC3x00/CC3x20
Based on the PCB trace capacitance and the 40 Mhz XTAL part used on final product , the shunt caps on the XTAL may need to be adjusted in order to make sure the net capacitance load on the XTAL is exactly what is required.
Adjust the values as per recommendation from crystal manufacturer. If the crystal specifies a 8pF load capacitor, then each capacitor on the crystal pin = 2x8 = 16pF as the capacitors would be in series. Out of this 16pF, account for 2 to 3pF for the board parasitic. Then the CC31xx/CC32xx device input has 5 to 6pF per pin. This calls for a total added capacitor of 7-9pF on each pin. Fine tune this based on TX output frequency. Never tune the crystal by monitoring the crystal itself. Loading the crystal with a oscilloscope probe will reduce the frequency of oscillation
Procedure to measure the ppm error:
- Connect the device using Radio Tool (CC3100/CC3200 Radio Tool shown). Transmit a unmodulated sin wave using TX CW mode. Select the “Tone” as 1.
- This would transmit a RF sine wave at Channel frequency + 312.5 Khz. Eg: On CH6 the sine wave would be at 2437.3125 Mhz.
- Measure the actual frequency on a spectrum analyzer and the delta from the expected frequency gives the frequency error. Eg: if the measured frequency is 2437.3225 Mhz then the frequency error = 2437.3225 – 2437.3125 = 0.01Mhz (10 Khz). For accurate measurement set a low span in the spectrum analyzer , like 100Khz and RBW of 100Hz.
- To convert this to ppm -> (Freq error/expected frequency) * 10^6 -> (0.01/2437.3125) * 10^6 = 4.1 ppm.
- Adjust the onboard shunt cap to bring the frequency error to as low value as possible (at 25C). A positive ppm error (larger frequency) indicates you need to increase the cap and negative ppm error indicates you need to decrease the cap. Note that the two shunt caps effectively come is series. For fine adjustments the two capacitors can have different values.