Due to tolerances of on board components and PCB materials, impedances found within the RF circuit path of a given design will vary from board to board. Additionally, the tolerances of the impedance matching components themselves allows a range of impedances between parts for a given component value, with wide tolerance parts being more likely to have inconsistent tuning results. For this reason, the procedures below should be applied to a minimal sample set of at least three to five boards of the same design in order to determine the most consistent component value to use in production. Note that component type and tolerance are factors that will need to be considered for higher end designs that require more precise RF performance targets.
The RF tuning of an EM3xx design consists of two steps: transmit power output and receive sensitivity. Transmit frequency offset (crystal) tuning is recommended prior to the RF tuning.
Transmit Frequency Offset / Crystal Tuning:
For proper operation between the radios in a network, it is very important to have transmit frequency offset as small as possible. For more details refer to following KBA article: http://community.silabs.com/t5/Wireless-Knowledge-Base/EM3xx-24MHz-Crystal-Tuning/ta-p/157154
Transmit Power Output Tuning (Ceramic Balun):
In an EM3xx ceramic balun design, the main objective is to minimize the RF circuit path losses by presenting the best impedance match between the EM3xx differential port pins and the antenna. This is facilitated by adjusting the value of the tuning inductor which is placed between the differential port traces which connect the EM3xx to the ceramic balun.
To accomplish this, the DUT is first configured for maximum transmit power level by issuing nodetest command ‘settxpower 8’ and for channel 0x13 (2445MHz, middle of the spectrum) by issuing nodetest command ‘setchannel 13’. The DUT is then placed into continuous unmodulated transmit mode with nodetest command ‘txtone’. The power output is measured at the output of the harmonic filter by connecting a spectrum analyzer through a RF connector interface placed for this purpose, such as a U.FL, SMA or any other preferred RF connector. The power output is then measured for a range of inductor values. The inductor value that provides the best impedance match will result in the highest power output. The goal of the tuning is to determine this value. Note that this value will differ from design to design as it is dependent on PCB layout which is generally not the same for each design.
Once the tuning inductor value is determined, we recommend to check TX output power on the low and high channel of the IEEE 802.15.4 band to be sure that TX output power does not vary much more than 1 dB across the band.
Transmit Power Output Tuning (Front End Module):
In an EM3xx front end module (FEM) design, the transmit power output tuning is performed by finding an inductor value that provides the best impedance match between the EM3xx and the FEM. The procedure is similar to that of the ceramic balun design except that care should be taken to keep the FEM’s power amplifier (PA) in its linear output range and avoid putting the PA into a state of compression. In order to avoid this, the transmit power setting of the EM3xx needs to be set much lower than normal. For example, if the typical transmit power setting for a FEM design with a gain of 25 dB is -5dBm for a desired output of +20 dBm, then -9dBm or lower power level should be used for tuning efforts. Refer to the ceramic balun section above for nodetest commands for configuring power level, channel, and transmit tone mode. The TOKEN_MFG_PHY_CONFIG need to be properly configured to route transmit signal onto the bidirectional port for single port FEMs or onto alternate TX port for dual port FEMs. Also, the FEM control signals need to be properly configured as per the FEM control logic table. Finally, it should be noted that the VDD_PADS supply should be set to 3.6V for maximum FEM output power, or to the maximum expected VDD_PADS supply voltage for the design being tuned.
The power output is measured for a range of inductor values. Similar to the ceramic balun tuning, the inductor value that provides the best impedance match will result in the highest power output. The goal of the tuning is to determine this value. Note that this value will differ from design to design as it is dependent on PCB layout which is generally not the same for each design. We recommend to check the TX power output in the lower and upper channels to make sure that the power level across the band is not varying much more than 1 dB. Once the inductor value is determined, the lowest EM3xx transmit power level that gives the highest FEM power output (depending on the desired power output and regional regulatory restrictions) can be determined. This power level should then be used in the customer application for proper configuration of the PA as the most efficient power level setting resulting in lower spurious emissions and current consumption. Once the appropriate EM35xx TX power setting is determined, we recommend to measure EVM of the DUT to make sure that there is no distortion resulting from compression.
Receive Sensitivity Tuning (Ceramic Balun or FEM with 1 RF Port):
Since there is only a single tuning inductor in these designs, the receive sensitivity is not tuned. The internal PA and LNA pads of the EM3xx have similar impedances. Due to it is more practical to measure the TX power output than it is to measure RX sensitivity during an impedance matching exercise, the inductor value is determined for the optimized TX power output. In this way, receiver sensitivity is optimized by default and therefore does not require further tuning.
Receive Sensitivity Tuning (FEM with 2 RF Ports):
For FEM designs with 2 RF ports, the receive port is able to be tuned separately from the transmit port. Differences in the PCB layout between the TX differential port and the RX differential port will generally require different tuning component values for each differential port due to impedance differences in the layout. The receive port tuning determines the value of tuning inductor which maximizes receive sensitivity (the minimum signal level which still achieves less than 1% packet error rate) by providing an optimized impedance match. In order to tune the receive side, the receive sensitivity should be measured with a range of inductor values in order to determine the value that maintains less than 1% packet error rate at the lowest signal level from the signal generator.
The DUT is placed in receive mode (nodetest command ‘rx’) with 1,000 or 10,000 valid packets sent from the signal generator. Normally the channel used for tuning is in the middle of the spectrum (2445MHz, in the case of ZigBee). We recommend to check the lower and upper channels before the tuning task is completed to be sure that the difference in RX sensitivity is less than 1 dB across the band. For more details on receive sensitivity testing, refer to section 2.10 Receive Sensitivity Test section in AN700: http://www.silabs.com/Support%20Documents/TechnicalDocs/AN700.pdf
For more information on EM3xx reference designs, refer to: http://www.silabs.com/products/wireless/zigbee/Pages/zigbee-reference-designs.aspx.
For information on setting the TOKEN_MFG_PHY_CONFIG, refer to:
For proper operation of the EM3xx RF block, it is very important to minimize transmit frequency offset, as well as comply with the 802.15.4 specification of +/-40ppm. The frequency offset is adjusted through the txtone (carrier tone) transmit function, as the 24MHz main clock offset directly relates to the transmitter frequency offset.
Use a spectrum analyzer to measure the frequency offset of the carrier. Set the amplitude reference level higher than the expected output power of the device in order to avoid possible damage to test equipment. For ceramic balun designs, this would be +10dBm, while for PA designs +30dBm should work fine as an initial setting and can be adjusted according to the actual PA TX power maximum capability. Configure the radio to transmit a continuous unmodulated tone at 2445MHz by issuing the commands ‘setchannel 13’ and ‘txtone’ with the nodetest application installed on the device. Measure the frequency offset by selecting the frequency counter setting on the spectrum analyzer followed by a marker peak search. To determine the PPM of the frequency offset, use the following equation:
Channel offset PPM = (expected / (measured – expected)) * 1E6
The frequency offset needs to be improved by adjusting the crystal loading caps only if it appears it will not comply with the +/- 40 PPM 802.15.4 specification. The intent is to tune the frequency to be as close to the desired frequency as possible. Refer to AN700 section 2.6, Transmit Frequency test http://www.silabs.com/Support%20Documents/TechnicalDocs/AN700.pdf and KBA article EM35xx-24MHz Crystal Selection http://community.silabs.com/t5/Wireless-Knowledge-Base/EM3xx-24-MHz-Crystal-Selection/ta-p/144602 for additional information.