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 and KBA article EM35xx-24MHz Crystal Selection for additional information.