Hi all,
We're in the process of having a FCC certification for our board which has a Xemics 1203 RF transceiver.
The problem is that our board has too much spurious emission around the desired signal frequency. FCC certification requires that all in band signal (within 902-928 MHz) has 20 dB lower than the carrier frequency. Out-of-band emission must be at -40 dB maximum.
This spurious emission occurs when we switch the Xemics chip from standby to transmitter mode - Semtech recommends to go from sleep to standby mode, then to transmitter mode. In continuous transmitter mode, the modulated signal is almost perfect, so it is a transition problem. I am not sure if it is a software problem or a hardware problem. We believe we follow the right code procedure recommended by Semtech when switching from standby to transmitter mode.
Here are more details:
The spurious emission was measured by a spectrum analyser around the desired signal with a long sweep time (120 s) with a span of 2.5MHz and 50 MHz. The data rate use is 38.4 kbps and the frequency deviation is 55KHz. I have changed several parameters such as those related to pre- filtering, frequency deviation, and bit rate, without success.
The devices are programmed to wake up every half a second to transmit a beacon and go back to sleep.
Has anyone have this kind of problem?
Thank you very much.