Hi,
I designed a tiny board based on the CC1200 transceiver.
The BOM, the PCB stack-up and the PCB layout are identical to the evaluation board.
When transmitting at 868 MHz we are measuring a spurious emission at 828 MHz.
The power level of the spurious emission is -54 dBm which is very close to the EN 300 220 limit of - 55 dBm.
TRANSMISSION DATA:
Carrier frequency: 868 MHz
Output power: 14 dBm
Modulation: 2-FSK
Frequency deviation: 4 KHz
Bit rate: 1.2 kbps
Mode: High performance
TCXO: CTS 520L15IA40M0000 - Frequency 40 MHz - Frequency tolerance +/- 2 ppm
Power supply: +3.3 V - LM317
Power supply filter - LC as in the evaluation board
INSTRUMENTATION:
Spectrum analyzer: HP 8566 B - Calibrated
RBW Filter: 100 KHz - As required by EN 300 220
10 dB attenuator - Non present even though it required by EN 300 220
SPURIOUS EMISSION:
Frequency: 828 MHz
Power level: -54 dBm
EN 300 220 limit: - 55 dBm.
I'm very close to the EN 300 220 limit of - 55 dBm.
Question 1
How can I fix the problem?
Question 2
With a 3 GHz scope I noticed that the +3.3 V supply line, during transmissions, has a residual ripple of 300 mV at a frequency of 868 MHz
Is it a power supply problem?
Question 2
868 - 828 = 40 MHz
It seems it is a problem related to the TCXO which works at 40 MHz.
Am I right?
Best Regards,
Enrico Migliore
