Transmitter's Output Spectrum of the BLE chipset CC2540
The transmitter of CC2540 was set to output a continuous modulated carrier, using the following codes in the firmware: HCI_EXT_ModemTestTxCmd( HCI_EXT_TX_MODULATED_CARRIER, 0).
Below is a spectrum plot of the modulated carrier, measured at the single-end output port of the balun transformer which was connected to the CC2540 balanced RF pins.
According to the BLE RF specs, the bandwidth-bit period product BT is 0.5, which is same as the standard Bluetooth; however, the modulation index is between 0.45 and 0.55 for BLE, while the standard Bluetooth's is between 0.28 to 0.35. The wider modulation index may widen the RF spectrum, but it should meet the 20-dB BW spec of +/- 500 kHz from the center frequency. However, my measurements showed that the CC2540 failed the 20-dB BW requirement.
I have the following questions, and please help me out if you can.
1. Do we need to calibrate the RF modulator inside the CC2540? The spectrum re-growth shows imbalanced IMD products on both sides.
2. whic HCI command/function should I use to set the CC2540 into a direct test mode so I can measure the modulation index?
3. In addition, which HCI command should I use to reduce the CC2540 RF output power?
Thank you in advance.
Minh To