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LMX2571 Analog FM through I2S mode

Hello,

I have been trying to modulate the device by using I2S data. But I couldn't get the results that I expected. Here is my setup:

840MHz external VCO with about 5KHz loop filter band width.

I2S one frame = 31KHz, Modulation data = 1KHz Sin wave 31KHz sampling, Deviation = 3KHz, FSK step = +17412 at 3KHz deviation   

I set modulation/FSK related registers to proper values.

I monitored the RF output signal using a spectrum analyzer and a modulation analyzer, but the RF signal is just CW, no FM modulation at all. The PLL loop filter band width should be wide enough to modulate something. I changed register values to several ways but no luck. I verified that FSK PIN mode works fine.

Am I missing something? Please advise.

NTN