LMX2571: LMX2571 about the frequency question

Hi team,

Would you support this question? The customer is waiting for my reply.

Hi Mickey,

I assume you are going to do 2FSK. In FSK SPI FAST mode, all you need to do is write to this register the equivalent FSK step (Equation 4 to 7 in the datasheet) for the two different FSK deviations.
For example, if the FSK steps for a +/-1000 Hz deviation are 4567 and 60969. Then when you write 4567 to this register, then the output frequency will be fo + 1000Hz. Similarly, if you write 60969 to this register, the output frequency will be fo - 1000Hz.
Please refer to section 8 in the datasheet for details.

Hi Noel,

Thanks for your support.
Q1: To enable the FSK SPI FAST mode, the customer needs to set FSK_MODE_SEL1 = 1, FSK_MODE_SEL0 = 1, FSK_EN_F1 = 1.

If set FSK_EN_F1=1, then the customer needs to set the register R8 bit[10]=1. If set FSK_MODE_SEL1 = 1 and FSK_MODE_SEL0 = 1,

then the customer needs to set R34 bit[1:0]=11. Is my understanding correct?

Q2: Now the customer needs to do 2FSK. If the customer uses the FSK SPI FAST mode, does he only write the FSK step to the register R33,
not use any other register?

Q3: Equation 4 to 7 in the datasheet means need to set the frequency deviation. So, how to determine the frequency deviation?

Does the customer need a reference frequency before set the frequency deviation?

Q4: Now the customer needs to switch the two output frequencies. How to switch the two output frequencies in the FSK SPI FAST mode?

As you said, if the customer would like to switch the two output frequencies in the FSK SPI FAST mode, such as +/-1000HZ, then he needs to writes two different

FSK step values to the register R33, that is 4567 and 60969 if he switches the +/-1000HZ frequencies. Is my understanding correct?

Hi Noel,

Would you support me ?The customer is waiting for my reply. Thanks.