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calculating carrier frequncy for wireless module RF96

sairfan1

Full Member level 1
I was reading datasheet for module RF96 (datasheet attached), by reading the datasheet I understand that we have to set register values to set carrier frequency as descrubed on the page 103, this topic is also described in some other sections of the datasheet like Pages, 13, 53, 77, 88

Formula described to calculate configuration value for registers RefFrLsb, RefFrMid, RefFrMsb is as below (ref page 103)
fRF = (XOSC) * Fr f / 2^19

Where they did not described what are these variables RF and Fr, through different tables in the datasheet I took the idea and tried to calculate the value as they described in the example:
OSC = 32MHz
Frequncy = 434 MHz

fRF = 32,000,000 * 434,000,000 / 524288

fRF = 13,888,000,000,000,000 / 524288

fRF = 26,489,257,812.5

which is 0x6 2AE1 FF54 and it does not match with the example in datasheet calculated value 0x6C 8000

I'm looking for help where am I making mistake. Thanks

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• RFM95_96_97_98W.pdf
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Solution
I was reading datasheet for module RF96 (datasheet attached), by reading the datasheet I understand that we have to set register values to set carrier frequency as descrubed on the page 103, this topic is also described in some other sections of the datasheet like Pages, 13, 53, 77, 88

Formula described to calculate configuration value for registers RefFrLsb, RefFrMid, RefFrMsb is as below (ref page 103)
fRF = (XOSC) * Fr f / 2^19

Where they did not described what are these variables RF and Fr, through different tables in the datasheet I took the idea and tried to calculate the value as they described in the example:
OSC = 32MHz
Frequncy = 434 MHz

fRF = 32,000,000 * 434,000,000 / 524288

fRF = 13,888,000,000,000,000 / 524288

fRF...
I was reading datasheet for module RF96 (datasheet attached), by reading the datasheet I understand that we have to set register values to set carrier frequency as descrubed on the page 103, this topic is also described in some other sections of the datasheet like Pages, 13, 53, 77, 88

Formula described to calculate configuration value for registers RefFrLsb, RefFrMid, RefFrMsb is as below (ref page 103)
fRF = (XOSC) * Fr f / 2^19

Where they did not described what are these variables RF and Fr, through different tables in the datasheet I took the idea and tried to calculate the value as they described in the example:
OSC = 32MHz
Frequncy = 434 MHz

fRF = 32,000,000 * 434,000,000 / 524288

fRF = 13,888,000,000,000,000 / 524288

fRF = 26,489,257,812.5

which is 0x6 2AE1 FF54 and it does not match with the example in datasheet calculated value 0x6C 8000

I'm looking for help where am I making mistake. Thanks
Just had a go at it:

dec2hex(434000000*2^19/32000000) = 0x6c8000

Last edited:

But you are not using the formula as sown in the datasheet? is there some math rule am I missing? I wanted to learn what made you understand to modify the formula and its till valid, and why i was not able to calculate by following what is said in the datasheet

fRF = (XOSC) * Fr f / 2^19

But you are not using the formula as sown in the datasheet? is there some math rule am I missing? I wanted to learn what made you understand to modify the formula and its till valid, and why i was not able to calculate by following what is said in the datasheet

fRF = (XOSC) * Fr f / 2^19

View attachment 185545
page 32:
FRF = Fstep × Frf(23,0)
and Fstep = Fxosc/2^19

Hence FRF = Fxosc/2^19 x Frf
Hence Frf(i.e. register value) = FRF*2^19/Fosc

You are confusing Frf (register value) and fRF (frequency) in post #1.