what is the nRF52840 in band image frequency for SIG RX C/I test and why?

[Frank Nie]

Hi Sir,

What's the nRF52840 in band image frequency for SIG RX C/I test? Learned from previous Q&A, looks like nRF52 series image frequency is fc -2MHz for 1Mbps and fc-4MHz for 2Mbps, suppose nRF52840 also follow this rule?

if so, why does this rule base on?

I leaned that the image frequency = 2fLO - fRX, it is related to the LO frequency, how to know the LO frequency at different fRX channels, such as my product is integrated one external 32MHz crystal and one 32.768KHz crystal. Is it the 75 times frequency of the 32 MHz (=2400MHz)? then how about other channels?

I am really confused many days for this question, looking forward for your reply, thanks so much!

BR

Frank

[ketiljo]

Hi Frank

The LO is always at fRX +  2 MHz in 2 Mbps mode on all nRF52 radios. So you will always find the image frequency at fRX - 4 MHz.

[Frank Nie]

Hi Ketiljo,

thanks for your kindly reply, 

for "The LO is always at fRX +  MHz in 2 Mbps mode on all nRF52 radios" suppose should be" fRX - 2MHz in 2 Mbps"?

fIF (intermediate frequency) is 2MHz in 2 Mbps mode, and fIF is 1MHz in 1 Mbps mode, so image frequency is fRX - 2MHz in 1 Mbps, below is my understanding about the relation among fRX,fIF & fIM, pls correct me if I'm wrong, thanks!

[Frank Nie]

so, the nRF52 radio receiver is superheterodyne low-side injection(fLO<fRX); and the nRF51 radio is superheterodyne high-side injection(fLO>fRX),that's why nRF51 radio image frequency is fRX+4MHz, nRF52 radio is fRX -2 MHz/4MHz for 1Mbps/2Mbps. right?

[ketiljo]

Hi Frank

Sorry, there was a typo. The LO is at fRX - 2 MHz in 2 Mbps mode. So low side injection, direct conversion with only one IF at 2 MHz.

nRF51 is a a superheterodyne receiver with high side injection and 2 IF frequencies. 

[Frank Nie]

Hi Ketiljo,

Thanks for your confirm, I'm already fully clear for this question, thanks again!