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While messing with Arduino I came across this wireless transceiver. As a newbie in all this stuff, it's written that it operates at 2.4 GHz, so the transmitter crystal and receiver crystal must operate at 2.4 GHz, right? From the photo I can see that the crystal is 16 MHz which is 0.016 GHz. Probably I'm not getting it right, where am I mistaken?

enter image description here NRF24L01+ 2.4GHz Antenna Wireless Transceiver Module

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3 Answers 3

up vote 7 down vote accepted

Most wireless transmitters (of this type) use a crystal and a phase-locked-loop (PLL) to generate much higher frequencies. See this article: -

enter image description here

Frequencies can be produced that are N*16MHz. This means, that for a 2.448GHz output, N equals 153. If N = 154, the output is 2.464GHz i.e. 16MHz higher.

There are other techniques that can do this but the PLL is most likely. Here's what the device says about itself: -

enter image description here

The type of PLL used is called "fractional N" because it has the ability to produce higher frequencies that are spaced at a fraction of 16MHz.

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+1 for fractional N –  Spehro Pefhany Mar 14 at 20:23
    
These PLLs are always confusing to me. I get how it creates a multiple with the divide by n part, but the VCO itself still needs to be able to create the desired frequency on its own. So which part of the device in this post is actually creating the 2.4 GHz? –  krb686 Mar 15 at 0:55
1  
The VCO is creating the hi frequency. It gets divided down to about 16MHz. The divided frequency is compared to "true" 16MHz and an error voltage representing the difference frequency is produced. This nudges the VCO in the right direction to be an exact multiple of 16MHz and the process repeats until the error is virtually zero. –  Andy aka Mar 15 at 1:19

They're probably using a phase-locked loop inside the chip to generate the RF.

enter image description here

The circuit shown produces an output \$F_O\$ that is N times the input frequency \$F_I\$. It does that by comparing the divided value of the output frequency with the input frequency and controlling the Voltage Controlled Oscillator (VCO) to equalize the two frequencies.

This allows the use of a convenient frequency for the crystal, and provides a way to tune the RF frequency by changing the divisor N.

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Standard PLL with pre-scaler & divided by N on the VCO inside, while the 16MHz XO is scaled down to 1MHz giving 1MHz resolution and 1 MHz Bandwidth. The FSK of the nRF24L01 transceiver chip is programmable for use in ISM Band up to 2Mbps.

http://www.nordicsemi.com/eng/nordic/download_resource/8041/1/16109348 < CHip specs show functional interface not Theory of Operation, which the other answers have shown.

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