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Thinking of cheap(er) consumer wireless products. Modulator aside, how is the RF carrier signal generated? Do they use really small capacitances/inductances on a chip die to make an RLC (say, Colpitts-like) oscillator? Phase shift? Do they use a DAC/R-network to synthetize the signal (with potentially added harmonics)? Or do they do it some other way?

Thanks!

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  • \$\begingroup\$ You'd have to look at some specific products to be sure but I believe most such devices today use a crystal-controlled oscillator that then is up-scaled using a frequency multiplier/PLL to get to the desired frequency. \$\endgroup\$ – jwh20 Apr 15 at 19:38
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Low Frequency crystal frequency feeding a phase locked loop integrated into the RFIC. An LC oscillator (Colpitts oscillators are a good example) is used to provide the on-channel (900MHz/1800MHz etc) RF carrier. All of these low cost/high volume systems use Zero IF Architectures, where the local oscillator is at the channel frequency.

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The carrier is generated by:

  1. Cheap quartz crystal from 26 to 40 MHz and +/- 10 ppm frequency deviation

  2. An internal PLL that pulls the frequency up.

There's no need for using a more precise TXCO with +/- 1 ppm frequency deviation because channels are pretty wide in frequency.

Harmonic filters are mandatory.


To answer your question:

The oscillator is a quartz crystal based CMOS oscillator.

The output is a square wave with a lot of harmonics.

The oscillator output is then brought to the input of the internal PLL.

The technological process is usually the well known RF CMOS.

Some, more expansive, integrated radio circuits are made in SiGe.

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  • \$\begingroup\$ Thank you! Would that PLL be analog or do they apply some filtering to mitigate the higher order harmonics? \$\endgroup\$ – user3765743 Apr 15 at 21:03
  • \$\begingroup\$ @user3765743 For many mixer topologies, filtering is not useful or even problematic; for example the noise characteristics of some passive mixers are best when the LO waves switch rapidly and are near-square. \$\endgroup\$ – nanofarad Apr 15 at 21:12
  • \$\begingroup\$ Would the square wave be fed directly to the antenna? \$\endgroup\$ – user3765743 Apr 15 at 21:26
  • \$\begingroup\$ No, the square wave goes to the PLL. Afterwards the signal goes to the Power Amplifier circuitry, then it's filtered to remove harmonics and finally it goes to the antenna. \$\endgroup\$ – Enrico Migliore Apr 16 at 4:41
  • \$\begingroup\$ Got it. So the oscillator is rectangular and the result gets filtered into a more sinusoidal output prior to being fed to the antenna. Is this understanding of your answer correct? \$\endgroup\$ – user3765743 Apr 16 at 20:13

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