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I know of a few ways to generate a sine wave; Op-amps, 555, LC, crystals, or microcontrollers. But none of those would really work for a frequency as high as 900 MHz. I have heard of phase lock loops, but I don't know how to use one. What's the highest factor I could multiply my reference frequency by and still have it end up with the expected output frequency, and how do I divide the frequency leaving the VCO? Is a VCO part of the PLL or are they separate components? Will my output be a sine wave if my reference frequency is a sine wave, or do these only output square waves?

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  • \$\begingroup\$ A PLL locks a high frequency to a low frequency reference. It is usefull if you need a precise high frequency. I do not think you need a PLL. You can make a simple 900 MHz oscillator based on a single transistor. But this is RF design, you must know what you're doing to be succesfull. So study how others make 900 MHz oscillators and try to do the same. \$\endgroup\$ – Bimpelrekkie Feb 26 '16 at 9:33
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    \$\begingroup\$ I would like to attempt to make a radio transmitter in the ISM band. It's not vital to have a specific frequency as long as it wont drift around. Don't worry about me failing, that's mostly what I expect from a first attempt anyway \$\endgroup\$ – griffin175 Feb 26 '16 at 9:37
  • \$\begingroup\$ There are PLL ICs that can make a 900 MHz sinewave. Analog Devices and Texas Instruments make them. These are complex ICs, if you needed these then you would know how to use them. If you want to start with PLLs, get a 4046 PLL-IC. It's very cheap and easy to use and teaches you the basics of PLLs. It can only go up to a few MHz though. \$\endgroup\$ – Bimpelrekkie Feb 26 '16 at 9:38
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    \$\begingroup\$ You can make a low-drift single transistor oscillator if you use a resonator to determine the frequency. Many 433 MHz transmitters use this, see: freeinfosociety.com/electronics/schemview.php?id=1837 \$\endgroup\$ – Bimpelrekkie Feb 26 '16 at 9:41
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    \$\begingroup\$ That 902-932 band, plus I already have a giant 900 MHz antenna lying around. I think 433 MHz is known as the "family band" and I know It's very well filled in my location. I live at a giant ski resort where they use radios for everything and I'd rather not interfere. But if I need to buy a new antenna, it would be a lot easier to work with a lower frequency \$\endgroup\$ – griffin175 Feb 26 '16 at 10:16
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One approach would be to use a simple single-chip transmitter designed for the 902-928 MHz ISM band. Here is a typical circuit:

enter image description here

This chip uses a PLL and internal divider to lock to a reference crystal. Power output is +23dBm (200mW) and it does not require a uC to set it up.

There are a number of app notes and a somewhat pricey eval board available from the supplier.

In answer to your question about a minimum comparison frequency for a PLL, there is no minimum, but the loop filter frequency and the lock-in time will increase proportionally to the inverse of the comparison frequency (all other things being equal), so if you compare at 100Hz vs. 10MHz it will take 100,000 times as long to lock in and stabilize.

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