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I am using an SX1272 and am having trouble locating viable distributors of any of the 32 MHz crystals recommended by Semtech, listed in the Crystal Oscillator Guidance documentation. I also can't find distributors for ant of the suggested TCXOs.

Thus, I am looking for alternative parts, with not much more success. The aforementioned documentation specifies that the ESR should be a maximum of 40 ohms, the shunt capacitance a maximum of 7 pF and the load capacitance a maximum of 12 pF. What I am confused about is that the frequency can be anything from 26 MHz to 32 MHz. How can this be? I haven't seen any registers in the device to specify the frequency. It seems that the suggested crystals are 32 MHz, but I can't find any 32 MHz crystals online that have <=40 ohm ESR and all of the other requirements as well.

I may switch to just using an RFM95W module instead; I thought that using the SX1272 by itself would have been a useful exercise.

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  • \$\begingroup\$ I thought that using the SX1272 by itself would have been a useful exercise An exercise in finding parts that is. Unless you're going to mass-produce a product using the SX1272 I see no reason why you should not use a ready made module. 32MHz crystals do exist, just make sure that they are not overtone crystals. Also in practice the ESR is usually not much of a concern. If you insist on the SX1272 contact Semtech for a solution. If not, just go for the module and save some time. \$\endgroup\$ – Bimpelrekkie Oct 22 '17 at 20:35
  • \$\begingroup\$ What I am confused about is that the frequency can be anything from 26 MHz to 32 MHz. How can this be? This can be done using a PLL. There will be a PLL anyway for generating the RF frequency. It is possible that there is no direct setting for the XO clock but that the some other settings depend on the actual XO frequency. \$\endgroup\$ – Bimpelrekkie Oct 22 '17 at 20:38
  • \$\begingroup\$ Use a module. Semtech licensed the Silicon to a bunch of the big boys for a reason. \$\endgroup\$ – Matt Young Oct 23 '17 at 5:12
  • \$\begingroup\$ If you do use a ready made module, be careful selecting the board. Many of the cheap modules available do not use TCXOs and are not capable of operating with a spreading factor of 6 (highest throughput mode). \$\endgroup\$ – Dean Franks Oct 23 '17 at 5:17
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If you use a TXCO, you only need to worry about the output swing (max 1.8V peak-to-peak) and frequency stability. The load and shunt capacitance and ESR are only relevant for a crystal. With a TXCO, you will take a single ended output of the oscillator and drive XTA, XTB remains unconnected.

Frequency is set by the Frf register which is in Fosc/2^19 units allowing for the use of a range of crystals/oscillators. If you are using prebuilt libraries, some of them assume a 32Mhz crystal.

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You need to contact a crystal manufacturer, Fox Technologies is a domestic one, and have them custom build the crystal. That's the deal. For boards I build, usually I custom order crystals. If they are not on Digi or Mouser, you gotta get them custom.

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