[SOLVED] Alternative for weird crystal frequency

[Carlos Navarro Astiasarán]

Hello everyone!

I have to use Melexis TH72032 ASK Transmitter @ 868MHz and the datasheet says I need a crystal @ 27.134375MHz, but all the manufacturers I found are not producing it anymore, and the stock at mouser, farnell, digikey, arrow,... is 0. I've search for two days now and found nothing.

So I have two alternatives:

- Use another crystal at a close frequency (e.g. 27.12MHz), but I don't know how this will affect the output frequency. Any help calculating this? In the datasheet I didn't find any info. Also, would I need to change the output matching circuit? (I guess so).

- Generate somehow that frequency. I have no idea on how to do this (except by using an external signal generator, which is obviously out of the table).

It is my first pcb design and it's driving me crazy!! Any help would be really appreciated.

I'm following the test circuit given in the HT72032 datasheet (which you can find here: https://www.melexis.com/Asset/TH72032-DataSheet-DownloadLink-4807.aspx )

Thanks!
Carlos

 

[betwixt]

The device uses a PLL to multiply the frequency 32 times. (27.134375 * 32) = 868.30.

If you use a different crystal the frequency will 32 times higher than it resonates at. You should be able to buy crystals in any quantity from a crystal manufacturer, they will make them to any frequency you want although I would guess they keep 27.134375MHz as a stock item as it is commonly used.

Brian.

 

[jiripolivka]

The device uses a PLL to multiply the frequency 32 times. (27.134375 * 32) = 868.30.

If you use a different crystal the frequency will 32 times higher than it resonates at. You should be able to buy crystals in any quantity from a crystal manufacturer, they will make them to any frequency you want although I would guess they keep 27.134375MHz as a stock item as it is commonly used.

Brian.

If you refer to Melexis data sheet, you can see that the crystal is connected in series with CX1, given as 15 pF. If you use a 3-30 pF trimmer, you can adjust crystal frequency to an exact output frequency as needed.
It is a good idea to buy several crystals, possibly from different dealers, and select one for the best results.
I have built many crystal oscillators with different crystal types. Selecting the good one was always important.