Alternative for weird crystal frequency

Discussion in 'The Projects Forum' started by cnavast, Mar 21, 2015.

  1. cnavast

    Thread Starter New Member

    Mar 21, 2015
    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. In theory, the output frequency is 32*fref according to the data sheet, so if I use 27.12 the output will be 867,84MHz, which is a difference of 0.46MHz. The question is: will I need to change the output matching filter, or with such a "little" difference won't be necessary?

    - 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: )

    Last edited: Mar 21, 2015
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
  3. DickCappels


    Aug 21, 2008
    You understand the relationship between crystal frequency and transmit frequency correctly. A shift of half a MHz would not require changes in the output matching network/filter (check table 6.2 and you will see how small the changes would be by shifting the frequency up from 868 MHz to 915 MHz. If you are going to use it with a crystal controlled receiver, then it might pay to adjust the crystal frequency of the receiver; if its a regenerative receiver, it won't know the difference.
    cnavast likes this.
  4. Papabravo


    Feb 24, 2006
    Normally crystals are custom made for this kind of application, not purchased from distributors. In addition to the crystal frequency there are a host of other parameters that you need to account for. Are you sure that the 27.12 MHz. crystal has the correct load capacitance and is pullable. The last feature is what allows a PLL to work and it involves changing or "pulling" the crystals frequency by upto 175 ppm in order to maintain phase lock.

    Another question. Do you have the test equipment necessary to verify the effects of component substitutions?
  5. cnavast

    Thread Starter New Member

    Mar 21, 2015
    Thank you for your responses.

    @Papabravo Yes, I do have Agilent equipment to test the effects of substitution, and the Load Capacitance is 12pF as required in the datasheet.

    @DickCappels I guess I will now have problems with the receiver... the thing is that Melexis does not have an rx equivalent for this tx (I will always wonder why!) so I'm using an Infineon rx which uses another crystal frequency to generate the same output frequency, so I can't use the 27.12MHz for both tx and rx, so I will have to deal with this problem as well. Thank you for solving my question!
  6. DickCappels


    Aug 21, 2008
    You might be interested in this tiny frequency synthesizer -you could probably use it to adapt the receiver to the transmitter or the other way around if the approach turns out ot be practical for your application.
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
    why not use a chanel 15 cb transmit crystal? at 27.135 mhz it would be quite close and probably easier to obtain. then you could try differing values of small pf caps across the crystal to trim it to the proper frequency.
    BR-549 likes this.