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TRF7970A crystal selection

Vitor Barbosa
Intellectual 495 points
Other Parts Discussed in Thread: TRF7970A

I designed a PCB that has embedded a RFID reader based on TRF7970A and an MSP430 MCU. The crystal that interfaces TRF7970A has a 3.2x2.5mm footprint and for my surprise it is being difficult find on the market crystals the characteristics required on the datasheet for this kind of footprint.

Just about ESR.

In table 5-8 of TRF7970A datasheet there are details of the minimum crystal requirements and the ESR required is 50 Ohm. In 3.2x2.5 footprint crystals I'm not finding any with such low value. Just a question before makiing mess with this: can I choose a crystal with ESR=80~100 ohm?

About the other parameters as load capacitance it is easy to adjust or compensate following the equation 1(C1 = C2 = 2 × (CL– CS))

Best regards,

Vitor Barbosa

  • 0
    TI__Guru* 91206 points

    Vitor - 

    have a look at TXC, ITTI, Crystek and ECS Crystals

    http://www.txccrystal.com/images/pdf/7a.pdf (we have used 7A-13.560MAAJ-T parts)

    http://www.itti-co.com/products.php?sid=33 we have used the 1300225-13.560MHz parts

    http://www.ecsxtal.com/store/pdf/csm_7.pdf (ECS-135.600-CD-0374)

    and http://www.digikey.com/scripts/dksearch/dksus.dll?FV=fff4000d%2Cfff8016d&k=crystek&vendor=0&mnonly=0&newproducts=0&ptm=0&fid=0&quantity=0&PV139=732

    i'm sure there are more suppliers out there, too. 

  • 0 in reply to Josh Wyatt
    Intellectual 495 points

    Hello Josh,

    I understand that the options you gave me have ESR=50 Ohm but demand a different footprint in  most of the cases, or are more difficult to get as ITTI. It's easier for me to get some units from Digi or Mouser.

    I choose this footprint (3.2x2.5 mm) because I have space constraints in the PCB board. The prototype PCBs are already manufacturing and I can't go back to redo.

    What I wan't to ask is if TRF79070A can work with a crystal with a higher ESR. Maximum ESR=50 ohm for the crystal is really an important constraint? With a crystal with ESR=60 Ohms could it work?
    Perhaps It was a design mistake but I have to turn around at this moment and I can not order another batch of prototypes.

    I hope you understand my constraints.

    BR

    Vitor Barbosa

  • 0 in reply to Vitor Barbosa
    TI__Guru* 91206 points

    Vitor - 

    the TXC crystal i pointed you to is same size, is higher ESR and seems to work just fine - sorry i was not clear on that before.

  • 0 in reply to Josh Wyatt
    Intellectual 495 points

    Thank you Josh. That's nice!

  • 0 in reply to Vitor Barbosa
    Prodigy 50 points

    IMHO,  is higher ESR - bad crystal.......

  • 0 in reply to Sergey58760
    Intellectual 495 points

    Josh, I took with a closer attention and the TXC crystal you pointed me is not the same size.

    Meanwhile ITTI sent me the datasheet of the crystal used ref 1300225-13.560MHz , this one has the same size but higher ESR. ITTI crystal 3.2x2.5mm ESR=100Ohm.

    One other doubt raised: in 1300225-13.560MHz datasheet that was sent to me CLoad=10pF, and according to the formula the capacitors used for the resonant circuit should be C=2(CLoad-CS) =11pF. Well on BOM of TRF7960ATB, where this crystal is used the capacitors used in the resonant circuit with crystal are C1=C2=27pF.

    Reversing C=27pF shoul give CLoad=18pF.

    Three things can happen:

    1. ITTI datasheet is wrong ( this is difficult for me to believe...)

    2. ITTI datasheet is OK and the resonant circuit can work with different capacitor values specifified by the formula.

    3. ITTI datasheet is OK but the TRF7960ATB BOM have a mistake on the resonant capacitors values used with crystal (C1 and C2)

    Filtering, I will trust this formula, as the other crystals obey to this, as all of them have CLoad=18pF.

    BR

    Vitor Barbosa