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Crystal Oscillator Circuit

Sailesh Vempaty
Prodigy 165 points
Other Parts Discussed in Thread: BUF634, BUF602

Hello,

I need to produce a 3.3Vpp [0 to 3.3V] square wave/clock signal at 60MHz and I plan to use the MXO45HSTLV-3C-60M0000-ND oscillator:


1) I need to know whether this oscillator will allow me to generate clean square-waves at 60MHz.

2) I have never really worked with this type of oscillator before so could someone provide me with a circuit which shows how to connect and apply this device?A circuit which will allow me to generate square waves to my specifications?

3) I need the generated square waves to drive a 50ohm load so will require approximately 65mA of output current. For this I plan to feed the generated square wave from the oscillator through a high speed, high current buffer such as the BUF634. Will this two stage set-up work?

BUF634:


 

  • 0
    TI__Genius 13935 points

    Hello Sailesh,

    I can't help you with the crystal oscillator, I am not familiar with this part and it is not sold by TI. 

    Two stage clock systems are common, especially using a buffer for a crystal oscillator to isolate the load from the oscillator.

    Generally for a square wave you want at least 7 harmonics for a fast edge (5 harmonics may be OK).  The BUF634 has up to 180 MHz of bandwidth and 2000 V/us slew rate.  This may not be enough to preserve a square clock. 

    You may want to consider an amplifier with at least 300MHz small signal bandwidth (harmonics will be much smaller amplitude than the fundamental so you can use SSBW for the harmonics). 

    Regards,

    Loren

  • 0 in reply to Loren Siebert 1
    TI__Genius 13935 points
    Hello Sailesh,

    You may want to look at the BUF602 for this application.

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    Prodigy 165 points

    Hi Loren,

    Thank you for your reply. The BUF602 buffer does look more appropriate for my application. However, are you able to clarify what the Vref pin on the device would be used for?

    Regards,
    Sailesh

  • 0 in reply to Sailesh Vempaty
    TI__Genius 13935 points
    Hi Sailesh,

    If you look at Figure 32 on page 13 of the datasheet there is a typical application for the Vref pin.

    The Vref pin is set to exactly 1/2 of the supply voltage by internal resistors and a buffer. This pin can be used to establish a DC operating point when using the buffer in an AC coupled application.

    Figure 34 on page 14 shows another use for the Vref pin. In this case the Vref voltage is buffered by the main amplifier to form a low impedance one half supply voltage.

    Best Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    Prodigy 165 points

    Hi Loren,

    So if I simply wanted to use this buffer to increase the output drive current capability of my square wave generator I would ignore this pin? My sole purpose for using the buffer is the following:

    1st stage: I will be generating a 50MHz, 0 to 3.3V, 50% duty cycle square wave clock signal using some sort of crystal oscillator and schmitt trigger device perhaps. The problem here is that this 1st stage alone won't be able to drive a 50 ohm load as it can only supply a limited amount of output drive current.

    2nd Stage: To solve the output drive current problem I was thinking of passing the output of the 1st stage through a high speed, high current buffer such as the BUF602 device which will simply allow me to produce the same 50MHz, 0 to 3.3V, 50% duty cycle square wave clock signal as the 1st stage but also now allow me to drive a 50 ohm load due to the buffer being able to supply a higher output drive current.

    Keeping in mind the considerations of what I would need from the second stage buffer, how would I wire up the BUF602 for my application? By this I mean would I need to supply +/- 5V to the supply rails or would a single 3.3V to the + rail and GND to the - rail be sufficient? What exactly would I do with the Vref pin?

    Regards,
    Sailesh

  • 0 in reply to Sailesh Vempaty
    TI__Genius 13935 points

    Hello Sailesh,

    You can ignore the ref pin.  You want a 0V to 3.3V square wave so you will need at least -1V on the negative supply and at least +4.3V on the positive supply.  I would recommend using the +- 5V supplies.  This is because the BUF602 is not a rail to rail amplifier. 

    Your basic idea is sound.  However, the BUF602 is a linear amplifier.  It was not designed to be a clock buffer.  This does not mean it won't work, it is just a non standard application.

    Texas Instruments does have a line of timing and interface products.  It is possible that we have a dedicated clock buffer that would work better.  I am not familiar with that product line.  If you are interested in doing some more research you may want to do a quick search for a clock buffer.

    Regards,

    Loren