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How to generate a 50 Mhz clock for TTL circuit

Discussion in 'Electronic Design' started by Lathe_Biosas, Feb 23, 2005.

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  1. Lathe_Biosas

    Lathe_Biosas Guest

    Hi

    Could anyone share some ideas or source of information on how to build
    a circuit to generate a 50 Mhz clock for a TTL circuit?

    Best Regards
     
  2. Ken Taylor

    Ken Taylor Guest

    http://www.foxonline.com/
    etc

    (Google is your friend, as they say).

    Ken
     
  3. What do you want that's different from a modestly priced "canned"
    50MHz xtal oscillator?

    A resonator, an unbuffered single inverter and a resistor could be
    cheaper.
     
  4. Lathe_Biosas

    Lathe_Biosas Guest


    Hi,

    Thank you very much for your answer.
    I have the SG-51 and SG-531 @ 50.00 Mhz
    http://www.eea.epson.com/go/Prod_Ad...Id=EEA.QD.Crystal_Oscillators.all_oscillators

    Question:
    If I wire the terminals V_DD to 5 volts, the GND to ground then it will
    generate a clock signal on the OUT pin of 50 Mhz to make a TTL counter
    work at that frecuency? (Sorry I haven't wired because I'm afraid to
    burn it)

    Best Regards
     
  5. Yes, assuming it's designed for 5.0V (not 3.3V or whatever) and
    assuming the TTL counter can clock that fast.

    For example, an SN7490 might only be able to clock at 32MHz, a 74HC74
    has a max toggle frequency of only 25MHz @~5V over temperature etc.
    Fast "TTL" logic families can do 50MHz easily.
     
  6. Lathe_Biosas

    Lathe_Biosas Guest

    Great, thanks for the helpful information

    The TTL is a 74HC4040 on page 5 of product specification
    http://www.semiconductors.philips.com/pip/74HC4040.html
    shows +25° C @ 82 Mhz @ 4.5 Volts, so I think the chip won't run
    so hot at 50 Mhz

    Best Regards
     
  7. Lathe_Biosas

    Lathe_Biosas Guest

    Hi again

    The crystal oscillator was wired, the signal is sinusoidal.
    How do I make the signal a square pulse to clock the TTL?

    Regards
     
  8. tlbs

    tlbs Guest

    It doesn't matter that the signal looks sinusoidal. I assume the
    output oscillates between 0V and Vdd (at 50 MHz)? correct?

    The output will still cross the TTL transition voltages (or in your
    case the HCMOS transitions) at 50 MHz +/- the tolerance of the
    oscillator you purchased, with the jitter spec. of the oscillator you
    purchased.

    Your counter will run at 50 MHz.
     
  9. John Fields

    John Fields Guest

    ---
    Yes, it will, _but_ if he's using the output of the oscillator to run
    anything else in parallel with the counter, skew might kill him since
    Vth will vary from chip to chip.

    Check this out:

    http://www.semiconductors.philips.com/acrobat_download/various/HCT_USER_GUIDE.pdf
     
  10. Quite true, John but I didn't see what scope he was using to look at
    his signal. My Tek 475 at full bandwidth would give a reasonable
    display but a slower scope would round off his squarewave. I've been
    using a 32 MHz canned oscillator that makes quite nice squares,
    considering the 475 can only show the harmonics up to 250 MHz.
    GG
     
  11. The crystal oscillator was wired, the signal is sinusoidal.

    How do you know? To realy appreciate the squareness of a 50 MHz signal
    you need at least a 200 MHz scope, more is better.



    Wouter van Ooijen

    -- ------------------------------------
    http://www.voti.nl
    Webshop for PICs and other electronics
    http://www.voti.nl/hvu
    Teacher electronics and informatics
     
  12. Lathe_Biosas

    Lathe_Biosas Guest

    Setup: FLUKE 192B Scopemeter 60 Mhz 500 MS/s

    The output oscillates as follows:

    Scope: Channel A 200 mV - 10 ns
    Peak Max: 0.63 V
    Peak Min: -0.24 V
    Peak-Peak: 0.84 ~ 0.91 V

    Multimeter: V_out= 2.476 Volts
    Great. I was on the verge of buying an LMV7219
    http://www.national.com/pf/LM/LMV7219.html to build
    the circuit to get the square wave.

    Thank you and best regards
     
  13. Lathe_Biosas

    Lathe_Biosas Guest

    Hi,

    The clock signal is only for the counter, the counter will feed other
    TTLs

    Regards
     
  14. Lathe_Biosas

    Lathe_Biosas Guest

    Setup:
    FLUKE 192B Scopemeter 60 Mhz 500 MS/s
    Channel A = 200 mV - 10ns
     
  15. Lathe_Biosas

    Lathe_Biosas Guest

    That means that the signal is square but as I have only a 60Mhz Scope,
    it is only possible to see a sinosoidal signal?
     
  16. page 30 sect 7.4
    "it is good practice to
    terminate all unused LSTTL inputs to VCC via a 1.2 kR
    resistor. Inputs should not be connected directly to GND or
    VCC, and they should not be left floating."

    I never knew that before.



    martin

    "An eye for an eye makes the whole world blind"
    Gandhi
     
  17. Rob Gaddi

    Rob Gaddi Guest

    Not only possible, but pretty near assured.
     
  18. That means that the signal is square but as I have only a 60Mhz Scope,
    Of course. Basic signal theory.



    Wouter van Ooijen

    -- ------------------------------------
    http://www.voti.nl
    Webshop for PICs and other electronics
    http://www.voti.nl/hvu
    Teacher electronics and informatics
     
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