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AD725 oscillator options

Hello,

I want to use AD725 and made circuit to automatically detect NTSC and PAL, so that after that it will feed correct crystal oscillator to AD725.

I have 2 options:

1- get 2 oscillators, feed them through SPDT analog switch and make the circuit activate either one automatically.

2- as seen in this image. I got LTC1799 and made 2 resistor lines according to its datasheet to have 14.318180MHz or 17.734475MHz (please check if my calculations are correct). Here this part will deliver the correct frequency instead of having 2 oscillators now I got only one. Still going to use the analog switch as seen.

My issue is that option 2 is expensive and I would like to have option 1 or anything cheaper than option 2. Most oscillators here are 3.3v and most of them are out of stock.

I selected these 2 oscillators:

NTSC: www.digikey.com/.../11400465
PAL: https://www.digikey.com/en/products/detail/sitime/SIT8208AI-82-33E-17-734475/11459897

are they good? they are 3.3v and do not go to 5v, are they good enough with ad725 or the 2nd option is better?

thanks

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  • The LTC1799 is a good part however it is not designed to generate color burst frequencies.  Color burst frequencies must be stable and accurate, else you might get color artifacts in the final image.

  • so what do you suggest? LTC1799 was my best attempt to avoid crystal oscillators. Kindly specify which crystal oscillator I should use so I can find them on digikey. I posted 2 oscillators one for ntsc and another for pal, kindly check if they are good or not.

    also, did you see my analog switch? will it be ok with passing the clock signal from oscillators? no other way to do the switching.

  • The AD725 needs a TTL input crystal source of 4 times the color burst frequency of the output format.  As shown in the data sheet this would be 14.318180MHz or 17.734475MHz.  

    Normally I would use crystal oscillators.  The AD725 is designed for +5V rails.  I can't find 5V oscillators for these frequencies.  However there are plenty of 3.3V oscillators available and they'll drive the AD725 just fine, just need a 3.3V rail besides the 5V rail.  

    Search for oscillator with Function=enable/disable, voltage supply=3.3V, Part Status=In stock, Frequency=14.31818MHz and base resonator=crystal

    Do the same except for 17.734475MHz

    I'm seeing stock problems on the PAL oscillator.  You can use the enable/disable pins on the oscillator to switch between the 2 frequencies.

    In the data sheet there's a low cost crystal oscillator circuit, Figure 20 which use readily available crystals from digikey.  Just might take a bit of cap tuning to get them to oscillate at first.  You can change U1B to a nand gate and use it as an enable/disable switch.  Any of the Epson FA-238 or any equivalent series will work

    Watch availability, it seems to come and go on most any of these parts.  check availability over manufacturer.

    Do not use MEMS oscillators, they have poor jitter and temp tolerance.  Always go with a crystal or crystal based oscillator.  Do not use FPGA oscillators even if you could get the right frequency.  They have similar problems that MEMs have, especially jitter.

    No analog switch needed.  You do need stable 4FSC so this leads to a crystal somewhere

Reply
  • The AD725 needs a TTL input crystal source of 4 times the color burst frequency of the output format.  As shown in the data sheet this would be 14.318180MHz or 17.734475MHz.  

    Normally I would use crystal oscillators.  The AD725 is designed for +5V rails.  I can't find 5V oscillators for these frequencies.  However there are plenty of 3.3V oscillators available and they'll drive the AD725 just fine, just need a 3.3V rail besides the 5V rail.  

    Search for oscillator with Function=enable/disable, voltage supply=3.3V, Part Status=In stock, Frequency=14.31818MHz and base resonator=crystal

    Do the same except for 17.734475MHz

    I'm seeing stock problems on the PAL oscillator.  You can use the enable/disable pins on the oscillator to switch between the 2 frequencies.

    In the data sheet there's a low cost crystal oscillator circuit, Figure 20 which use readily available crystals from digikey.  Just might take a bit of cap tuning to get them to oscillate at first.  You can change U1B to a nand gate and use it as an enable/disable switch.  Any of the Epson FA-238 or any equivalent series will work

    Watch availability, it seems to come and go on most any of these parts.  check availability over manufacturer.

    Do not use MEMS oscillators, they have poor jitter and temp tolerance.  Always go with a crystal or crystal based oscillator.  Do not use FPGA oscillators even if you could get the right frequency.  They have similar problems that MEMs have, especially jitter.

    No analog switch needed.  You do need stable 4FSC so this leads to a crystal somewhere

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