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Building a s-video circuit - which caps to use?

Discussion in 'General Electronics Discussion' started by NeilTheDruid, Jun 21, 2011.

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  1. NeilTheDruid

    NeilTheDruid

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    Jun 21, 2011
    Hello everyone. This is my first post here, I am looking foward to learning from this community. :)

    I have an old Atari 7800 here that I am building a svideo circuit for to make the picture better than the original RF signal. For the chroma portion of the circuit it has been recommended to me that I should use a 1uf tantalum bead style cap. Unfortunately, all I have are 1uf electrolytics and I am having trouble finding a tantalums.

    Is there any reason why I couldn't use my electrolytic caps, if so what would you recommend as an alternative?

    If you'd like to check out the modification I am doing it's right here.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'm surprised you need something as large as 1uF. Having said that, You would want something with lower inductance, so an electrolytic is probably not the best option.

    I'd stick a 0.1uF ceramic cap in there, but I can't tell you if that's good advice :)
     
  3. Resqueline

    Resqueline

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    Jul 31, 2009
    Since the chroma signal to pass is only 1MHz wide & lying around the 4MHz range you can use any value from 5nF to 500uF - as long as you use a low-inductance layout.
    So like Steve I wouldn't recommend an electrolytic (though it would surely work well enough) but rather put a 10-100nF ceramic or plastic cap in there.
     
  4. NeilTheDruid

    NeilTheDruid

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    Jun 21, 2011
    Cheers guys. That's handy because I've got a big bag of 100nf right here :) I'll let you know how it goes.

    BTW, I'm a bit of a hack with only limited knowledge on electronics. If someone could explain to me why a 0.1uf cap would be recommended that would be top
     
  5. Resqueline

    Resqueline

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    Jul 31, 2009
    It's about the wanted impedance of the cap at the pass frequency range in question. You don't want the cap to attenuate the wanted frequencies too much.
    Since the impedance of video inputs are 75 ohm you'll want the cap to be 1/10th or less of that, at the chroma frequencies (4.43 MHz for PAL / 3.58 MHz for NTSC).
    The chroma has around 1Mhz bandwidth so choose 3MHz as the lower frequency, where the cap should have 7.5 ohms or less impedance (Z).
    The formula is C = 1 / (2 ' pi ' f ' Z) which gives 7 nF as the minimum capacitance using the suggested values.
    These are all ballpark figures just to "get in range". Since you're not making a filter needing an exact transition frequency you can choose a substantially larger capacitor.
    A 100nF cap (being small, cheap, & easy to get) will have a 0.5 ohm impedance at 3MHz, which fits the bill just fine. It also makes sure there's no phase shift at 3MHz.
     
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