Odd load capacitance

Go To Last Post
5 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

While messing around with one of my projects, I took out the clock oscillator and connected up a 3.579MHz crystal. I had to run a set of 6 inch wires form my small board to a breadboard, where I jammed in the crystal. Typically, I have seen loading caps in the 15-27pf value used, so I dropped on a set of 22pf caps, thinking it would work fine since the long wires and breadboard rails would probably add a good 10-15pf as well.

At this point, the XMega started fine, but the 3.579MHz color burst was off enough to cause the TV to drop the color reference.

In fact, I could not only get the color burst back if I loaded up with 100pf caps.

Not that this really matters as I am just messing around, but doesn't it seem odd? I would have expected to not even need caps with all that extra parasitic capacitance. I must be running at least 120pf now with those long wires and the breadboard rails. The lowest caps I can jam in and get a color lock are 68pf.

The load capacitance on the xtal is 20pf, so using the formula in the datasheet, 22pf should be just fine.

I intend to replace my clock oscillator with this xtal when I do my PCBs, so I wonder if the 22pf will actually work?

Has anyone here experienced this need for huge values on the xtal caps? If it wasn't for that 3.579MHz frequency having to be ultra precise, I probably would not have even noticed. 3.578MHz or 3.580MHz would not be good enough for a TV color PLL.

Brad

I Like to Build Stuff : http://www.AtomicZombie.com

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I know nothing about crystals, but I do know that there are so-called parallel and serial resonant types.
Maybe this is relevant?

Four legs good, two legs bad, three legs stable.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Second time I type this :(
Win7 & Chrome make that I loose what I typed at unknown intervals. Grrrr.

Brad, the 6" wires represent more inductance than capacitance ! Unless you twisted the wires ;)
The capacitance of a breadboard, row to row is just a few pF. I measured that when this matter came up earlier. Searching ..... https://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&p=947444#947444

cheers

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi Brad,

Lay out your board and use the recommended caps, +/- a small amount. It ought to work fine.

For your TV circuitry and precise timing requirements you probably want to put a ground plane under the Xtal, as per Atmel Ap Note 042.

I would not worry at all about your above test results. I am surprised the clock ran at all with 6" of wire to the Xtal. A "Dead Bug" connection, right at the micro's pins, would be a better test.

JC

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Thanks, I will have to try some more experimentation when I have the chance. I have been an advocate of using only clock modules, so this is my first experience with a crystal.

Brad

I Like to Build Stuff : http://www.AtomicZombie.com