Why a small cap at oscillator output?

M

Mark

Guest
Hi,

In some design I use a crystal oscillator. The oscillator works with
two nands. The second nand is a buffer/driver. In some schematic you
see a small capacitor on the output of the buffer (47pf/68pF). What is
the function of this capacitor?

Regards,
Mark
 
Mark wrote:
Hi,

In some design I use a crystal oscillator. The oscillator works with
two nands. The second nand is a buffer/driver. In some schematic you
see a small capacitor on the output of the buffer (47pf/68pF). What is
the function of this capacitor?

Regards,
Mark
Click to expand...
....to introduce Vcc spikes that keep the oscillator section going....
 
I mean that the capacitor is connected between the output of the
buffer to ground. Is this to have slower rising/falling slope for EMC?
Filter the high harmonics of a block signal?

On Thu, 13 May 2004 14:34:37 +0200, Mark <markbng@hotmail.com> wrote:

Hi,

In some design I use a crystal oscillator. The oscillator works with
two nands. The second nand is a buffer/driver. In some schematic you
see a small capacitor on the output of the buffer (47pf/68pF). What is
the function of this capacitor?

Regards,
Mark
Click to expand...
 
On Thu, 13 May 2004 16:00:08 +0200, Mark <markbng@hotmail.com> wrote:

I mean that the capacitor is connected between the output of the
buffer to ground. Is this to have slower rising/falling slope for EMC?
Filter the high harmonics of a block signal?

On Thu, 13 May 2004 14:34:37 +0200, Mark <markbng@hotmail.com> wrote:

Hi,

In some design I use a crystal oscillator. The oscillator works with
two nands. The second nand is a buffer/driver. In some schematic you
see a small capacitor on the output of the buffer (47pf/68pF). What is
the function of this capacitor?
Click to expand...
Well what does the stage it's driving consist of? Can you post a
schematic?
--

"What is now proved was once only imagin'd." - William Blake, 1793.
 
"Mark" <markbng@hotmail.com> wrote in message
news:fhq6a0d7u2d3kva2j97fn0rghrgcqo38oh@4ax.com...
Hi,

In some design I use a crystal oscillator. The oscillator works with
two nands. The second nand is a buffer/driver. In some schematic you
see a small capacitor on the output of the buffer (47pf/68pF). What is
the function of this capacitor?
Click to expand...
Maybe this will help.
The crystals are there to provide loading capacitance that isn't practical
to include inside the crystal.

http://www.dvanhorn.org/Micros/All/Crystals.php
 
...to introduce Vcc spikes that keep the oscillator section going....
Click to expand...
Now that's funny.

Like when a senior VP from sperry told me that flyback converters need
"noise currents" to start up. He was trying to explain intermittent
flameouts in a production design of a 5V-30V/10V converter. The problem
turned out not to be "lack of suficient noise current", but a reverse
polarized screw.
 
On Thu, 13 May 2004 18:18:12 -0500, "Dave VanHorn"
<dvanhorn@cedar.net> wrote:

"Mark" <markbng@hotmail.com> wrote in message
news:fhq6a0d7u2d3kva2j97fn0rghrgcqo38oh@4ax.com...
Hi,

In some design I use a crystal oscillator. The oscillator works with
two nands. The second nand is a buffer/driver. In some schematic you
see a small capacitor on the output of the buffer (47pf/68pF). What is
the function of this capacitor?

Maybe this will help.
The crystals are there to provide loading capacitance that isn't practical
to include inside the crystal.

http://www.dvanhorn.org/Micros/All/Crystals.php
Click to expand...

From the description given, this isn't a loading cap. It's not even in
the osc. stage. That's why I would have liked to have seen a circuit.
--

"What is now proved was once only imagin'd." - William Blake, 1793.
 
"Dave VanHorn" <dvanhorn@cedar.net> wrote in message
news:qMudnQlY6IqKnzndRVn-gQ@comcast.com...
"Mark" <markbng@hotmail.com> wrote in message
news:fhq6a0d7u2d3kva2j97fn0rghrgcqo38oh@4ax.com...
Hi,

In some design I use a crystal oscillator. The oscillator works with
two nands. The second nand is a buffer/driver. In some schematic you
see a small capacitor on the output of the buffer (47pf/68pF). What is
the function of this capacitor?

Maybe this will help.
The crystals are there to provide loading capacitance that isn't practical
to include inside the crystal.

http://www.dvanhorn.org/Micros/All/Crystals.php
Click to expand...
In a parallel resonant circuit using an inverting amplifier, the capacitors
provide a central tap much in the same way a centre tapped coil provides
anti-phase voltages on each end of the winding.

The crystal is designed to resonate at the correct frequency with the design
capacitive load.
 
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