Why a small cap at oscillator output?

[Mark]

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

 

[Fred Bloggs]

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

....to introduce Vcc spikes that keep the oscillator section going....

 

[Mark]

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?

 

[Paul Burridge]

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?

Well what does the stage it's driving consist of? Can you post a
schematic?

 

[Dave VanHorn]

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

 

[Dave VanHorn]

...to introduce Vcc spikes that keep the oscillator section going....

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.

 

[Paul Burridge]

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

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.

 

[Fred]

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

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.