Can I swap a cer. resonator for an xtal?

[Isaac Wingfield]

In article <6DadnbZKbL8AEN6iXTWJjQ@comcast.com>,
"Dave VanHorn" <dvanhorn@cedar.net> wrote:

http://www.jameco.com/cgi-

bin/ncommerce3/ExecMacro/jameco/searchResult.d2w/report?sort=BPA&search=1378
16
which states 17pF, the two 30pF caps currently in the circuit would be
fine?

Sounds about right.

There are two main types of crystals. Parallel resonant, and series
resonant.

That's a common belief, but it's not true. A crystal can be operated in
either mode; what that specification tells you, is which way to operate
the crystal to have it oscillate at its specified frequency (in other
words, that's how the manufacturer operated it, to measure it, and
*that* is why the parallel capacitance is specified). It'll operate in
the other mode just fine, but at a slightly different frequency; that's
all. Many times, that difference in frequency is of absolutely no
significance.

Isaac

 

[Isaac Wingfield]

In article <qgd9kvcbaojti5ipt7rrliq0odo4emqhff@4ax.com>,
Spehro Pefhany <speff@interlog.com> wrote:

On Thu, 21 Aug 2003 04:17:24 GMT, the renowned Isaac Wingfield
isw@witzend.com> wrote:

That's a common belief, but it's not true. A crystal can be operated in
either mode; what that specification tells you, is which way to operate
the crystal to have it oscillate at its specified frequency (in other
words, that's how the manufacturer operated it, to measure it, and
*that* is why the parallel capacitance is specified). It'll operate in
the other mode just fine, but at a slightly different frequency; that's
all. Many times, that difference in frequency is of absolutely no
significance.

Certainly it wouldn't be of significance if a resonator was good
enough.

No real-life resonator is that good. What matters is the precision of
frequency necessary for the task at hand. Sometimes the difference
between the series resonant frequency and the parallel resonant
frequency *for any crystal you can buy*, is an error that is simply not
tolerable.

OTOH, sometimes the precise frequency is of no concern whatsoever, and
it's fine to be somewhere close, but fairly stable. That would be the
case for almost all computer system clock applications, for example.

Isaac

 

[Da Man]

"DaveC" <dave-usenet3016@mailblocks.com> wrote in message
news:0001HW.BB6786AE0029A813072A6E70@news.individual.net...

On Tue, 19 Aug 2003 4:42:44 -0700, Tim Shoppa wrote
(in message <bec993c8.0308190342.2ee4b60b@posting.google.com&gt:

DaveC <dave-usenet3016@mailblocks.com> wrote in message
news:<0001HW.BB67089F00294CF5072A6E70@news.individual.net>...

Symptom is that the microcontroller (that this clock drives) is driving
the output relays to chattering (about 10 Hz).

10 Hz is a pretty typical frequency for motorboating. The regulator
might
be putting out 5VDC, but is it really flat 5VDC? I know you checked the
ground, that's a very good thing to check in case of motorboating, but
you
also should check for a good clean low-impedance connection on the 12V
side
too.

Removed all caps & tested with cap tester.

What's the ESR of the electrolitic caps?

All within 5 percent. Voltages at
the chips are solid.

Thanks,
--
Dave C.
dave-usenet3016@mailblocks.com

 

[DaveC]

On Sat, 23 Aug 2003 19:46:40 -0700, Da Man wrote
(in message <kGV1b.8146$qA3.652809@ursa-nb00s0.nbnet.nb.ca&gt:

What's the ESR of the electrolitic caps?

Criminy, I don't know. Can the ESR really change that much without the
capacitance value not changing?

Is there a way to measure this with a meter -- that is, a commonly-available,
meter, not an ESR meter (if there is such a thing...)?

Regards,
--
Dave C.
dave-usenet3016@mailblocks.com

 

[James Sweet]

"DaveC" <dave-usenet3016@mailblocks.com> wrote in message
news:0001HW.BB6EB82500C49CA202368A30@news.individual.net...

On Sat, 23 Aug 2003 19:46:40 -0700, Da Man wrote
(in message <kGV1b.8146$qA3.652809@ursa-nb00s0.nbnet.nb.ca&gt:

What's the ESR of the electrolitic caps?

Criminy, I don't know. Can the ESR really change that much without the
capacitance value not changing?

Is there a way to measure this with a meter -- that is, a
commonly-available,
meter, not an ESR meter (if there is such a thing...)?

Yes and no, if in doubt try replacing caps.

 

[Spehro Pefhany]

On Mon, 25 Aug 2003 02:12:21 GMT, the renowned "James Sweet"
<jamessweet@hotmail.com> wrote:

"DaveC" <dave-usenet3016@mailblocks.com> wrote in message
news:0001HW.BB6EB82500C49CA202368A30@news.individual.net...
On Sat, 23 Aug 2003 19:46:40 -0700, Da Man wrote
(in message <kGV1b.8146$qA3.652809@ursa-nb00s0.nbnet.nb.ca&gt:

What's the ESR of the electrolitic caps?

Criminy, I don't know. Can the ESR really change that much without the
capacitance value not changing?

Is there a way to measure this with a meter -- that is, a
commonly-available,
meter, not an ESR meter (if there is such a thing...)?


Yes and no, if in doubt try replacing caps.

Or just tack one of roughly equal value and at least as high voltage
rating across the suspect cap....


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Fred Abse]

On Sun, 24 Aug 2003 03:46:40 +0100, Da Man wrote:

What's the ESR of the electrolitic caps?

We don't live in a perfect world, hence the perfect capacitor of circuit
theory doesn't really exist. Capacitors are made out of real materials
which are less than perfect. The leads are made of some metal or another,
which has resistance, they are attached to the plates, which are made out
of something which has resistivity, in the case of electrolytic
capacitors, one "plate" is actually a solution of chemicals in one form or
another. No dielectric is perfect either, all have losses which can be
expressed as a resistance.

All these imperfections add up, so what we have is what appears at the
terminals as a "perfect" capacitor in series with a resistance.

This is called the equivalent series resistance (ESR). "Equivalent"
because it is the cumulative effect of many different losses.

This is true for all capacitors, but electrolytic capacitors exhibit the
highest values of ESR.

You will also come across the term "loss angle". This is just a different
way of expressing the same thing, and is the difference between the actual
phase angle of the component and the -90 degrees of a perfect capacitor.

Summing up; a perfect capacitor does not dissipate power, real capacitors
do, and ESR is one way of expressing this.


--
Then there's duct tape ...
(Garrison Keillor)
nofr@sbhevre.pbzchyvax.pb.hx

 

[Asimov]

"DaveC" bravely wrote to "All" (24 Aug 03 18:44:53)
--- on the heady topic of "Re: Can I swap a cer. resonator for an xtal?"

Da> From: DaveC <dave-usenet3016@mailblocks.com>

Da> . Can the ESR really change that much without the
Da> capacitance value not changing?

Yes, esr is like adding a resistor in series with the capacitor. In
truth the capacitance will often decrease as esr increases but not
in all cases.


Da> Is there a way to measure this with a meter -- that is, a
Da> commonly-available, meter, not an ESR meter (if there is such a
Da> thing...)?

There are a number of commercial esr meter types available but one can
easily make the same measurement with a generator, a couple of
resistors, and a dmm. It's basically an ac voltage divider, a very
simple circuit, inject a known ac current into the capacitor and measure
the voltage drop across it. Calculate voltage divided by current = esr.
Naturally the frequency of the ac has to be great enough to swamp the
capacitive reactance but it's really that simple.

But there are pitfalls with esr, for example we want a cap to show a
zero reading but in fact a zero reading may be hiding a shorted cap.
Another effect is if the frequency is very high then large capacitors
may indicate as good because part of its outer layer capacitance is able
to swamp out the internal esr of the rest of the cap but such a cap
would have a lower capacitance. Lowering the measuring frequency would
show the higher esr and not necessarily a higher reactance. IOW even a
commercial esr meter can fool one into thinking a cap is good. Besides a
cap could be intermittant!

Asimov
******

.... I am Ohm of Borg. Resistance is V/I...