L
Lionel
Hm, looks like you need to run for another kook award, Philbert.
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Hm, looks like you need to run for another kook award, Philbert.
V
Vegetarian Plow
A revenge nomination so soon. He hadn't even started to tee off on you.
M
Michael A. Terrell
Phil said:
What's wrong, Phyills, didn't your boyfreind show up last night? We
know how cranky you get when you don't get....
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida
P
Phil Allison
"Michael A. Terrell, Psychotic, Autistic FREAK "
YOU can go a shove your pointy head up a dead donkey's backside where you
normally hide it.
**** BRAIN !!
........ Phil
YOU can go a shove your pointy head up a dead donkey's backside where you
normally hide it.
**** BRAIN !!
........ Phil
A
Archie Leach
Phil Allison said:
I think you just need to tell us (and Michael) how you really feel.
J
John Fields
M
Michael A. Terrell
Archie said:
Syphilis doesn't feel, she just flames.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida
L
Lionel
Hm. Why am I getting a sudden urge to start reposting all that stuff
about Dean Humphries catching his crackwhore wife blowing one of his
friends?
What a very strange impulse to have after all these years.
V
Vegetarian Plow
Do tell!
Lionel said:
J
jcomeau_ictx
Well, I have something, admittedly not much, but it works for one of
my junkbox crystals. See http://www.jcomeau.com/blog/crystal_tester.shtml.
L
Lostgallifreyan
Build a crystal oscillator with an inverter (logic gate NOT) like this:
http://www.eepn.com/images/Articles/32303.jpg
Usually best buffered by an extra gate if you're going to load it a bit:
http://www.ozhiker.com/electronics/archive/Oscillators/Crystal5.gif
74HC04 or 74HCT04 devices can work to over 100 MHz. NAND gates with inputs
tied can be used as a NOT gate if spare. NAND gates are amongst the
cheapest IC's ever.
J
jcomeau_ictx
Nice! Those circuits are about my speed. I'm headed out of town for a
week, but plan to update my page when I get back. Thanks -- jc
J
John Fields
J
jcomeau_ictx
Not to rain on your parade, but the difference between 7.15625 and
7.19780 is 0.04155, which is an error of 0.58%, a _huge_ error in
the land of crystals.
No, that doesn't rain on my parade, as the calculation was based on
eyeballing a moving LED display. If I wanted accuracy I would have
rewritten the program to calculate the result itself with a stable
timebase, and displayed it in binary on the LEDs. This was just a
proof-of-concept, and since I had only one crystal that worked, it
doesn't really qualify as that, but the amount of accuracy does make
it seem a plausible approach.
L
Lostgallifreyan
Bear in mind that while logarithms involve precision that justifies 12 bit
ADC/Process/DAC work, getting accurate voltage conversions from frequency
is far less demanding. It's linear, and you can get a 1 Hz to 1 MHz range
within the range of 20µV to 20V, a fairly easy task with one of various
cheap freq/volt converter IC's and a cheap op-amp like an LF411, let alone
something more exotic. You can make switched ranges if needing more, or
sample a pulse train digitally. Either way, you can start with one of those
circuits I linked to from my other post, the one with the hex inverters in
it. Why settle for an estimate when you can get high precision for less
than most kids would consider as adequate pocket money?
If you can find a freq/volt IC that has connections for a crystal to be
added, and a variable gain on the output, you could do everything with that
IC and a voltmeter. I don't know if such a singular IC exists, but if I had
a lot of crystals to test, I'd be looking for one.
J
John Fields
---
The insurmountable problem I see with your approach is that no
matter what you do you can't use the µC's time base to determine its
own frequency since you'll either have to have a reference time base
running independently from your µC time base or, knowing the
frequency and accuracy of your µC time base, use it as the
referenced against which to measure the period/frequency of the
crystal being tested in a _separate_ oscillator. In either case what
you'll have built is a conventional frequency or period counter.
J
John Fields
---
The circuits you're referring to are merely oscillators, and will do
nothing except get the crystal to oscillate so that its frequency
can be measured.
As far as I know there are _no_ frequency-to-voltage converters with
the accuracy required to measure frequency with anything coming even
close to the crystal's own accuracy, and if there are they won't be
cheap.
I could be wrong, of course, so if you can find one and post which
one it is, I'll gladly admit to being wrong.
J
jcomeau_ictx
That looks like an interesting approach. I'd need a better VOM than I
have now, though, to be able to take advantage of the high precision.
And it seems most kids nowadays have more pocket money that I do.
J
jcomeau_ictx
Yes, that's a problem for sure. I wonder how accurate is the 60 Hz
from the power company? A quick websearch didn't come up with
anything. If it were reliable to a few PPM that might work as a
timebase. And IIRC the PIC devices have protective circuitry on the I/
O pins so all you need is a current-limiting resistor, say ten
megohms.
Right. One that anyone with a PIC device could make in a few minutes
when needed, and repurpose for other uses afterwards.
In any case, I probably won't pursue this any more unless I have need
for it myself. If the OP really gives a damn about his crystals,
there's enough useful info in this thread now.
L
Lostgallifreyan
I never said otherwise.
That's true too, but it would still be better than the kind of error you
showed existed in the other method. Not over the entire range, perhaps, but
it should be a reasonable match for the 0.5% typical of many multimeters.
My emphasis was on cheap, and easy, and how you can make a significant
improvement on accuracy this way.
