Hiya,
A quick observation, though I'm mindful we may be drifting (sorry...)
from the charter of time-nuts somewhat.
I worked with 'HC11s a fair bit when they were current and seem to
recall there were some subtle variations on the load capacitance
requirements for the different variants (68HC811E2 vs 68HC11A1 etc. etc)
It may be worth tracking down the electrical specs datasheet for the
specific variant you are working with and comparing this with what you
have to hand.
Apologies if I'm preaching to the choir so speak though!
Kind Regards/vy 73
Hugh
On 7/3/19 06:47, Bob kb8tq wrote:
Hi
Load capacitance on a crystal is not at all easy to guess from the “stuff”
hanging
off of an MCU chip. There’s simply to much inside the chip that you have no way
of knowing about. If indeed you *need* a very accurate crystal, it gets custom
made
for whatever that particular circuit happens to do. Generally that is taken
care of
with a PPM offset on the spec drawing. Rather than being +/- 30 ppm at some
load, it will be +150 to +210 ppm. At least that’s the way Motorola always used
to do it …..
Bob
On Mar 7, 2019, at 1:45 AM, Joe Leikhim <[email protected]> wrote:
The circuit was an attachment and it showed 24 pf caps on the legs of the
micro. It appears it is being scrubbed in the mailserver.
Am I using the correct load capacitance for the application?
Here are my assumptions:
CL=(24pf x 24pf)/24pf+24pf) + 6pf (stray is a guesstimate) = 18 pf
On 3/7/2019 1:41 AM, Joe Leikhim wrote:
Roger, does the 18 pf load, crystal I have chosen for replacement seem correct
for the design (attached)?
Joe
e:
"Greetings Joe,
Except for the difference in the marked frequency, there is no difference between the so-called "parallel resonant" and
"series resonant" crystals. There is a minute difference in the physical dimensions and/or the angle of the cut(s)
relative to the crystallographic axes to hit the desired frequency. The description of the OEM part "XTAL ANTIRES"
shows that it is "parallel resonant" with the capacitance specified by the crystal manufacturer (typically 20 pF
(sometimes less), 30 pF, or 50 pF) appearing in parallel with the crystal. This is the reason behind the use of a small-value
variable capacitance in parallel with the crystal to trim the frequency to exactly that specified or desired at constant
temperature though the trim range is relatively small. The design of crystals is something of a "black art".
The so-called "series resonant" crystals are sometimes described as for use in a "resonant" mode while the so-called
"parallel resonant" crystals are sometimes described as for use in an "anti-resonant" mode. This is technical
gibberish but the "parallel resonant" and "series resonant" descriptions are a useful guide for the designer of the
amplifier in which the crystal acts as narrow band filter in the feedback circuit and controls the frequency of the resultant oscillation.
There are many considerations, such as the drive level (particularly for
physically very small and very large crystals!) which have to be considered but
if the equipment used to work correctly in the long term it is unlikely that
there is a problem with the crystal. From your description, I doubt if the
fault is in the crystal and you will need to look elsewhere for the fault.
I hope that this may help you."
Regards,
Roger
--
Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida
[email protected]
407-982-0446
WWW.LEIKHIM.COM
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
