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
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