[time-nuts] Re; Motorola MC68HC11 Crystal

[Bob kb8tq]

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 <jleikhim at leikhim.com> 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
> 
> JLeikhim at Leikhim.com
> 
> 407-982-0446
> 
> WWW.LEIKHIM.COM
> 
> 
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