[time-nuts] Re; Motorola MC68HC11 Crystal

[Joe Leikhim]

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

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