[time-nuts] Re; Motorola MC68HC11 Crystal

[Joe Leikhim]

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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