[time-nuts] Motorola MC68HC11 Crystal

[Roger Tilsley]

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

On Tue, 5 Mar 2019 17:58:58 -0500, Joe Leikhim <jleikhim at leikhim.com> wrote:

> It seems some Crystal experts are on line, so here goes with my question.
> 
> I have this microcomputer circuit (attached) that is in a Motorola 
> Systems Saber radio.  It contains a 68HC11 uC that requires Y400 which 
> is a 7.3728 MHz crystal.  Motorola no longer provides an OEM replacement 
> for this part number 4805664G32. The description is simply "XTAL 
> ANTIRES". -- Is that Parallel Resonant? These are those tubular 3 x 8 mm 
> style like the 32Khz crystals used in early watches.
> 
> There are two critical stability requirements of this crystal. _First 
> _it runs the external RS232 programmer at 9600 baud. If bad the radio 
> cannot be read or written to, and _secondly_ the harmonics could fall on 
> operating channels. The network of Q403/L400 and C409 are to shift/pull 
> the frequency where a known harmonic might occur.  -- I don't know the 
> normal stray load this network imparts.
> 
> So far I have purchased some generic crystals from e-bay but they don't 
> seem to be solving the immediate problem which is serial communications 
> error.
> 
> The crystal I removed from circuit oscillated at 7.3708 MHz per the CW 
> zero beat on my Icom shortwave radio. (I know, yes, it is set to WWV)  
> or 2 KHz low. The 10 replacement crystals sampled in at around 2.47 KHz 
> low. An OEM crystal that I have (the last one) 4.36 KHz low. In my 
> estimation, these parts should be within 30 ppm or +/- 222 Hz
> 
> I do not know if the crystal pull network is running when I made these 
> measurements. It is a possibility. It is switched in and out depending 
> upon the radio RX frequency. I have no control over those until the 
> radio is read and rewritten to which I cannot in this condition.
> 
> ---------------------------
> 
> The E-bay generic parts:
> 
> Frequency: 7.3728Mhz
> 
> Frequency Tolerance: ±30ppm
> 
> Load Capacitance: 18pF
> 
> Mounting Type: Through Hole
> 
> ---------------------------------
> 
> Am I using the wrong load capacitance for the application?
> 
> Here are my assumptions:
> 
> CL=(24pf x 24pf)/24pf+24pf) + 6pf (stray is a guesstimate) = 18 pf
> 
> There is not enough board space to add parts. My estimation of stray 
> capacitance may be high.
> 
> I am trying to get to square one and buy the correct parts. Or maybe 
> this is as good as it gets and my problem is elsewhere?
> 
> -- 
> Joe Leikhim
> 
> 
> Leikhim and Associates
> 
> Communications Consultants
> 
> Oviedo, Florida
> 
> JLeikhim at Leikhim.com
> 
> 407-982-0446
> 
> WWW.LEIKHIM.COM
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