[time-nuts] Motorola MC68HC11 Crystal

[Joe Leikhim]

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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