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
jleik...@leikhim.com
407-982-0446
WWW.LEIKHIM.COM
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