Hi First off, the programmer will work with any of the crystals you are talking about. You should be fine at 0.1% which would be +/- 7.4 KHz relative to 7.3728 MHz. Indeed you should find that serial com will work fine at offsets even greater than this.
In terms of birdies - does the radio work? If not is it an IF issue or a front end problem? If none of the replacement crystals oscillate, that’s probably a good sign that the problem is not the crystal ….. Bob > On Mar 5, 2019, at 5:58 PM, Joe Leikhim <[email protected]> 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 > > [email protected] > > 407-982-0446 > > WWW.LEIKHIM.COM > > <Y400 Circuit.JPG>_______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
