[time-nuts] Motorola MC68HC11 Crystal

[Bob kb8tq]

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 <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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