On 4/11/2020 12:15 AM, John Moran, Scawby Design wrote:
During my 50 years in the electronics industry I have always been puzzled about one aspect of crystal oscillators. They go to great lengths to use a precise piece of quartz as the heart, because of its unique properties, and then add standard external components - capacitors, varactors, Zeners, etc. to tweak its frequency. All these components vary far more than the original piece of quartz ... hence my confusion. I know it is practically impossible to grind a crystal to exactly the frequency you want, and it then drifts over time, but what is the logic of using relatively wildly varying components to adjust the quartz? Are their temperature and ageing characteristics swamped by the superior crystal? In all the papers I have ever read, the subject is never mentioned ... you just add a variable capacitor and/or an EFC circuit and job done. I guess this is showing my total ignorance here, but I would like to know. Maybe this is at the heart of Rick's usual speech? John
For non-oven oscillators of ordinary precision (say 10 PPM over 0 to 50°C), the low pullability of the crystal is such that adding
adjustment capacitors is not a big hazard. An EFC circuit that is inside some PLL of course is only at risk of adding some noise from the drive circuit. I can't remember ever seeing an EFC'ed oscillators where the EFC was driven by, say, a pot. In the case of the 10811, I have already posted about the reference diode of special characteristics. I don't remember all the exact details of how it was chosen, but it was based on proprietary knowledge. Another anecdote of interest is that when the first 10811's were being tested, they exhibited very bad aging. It was eventually determined after a lot of investigation that oil in the piston trimmer was migrating around and tweaking the frequency. I don't remember whether the fix was using a different type of oil, or having the vendor apply it selectively, or if they deleted the oil completely. This type of knowledge can basically only be learned by getting a job working with the top experts in the field. Before I worked for HP, it became clear that I needed to get a job there in order to figure out how their stuff worked. I read HP manuals as much as I could, but actually being there was the real secret. I asked as many people as possible about their particular expertise in an effort to learn as much as possible. The right approach is necessary to get these experts to open up. I was careful not to wear out my welcome. Every person is an expert in their particular field, and there is always something to be learned from them. A couple of ground rules I formulated from experience: 1. If they tell me something that I know is wrong, I usually just thank them for the advice and then ignore it, rather than arguing. Or "correcting" them! Let them continue to believe what they want to believe. Sometimes I find out later that actually the person was correct and I was wrong. 2. Be extremely careful about asking "why did you design it that way?" This usually causes the other person to get defensive. Often, revealing the true reason would be embarrassing. If the true reason is not embarrassing, the person will volunteer it if you talk to them long enough. For example, the fact that many HP counters used up to four identical microcontrollers is a very touchy subject. I was, myself, guilty of doing this in the 5334B. My defense was that I inherited this from the 5334A. The project manager of that product himself inherited it from previous counters. The root cause, in case anyone is interested, is that the microcontrollers were legacy models with only 2k of memory. The sole vendor was not making any newer models with additional memory. So the only way to get more memory was to use additional microcontrollers. We needed more memory for necessary features. Since the code was written in assembly language, it would have been a major effort to port it to a different controller. The person who wrote the code was a major guru in those controllers and was the only person who could do the porting. Kind of like the Y2K Cobol coder. Rick N6RK _______________________________________________ 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.
