[time-nuts] OCXO and fluctuations after EFC adjustment

[David C. Partridge]

Well, that's how it's supposed to be done, but these days the usual (and often only) criterion other than part value (e.g. 15V 200uF +/- 10%) seems to be cost (cheapest == best).

Sad isn't it.

David
-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at lists.febo.com] On Behalf Of Bob kb8tq
Sent: 11 April 2020 14:05
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] OCXO and fluctuations after EFC adjustment

Hi

Bottom line is that, as long as one is careful about *which* vendors supply which
parts, normal parts do the job. Nobody is going to publish that selection process 
or the results. They very much want the “other guy” to have to do it on their own. 

The 78L12 might look just like one from 5 other vendors. It also might work 10X 
better than the others. Those caps may look pretty normal. They came from 
“this guy” and not “those guys”. That cheap looking thermistor might have spent 
a few years in evaluation before it was approved for use. 

There is a lot of work that goes into component selection. It simply does not 
result in $20 bulk metal film parts with 0.2 ppm/ C specs getting used. It is a 
lot more difficult to spot in the finished product. 

Bob





> On Apr 11, 2020, at 3:15 AM, John Moran, Scawby Design <john at scawbydesign.co.uk> wrote:
> 
> On Fri, 10 April 2020 14:31:53 -0700 Rick wrote:
> 
> 
>> At this time, I will give my usual speech about IMHO the fact that
> 
>> since the invention of the DDS on a chip, EFC should no longer be used
> 
>> for high performance oscillators.
> 
> 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
> 
> 
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