[time-nuts] Re: Crystal oscillator for a begginer

[Bob kb8tq]

Hi

Setting up a bench to measure clock sources can indeed be expensive.
eBay is one of the ways to find the gear you need at a semi-rational price.
The gotcha there is that the gear may or may not be fully functional. Stuff
from folks like TAPR might cost a bit more. The hassle level often makes
this worth every penny ….

The most basic thing you need for your bench is some sort of standard
to compare things to. It needs to be more accurate / stable than the devices
you are testing. A GPSDO is often the answer. Indeed folks do go to somewhat
insane lengths coming up with standards.

How complicated this gets is very much up to you. A $60 counter and an 
$80 GPSDO might well do the job. You could spend 10X that and still 
not be into the “crazy” level of bench equipment. 

Bob

> On Jan 6, 2022, at 12:50 PM, Adam Space <time.isanapp at gmail.com> wrote:
> 
>> How would you quantify "pretty weak?"  You could connect up the components
>> correctly, but you could not design a circuit from scratch?  Or you are
>> worried you will let the smoke out the first project you start?
> 
> I just mentioned this to indicate that I have only basic knowledge, as
> compared to many on this list, I assume. I'm sure I could understand most
> concepts if explained to me, and can connect things together. But I've seen
> videos where they talk about resistors, circuits, etc., and my knowledge of
> these things is limited to my physics class several years ago (not very
> good). Things like soldering and PCB are things I've heard of, but they're
> still very new to me and it would be a big step up to do those things on my
> own.
> 
> Regarding your other explanations, thank you, this cleared a lot of things
> up. To summarize, an oscillator will generate electrical signals at a given
> frequency, but a frequency counter and something to subdivide the frequency
> down to e.g. one second would be needed to get closer to a proper clock.
> (And finally of course something to display the count). It seems like the
> oscillator is by far the cheapest part of this: they can cost as little as
> under a dollar to more for better ones. But it seems like frequency
> counters and subdividers are much more expensive, no? Someone linked a
> subdivider to 1pps, which looks like a good place to start.
> 
> Adam
> 
> On Wed, Jan 5, 2022 at 10:33 PM Chris Caudle <6807.chris at pop.powweb.com>
> wrote:
> 
>> On Wed, January 5, 2022 5:01 pm, Adam Space wrote:
>>> My knowledge of hardware is pretty weak
>> 
>> How would you quantify "pretty weak?"  You could connect up the components
>> correctly, but you could not design a circuit from scratch?  Or you are
>> worried you will let the smoke out the first project you start?
>> 
>>> But I am confused on how I could exactly use this [crystal oscillator]
>> 
>> A "canned" crystal oscillator is a quartz crystal and circuitry together
>> in a package that provides an oscillating electrical output when connected
>> to power.
>> For the specific device you mentioned, you connect it to a 5V power
>> supply, and the output will vary between 0.5V and 4.5V 10 million times
>> per second.
>> 
>>> if I have a crystal oscillator going, how could I compare it or adjust
>> it to
>>> other clocks on my network, or to UTC for example?
>> 
>> First, a crystal oscillator has no concept of time, so discussion of
>> adjusting to UTC does not make sense in that context.  The output swings
>> low to high to low and that is it.
>> 
>> If you want a clock, you need something which oscillates (quartz
>> oscillator, atomic states of cesium atoms, pendulum, etc.) and you need a
>> mechanism to count and display the oscillations.
>> So to make a clock with your 10MHz quartz oscillator you would need an
>> electronic circuit compatible with the electrical levels of that
>> oscillator, which includes a counter that can increment at that rate, and
>> probably circuitry to divide down the 10 million cycles per second to more
>> usable rates (1 cycle per second, 1 cycle per hour, etc.).  Unless you are
>> satisfied just knowing that the circuit is busy counting cycles, you
>> probably also want a way to either display the current count, or output
>> the count in some electrical format for use elsewhere.
>> Unless your counter is completely self referenced, you will probably also
>> want a way to set the count to a predetermined count right after it powers
>> on so that it matches your other clocks.
>> 
>>> Or to take another example, suppose I have a few crystal oscillators like
>>> the one above. Is there a way I could compare them to each other, or log
>>> the offsets from each other, and so on?
>> 
>> Yes, and depending on how quickly you need to know, how precisely you need
>> to know, and over what time periods you need to know (or alternately what
>> frequency offset if viewing in frequency domain rather than time domain)
>> there are quite a few ways.
>> The most basic is to use a frequency counter instrument with a time base
>> much better than the device you are trying to measure and count how many
>> cycles occur over a particular time period.  For example, you could could
>> the transitions on the output of your oscillator for 1 second, and display
>> the count.  You expect it to be around 10 million, but oscillators vary in
>> accuracy as built, and drift with time, temperature, power supply
>> variations, etc.  Unless your counter can generate the start and stop
>> counting signals very accurately 1 second apart, you can't be sure how
>> much of the difference between the actual count and 10 million is due to
>> the device you are testing, and how much is due to inaccuracy in the
>> measuring device.
>> For the oscillator you mentioned the problem is not very difficult, most
>> good quality test equipment has much better accuracy oscillators
>> internally, but as you attempt to measure higher and higher quality
>> oscillators it becomes more challenging making the test equipment good
>> enough to be sure you are actually measuring your device, and not just
>> displaying the limits of the test equipment.
>> 
>>> Ultimately, it would be nice to
>>> compare the frequency and time offsets to a reference source that is
>>> accurate long-term, like my GPS hat Raspberry Pi.
>> 
>> You have the right idea, although a Rasberry Pi is probably not the ideal
>> tool for that job.
>> There are different tools needed as well for comparing how far apart two
>> devices have drifted over the course of a day, week, or month compared to
>> what you would use to determine how much the frequency varies second to
>> second, or millisecond to millisecond, or how the time period between any
>> two positive going edges of your 10MHz signal compares to the time between
>> any other two consecutive edges.
>> 
>>> Any ideas, suggestions, or clarifications are welcome. Additionally, if
>>> anyone knows of any guides (either text, video, or whatever), that would
>>> be great too.
>> 
>> You could start with this as a breezy introduction:
>> http://leapsecond.com/ten/
>> 
>> This will keep you busy for a long time:
>> 
>> http://www.resonal.com/Downloads/John%20R.%20Vig%20-%20tutorial%20on%20Quartz%20Crystals%20and%20Oscillators.pdf
>> 
>> (apologies if my mail editor mangles that link, hopefully you can put it
>> back together successfully if that is the case).
>> 
>> --
>> Chris Caudle
>> 
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