[time-nuts] Low cost oscillator packages

Bob Camp kb8tq at n1k.org
Fri Nov 28 19:42:25 EST 2014


> On Nov 28, 2014, at 6:02 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> Assume the temperature is stable

Including self heating?

Stable to micro-degrees / 100 seconds?

Simple math:

0.1 to 1 ppm / C slope (doing quite well for this sort of part)

1 mC per second random thermal noise (not uncommon)

ADEV 1x10^-9 to 1x10^-10 just due to the thermal noise and static (not dynamic, it will be higher) temperature coefficient.  

> and the power supply is clean

Same deal - how clean?

1 ppm / V voltage stability (not un-heard of on this sort of part)

1 mV / sec random voltage noise -> 1x10^-9 ADEV

> and we aren't 
> testing one of the PLL units and …

Sometimes it can be *very* hard to tell that in advance. The PLL / DDS units are all over the place.

> What's the ADEV and/or phase noise of a typical low cost oscillator package?  

One second ADEV (from the above and other things) ~ 1x10^-9 at 1 second +/- an order of magnitude. 

Phase noise depends very much on the chip used in the device. Floor numbers range from -120 dbc/ Hz to -155 dbc / Hz. Knowing which chip is in which part is nearly impossible, unless you are buying thousands of the same part a week. 

> Are there slightly more expensive parts that are a lot better?  Is there 
> something in the data sheets to look for?

A part with an ADEV or phase noise spec would be a good sign.

> What sort of toys do I need to measure ADEV and/or phase noise?  I'm 
> interested in hobby prices rather than expensive high-end gear.

ADEV - DMTD is about the easiest thing to set up in a basement. 

Phase noise - quadrature lock double balanced mixer + preamp running into an audio spectrum analyzer or sound card. 

Both setups will be a bit easier / more general useful if you are testing at 10 MHz. If you are off at some odd frequency you may have fun finding a low noise VCXO / OCXO to test against. 


In general it’s a *lot* easier to build up an oscillator with discreet parts and a cheap crystal than doing all the testing of pre-built parts. Having been through this sort of sorting / testing thing before: you’ll get the testing done and order some of the “good parts”. Maybe two days have gone by. The parts you get on the second order *look* identical to the first ones. They behave completely differently…. There are a lot of circuits out there in the various magazines. Total cost to build from scratch should be < $5. The one you build will blow away anything you would buy for the same cost. Phase noise and ADEV simply are not “target specs” on a simple / low cost XO. 


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