Hi The 1/F noise vs beat note “amplification” tradeoff is what pushes me up to 10 Hz rather than staying down around 1 Hz with most setups. It’s also a rational offset to achieve at 10 MHz with common OCXO’s. Once you get past about 20 Hz, your OCXO choices diminish.
Bob On Oct 12, 2014, at 7:57 PM, Magnus Danielson <[email protected]> wrote: > Increasing the beat frequency to find a balance between 1/f noise and > f/delta-f amplification may be worth doing and have been seen done to find > "optimum" performance. If you use hard limiters or audio channels to achieve > it is however a little detail. > > The benefit of audio channels is that the A/B channels does not disperse out > in time, such that you loose cross-correlation of transfer oscillator noise. > > Some AD inputs may need to be modified to remove DC-blocking cap. Not all > ADCs is happy with this. Some boards already have that and do DC-removal in > digital filters. > > Cheers, > Magnus > > On 10/12/2014 11:09 PM, Bob Camp wrote: >> HI >> >> A little more information: >> >> If you are doing the ADC thing, you still need to estimate zero crossings. >> In all likelihood you would be doing bandpass filtering first (say 8 Hz to >> 12 Hz) on your 10 Hz note. Next you would do some sort of estimator to get >> the zero cross. A curve fit is one sort of estimator, there are others. A >> simple straight line fit over 4 or so points might do it. A higher order fit >> over a few more points is possible. Why does that matter? The fit improves >> your accuracy quite a bit. It also reduces your vulnerability to odd single >> sample issues like popcorn noise. Since you are running at a very low >> frequency 1/f noise can be an issue. >> >> Bob >> >> On Oct 12, 2014, at 2:37 PM, Hal Murray <[email protected]> wrote: >> >>> >>> [email protected] said: >>>> Does it matter that the ADC in the sound-card is probably clocked by a >>>> crystal clock that is 50ppm off and has bad ADEV? >>> >>> You can calibrate the clock on the ADC. >>> >>> One way is to feed a known reference frequency in on the other channel. >>> (That's assuming you have a stereo setup and don't need the second channel >>> for something else.) >>> >>> Another way is to compare the sample rate with the PC clock. That will >>> correct for any long term drift but may not track shorter transients. >>> >>> -- >>> These are my opinions. I hate spam. >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
