[time-nuts] Digital Mixing with a BeagleBone Black and D Flip Flop

Bob Camp kb8tq at n1k.org
Sun Oct 12 20:30:15 EDT 2014


Hi

If odd “almost 10 MHz OCXO’s were more common, you could indeed have a bit more freedom on the offset. DDS is sometimes used. DDS spurs (which can be *very* close in) can be both hard to predict and hard to spot in the data. An OCXO is a much better bet unless you have a lot of time on your hands or a really good spectrum analyzer. 

Much easier to get a $20 Trimble OCXO at auction and ground the EFC pin….

Bob

On Oct 12, 2014, at 8:25 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> Bob,
> 
> I know, and I know you know. Just let others see how things connect up.
> 
> Still have some 10.000110 MHz OCXOs lying around.
> 
> Cheers,
> Magnus
> 
> On 10/13/2014 02:15 AM, Bob Camp wrote:
>> 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 <magnus at rubidium.dyndns.org> 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 <hmurray at megapathdsl.net> wrote:
>>>> 
>>>>> 
>>>>> anders.e.e.wallin at gmail.com 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.
>>>>> 
>>>>> 
>>>>> 
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