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Frequency reference clocks should ..
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edpalmer42:

--- Quote from: Bud on January 26, 2023, 10:19:52 pm ---If you think logically for a sec, you want to lock your receiving device to Fref. You do not want it to lock to 2Fref, 3Fref, etc. Therefore having a clean sine wave Fref makes sense.

--- End quote ---

It depends what you want to use the reference for.

If all you're interested in is frequency, many pieces of equipment used some variation of a comparator to square up the reference so only the zero crossings were significant and a 'distorted' sine wave or square wave might be desirable.  But if you wanted to multiply the reference, then you might want a clean sine wave.

A common architecture has the internal reference phase-lock to the external signal.  That gives you a known quality reference - the internal one - that matches the frequency of the external reference.

Some equipment will lock to harmonics or subharmonics, some ignore them, some have that as an option.  Input impedance for the Ref-In jack varies from 50 ohms to hundreds of Kohms.  Amplitudes are similarly variable.  I typically run a daisy-chain from unit to unit and then terminate the chain with a 50 ohm terminator.  (Anyone remember 10Base2 ethernet?)  ;)

Ed
BillyO:

--- Quote from: Bud on January 26, 2023, 10:19:52 pm ---If you think logically for a sec, you want to lock your receiving device to Fref. You do not want it to lock to 2Fref, 3Fref, etc. Therefore having a clean sine wave Fref makes sense.

--- End quote ---
A square wave is a superposition of the fundamental and it's odd harmonics at specific amplitudes.  When we say it's a superposition it means their amplitudes are added together.  Those frequencies are not there on there own, so unless there is some strange filtering or math (like a Fourier transform) going on those harmonics will not be apparent in the waveform.  Logically then, it does not follow that it has to be a sine wave for the reasons you give.  And if there was filtering going on, it would be fairly steep filtering at precisely the required frequency.  Jus' sayin'
TimFox:
As I mentioned above, if the external reference input (driven by your reference clock) goes through a band-pass filter, which would be fixed-tuned, it only has to be good enough to be far down at twice the reference frequency, and only at the third harmonic for a good (symmetric) square wave.
BillyO:

--- Quote from: TimFox on January 27, 2023, 12:15:10 am ---As I mentioned above, if the external reference input (driven by your reference clock) goes through a band-pass filter, which would be fixed-tuned, it only has to be good enough to be far down at twice the reference frequency, and only at the third harmonic for a good (symmetric) square wave.

--- End quote ---
So 18dB/octave .. 24dB/octave?  This would only really be needed if you were expecting substantially less than ideal signals.  A well behaved Sq.W. should have a nice clean rising edge to trigger the phase measurement off as would a pure sine/triangle/saw.  But real life being real life I suppose these inputs need to be designed to extract the fundamental from potentially sub-par signals.

I've looked at the specs of the equipment I have to see what they demand for an external reference signal and all they ever give is a frequency and a minimum signal level.

Below is the signal I get from my HP 5300B.  It's not symmetrical, but nothing I have connected it to yet has complained.

Detlev:
Hi all,

Last year I tried to determine the jitter of audio DACs. At the time, I noticed that square-wave signals have a significantly better Allan-deviation than sine signals.

During the setup, the SDG6022 and the FCA3000 are both attached to the Leo Bodnar GPSDO. Nevertheless, the time deviations in the signal for sine waves are worse by a factor of almost three. The Allen deviation is 150ps for sine and 60ps for square.

I think the voltage noise is noticeable due to the flatter edge and worsens the triggering of the connected device. So I would always prefer a square to a sine.



Have a nice weekend  :)
Detlev
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