How master clock oscillators work
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hagtech
diyAudio Member

Join Date: Mar 2002
Location: Hawaii
Quote:
 all DACs drive charge into a capacitor
This is not true. Typically they output current, not charge. Often this current is converted to voltage internally via a transimpedance amplifier. The charge thing might be valid for sigma-delta types.

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 The voltage across a capacitor is proportional to its charge
Sort of misleading. I = Q/T. I = C*V/T.

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 but practical springs do not behave with the linearity implied by Hooke's Law
Correct, their force is proportional to the square of the displacement.

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 A quartz crystal is a more complex resonator, but it still suffers from the preceding limitations
They do not suffer from the same non-linearity as a spring.

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 The ideal master clock for a digital system would combine the high Q of the crystal with a good amplitude stabilisation system to produce a sine wave
Ok. I'm with you here. Now you're actually getting into Shannon theory, too (optimal bandwidth for BER).

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 followed by a buffer to isolate it from the comparator
Very good! Most folks miss this.

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 Even so, if the amplitude of the sine wave changes, then dV/dt at the zero crossing also changes
Frequency is independent of amplitude. Any hysteresis on the comparator works in both directions and hence cancels out. Oh wait, I see your point. Yes, if the amplitude is varying with time then it will directly lead to a corresponding jitter component.

Hence, I believe you summarize that a good clock will be a sinewave type with amplitude stability (voltage regulation), and a buffer to drive the comparator. I gotta agree with you there! There's more to the story, though. You have to transfer that quality signal into another circuit without distortion. That requires transmission lines and impedance matching, etc.

jh

 9th March 2006, 10:49 AM #3 EC8010   diyAudio Moderator Emeritus     Join Date: Jan 2003 Location: Near London. UK Yes, it's true that DACs output current, but many produce variable width pulses (PWM) so that means they deliver charge (Q = It), rather than current, and when connected to a capacitor; V = Q/C. Springs might not be perfectly linear, but it certainly isn't a square law. Otherwise, how would kitchen scales based on springs work? Yes, you're right, my conclusion is that an ideal clock would start with an amplitude stabilised sine wave oscillator buffered away from the following comparator. And yes, we're now into transmission line problems in actually using the output. All the more reason to put the clock next door to the DAC. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
 9th March 2006, 11:25 AM #4 Elso Kwak   Banned   Join Date: Dec 2001 Location: Zamboanga, City of Flowers, Mindanao Masterclock? Hi EC8010, I thought you were going to explain how a Colpitts oscillator works..... Better build and give us an example of a circuit with your ideas implied. I am missing the analysis of various types of crystals: AT, BT, SC-cut. Comparing with mechanics/ physics/ springs is confusing to me. Cannot not confirm your findings about buffers, tried them though. My 45.1584MHz clock has a buffer though (for other reasons). High input currents into the comparator?? AD8561's input capacitance is 3pF typical. [current is a stream of charges, hahahaha] The masterclock is also needed in the transport as all the decoding and processing is done there. If you find a way to transport the clock without losses through a transmission line it does not matter if it is located in the player or in the separate DAC. Best is anyway using I2S Direct if possible and as I am using a NON-OS DAC I don't need the masterclock in the DAC (only wordclock and bitclock) so I leave the masterclock in the transport.
hagtech
diyAudio Member

Join Date: Mar 2002
Location: Hawaii
Quote:
 Springs might not be perfectly linear, but it certainly isn't a square law
Really, they are! I had it wrong (from memory), though, I said 'force'. Actually, it is the 'energy' that is proportional to the square of the spring displacement.

U = 1/2 * k * x*x;

where k is the spring constant. To get force you then differentiate (dU(x)/dx).

No idea how they get scales to work. Maybe you just keep displacement really low?

jh

 9th March 2006, 05:44 PM #6 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland Hooke's Law really does work pretty well for elastic materials (like spring steel). And since weight is force, kitchen scales work pretty linearly. As do meter movements. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 9th March 2006, 10:52 PM #7 andrei   diyAudio Member   Join Date: Dec 2002 Location: Canada Look up to hams to see some serious low noise oscillator designs - for example, the one for Linrad hardware. If you wonder why running the oscillator in Class C is a good thing, check out this paper.
Elso Kwak
Banned

Join Date: Dec 2001
Location: Zamboanga, City of Flowers, Mindanao
Quote:
 Originally posted by andrei Look up to hams to see some serious low noise oscillator designs - for example, the one for Linrad hardware. If you wonder why running the oscillator in Class C is a good thing, check out this paper.
A FET works better in the oscillator.
I could not open the second file!

 9th March 2006, 11:12 PM #9 EC8010   diyAudio Moderator Emeritus     Join Date: Jan 2003 Location: Near London. UK Neither could I! __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
andrei
diyAudio Member

Join Date: Dec 2002
Quote:
 Originally posted by Elso Kwak A FET works better in the oscillator.
Take a closer look at the phase noise measurements of Linrad oscillator - they are exceptionally good. Elso, if you have measured the phase noise of your clock, I would love to see it, especially if you think they are better.

Quote:
 I could not open the second file!
I guess I am enjoying an institutional subscription... Here is the direct link to the paper.

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