Re: faster DDS clock

On 9/22/24 03:40, john larkin wrote:

On Sat, 21 Sep 2024 18:37:26 -0700, john larkin <JL@gct.com> wrote:
 

On Sun, 22 Sep 2024 03:11:53 +0200, Lasse Langwadt <llc@fonz.dk>
wrote:
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On 9/21/24 17:42, john larkin wrote:

On Fri, 20 Sep 2024 00:30:28 +0200, Lasse Langwadt <llc@fonz.dk>
wrote:
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On 9/19/24 05:57, john larkin wrote:

On Thu, 19 Sep 2024 03:28:09 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
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john larkin <JL@gct.com> wrote:

On Wed, 18 Sep 2024 21:56:59 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
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john larkin <jl@650pot.com> wrote:

Assume a DAC being driven with an n-bit sine waveform at some clock
frequency, and then a lowpass filter and a comparator, generating a
programmable frequency clock.
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Why not use both edges of the comparator output as our clock? That
de-stresses everything by 2:1, which could well be a net win on jitter
and such. Or gives twice the clock frequency with the same parts.
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The usual trouble is that you have to get the other edge from somewhere. An
xor gate and an RC is typical.
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Any asymmetry in the square wave turns into subharmonic jitter.
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A 2:1 PLL would probably get my vote.

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I'm trying to make things cheaper and simpler. I need a clock that's
programmable up to maybe 20 or 25 MHz, and it would be nice to use
some relatively cheap dual DACs.

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Understood.  A Joergesque solution would be to use a discrete FET as part
of the RC + XOR, and dork the ON resistance to square up the duty cycle.
(He’d probably use a CD4007 DIY gate package to do a few at once. Maybe
it’s possible to use a TinyLogic inverter with VDD open.)
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Cheers
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Phil Hobbs

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An LVDS line receiver would make a pretty good comparator, after the
filter.
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If I have enough balls (no pun intended) I can use an LVDS input of my
FPGA. One could even servo that to exactly 50%.
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I don't know if this FPGA could internally clock on both edges.
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But I can get a TI DAC908 for under $5, so may just clock that fast,
brute force at 100 MHz or so. That would make 20 MHz with a dinky
filter.
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this will give you 3x10bit@140MHZ DACs for about the same price
https://www.lcsc.com/product-detail/Digital-to-Analog-Converters-DAC_Analog-Devices-ADV7123KSTZ140-RL_C172724.html
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or 3x8bit@330MHz
https://www.lcsc.com/product-detail/Digital-to-Analog-Converters-DAC_Analog-Devices-ADV7125JSTZ330_C662165.html
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if you opt for the Chinese clone, less than half for 3x10bit@240MHz
https://www.lcsc.com/product-detail/Digital-to-Analog-Converters-DAC_HTCSEMI-HT7123ARQZ_C2886392.html
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It occurrs to me that the use for a 3-channel fast 10-bit DAC is to
drive a color CRT monitor, which I expect nobody makes any more.
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It's for VGA (that's why it has sync and blank input)
While VGA is old I doubt it is going anywhere soon, it still widely
used, go buy a server and it has VGA
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Seems silly to take digital data, convert it to analog, ship it six
feet, and convert it back to digital.

 And why do we have those firehoses of HDMI connectors and cables? Why
not use Ethernet or USB  out to a monitor?

HDMI  is ~10-20 times the bandwidth of regular ethernet
there are plenty of monitors that can use USB-C