Re: faster DDS clock

On 21/09/2024 12:49 am, john larkin wrote:

On Fri, 20 Sep 2024 00:30:28 +0200, Lasse Langwadt <llc@fonz.dk>
wrote:
 

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:
>

john larkin <JL@gct.com> wrote:

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

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.
>
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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>

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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.

>
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
>
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.
>

>
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

 That's cool. I need 4 DDSs so I'd use two of them, but it
is still appealing. It looks like I'll have to use an Efinix T130 FPGA
to get the RAM I need for waveform storage, so I'll have tons of logic
and i/o's to go hard parallel to the DACs.
 I could use 10 or 9 or 8 bits if that is easier to route, and the
current outputs dump right into the right kind of filter.
 I have a Spice model of a DDS clock generator. I wonder how awful a
lowpass filter I can get away with. CLC? Or even RC?

Active RC filters are a lot easier to design than anything involving an inductor. Close tolerance inductors are rare and expensive when they are available. 25 MHz calls for fast op amps, but you can find them.
Williams and Taylor is a useful reference - not as user-friendly as Don Lancaster, but a whole lot more comprehensive. And they do cover finite impulse response filters.
--
Bill Sloman, Sydney