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From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: faster DDS clock
Date: Sun, 22 Sep 2024 13:32:11 -0700
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On Sun, 22 Sep 2024 18:37:54 +0200, Lasse Langwadt <llc@fonz.dk>
wrote:

>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:
>>>
>>>> On 9/21/24 17:42, 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.
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> The usual trouble is that you have to get the other edge from somewhere. An
>>>>>>>>>> xor gate and an RC is typical.
>>>>>>>>>>
>>>>>>>>>> Any asymmetry in the square wave turns into subharmonic jitter.
>>>>>>>>>>
>>>>>>>>>> A 2:1 PLL would probably get my vote.
>>>>>>>>
>>>>>>>>>
>>>>>>>>> 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.)
>>>>>>>>
>>>>>>>> Cheers
>>>>>>>>
>>>>>>>> Phil Hobbs
>>>>>>>
>>>>>>> An LVDS line receiver would make a pretty good comparator, after the
>>>>>>> filter.
>>>>>>>
>>>>>>> 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%.
>>>>>>>
>>>>>>> I don't know if this FPGA could internally clock on both edges.
>>>>>>>
>>>>>>> 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
>>>>>>
>>>>>> or 3x8bit@330MHz
>>>>>> https://www.lcsc.com/product-detail/Digital-to-Analog-Converters-DAC_Analog-Devices-ADV7125JSTZ330_C662165.html
>>>>>>
>>>>>> 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
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>> 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.
>>>>>
>>>>
>>>> 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
>>>>
>>>
>>> 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

Maybe some gamers need multi-gigabit bandwidth. 

I can watch a movie that arrives at my house over a cable modem and
CAT5 or WiFi to my computers. So a CAT5 from the computer to a monitor
should be OK.


>
>there are plenty of monitors that can use USB-C
>

That's sensible. The HDMI connectors and cables are klunky.

We really only need ethernet, USB-c, and one unified wireless/wifi
network.