[time-nuts] Clock level conversion 5V -> 3.3V

[Bob Camp]

Hi

It will indeed be better for phase noise to do away with the resistive divider and get faster edges. 

Of course there are indeed resistive dividers that don’t slow things down. It’s unlikely that a divider with a 10 ohm output impedance is going to tack on to the output of an OCXO.

————————

The real point is that (in this case) you can get the job done for less than 10 cents with a single gate chip. They are available in many packages from many people. There are indeed chips that have a bit less noise than others. With the OCXO that we’re talking about here, it’s not worth going crazy to find this or that. This being Time Nuts, if you do decide to go crazy - as long as it’s saturated silicon, faster is more quiet than slower. 

Bob

On Oct 1, 2014, at 9:21 AM, David McGaw <n1hac at dartmouth.edu> wrote:

> Would it not be better for phase noise to use a logic gate with a fast transition than a resistive divider that would be slower due to the load capacitance?
> 
> David
> 
> 
> On 10/1/14 7:09 AM, Bob Camp wrote:
>> Hi
>> 
>> Ok, so it’s not a super duper low phase noise OCXO. It’s also at a reasonably high frequency.
>> 
>> I’d just drive it into a 5V tolerant input and move on. There are lots of logic gate chips out there that will run from 3.3 and accept 5V inputs. Use something reasonably fast and it will do a pretty good job.
>> 
>> Bob
>> 
>> On Sep 30, 2014, at 10:11 PM, Mark A. Haun <haunma at keteu.org> wrote:
>> 
>>> Hi Bob,
>>> 
>>> The OCXO is one of those 26-MHz ebay Pletronics from a couple years
>>> back.  I would like to not degrade its close-in phase noise (quoted as
>>> -100 dBc @ 10 Hz, -130 dBc @ 100 Hz).  Thinking about Said's suggestion
>>> to phase lock a higher-frequency sampling clock to this, with a loop BW
>>> somewhere in the 10-100 Hz range.
>>> 
>>> I have seen a resistive divider used in a similar application, but
>>> wondered if I could save the couple dozen mA they were spending.
>>> 
>>> Mark
>>> 
>>> On Tue, 30 Sep 2014 20:18:56 -0400
>>> Bob Camp <kb8tq at n1k.org> wrote:
>>>> Hi
>>>> 
>>>> How quiet does it need to be?
>>>> 
>>>> Put another way - how good is the OCXO?
>>>> 
>>>> What frequency are we talking about?
>>>> 
>>>> What is the phase noise “need” after you get to 3.3V (is there a
>>>> system spec)?
>>>> 
>>>> Bob
>>>> 
>>>> On Sep 30, 2014, at 5:46 PM, Mark Haun <haunma at keteu.org> wrote:
>>>> 
>>>>> Is there a "best" way to do this without adding phase noise?  For
>>>>> example, a 5V OCXO into an ADF4002, or a 3.3V or even 1.8V logic
>>>>> input.  Is a resistive divider the way to go?
>>>>> 
>>>>> Mark
>>>>> 
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