[time-nuts] Unified VCXO Carrier Board

Bob Camp kb8tq at n1k.org
Fri Oct 23 11:40:56 UTC 2015


Hi

There are a wide range of OCXO’s listed for this project. I certainly
do not have a sample of ever single one of them. For the ones that 
I *do* have samples of, a properly done CMOS gate sine to square
converter will not degrade the close in phase noise or ADEV of the OCXO.
Based on TimePod measurements, I believe it would be adequate for 
all the ones I’ve seen specs for. 

With far removed phase noise spec’d into the “past 180 dbc/Hz” range on
some parts - no logic is going to handle that. Since the CPLD on the board
will floor out well before that, doing a “perfect” conversion and then degrading
it as soon as you hit the bulk logic does not make a lot of sense. 

Bob

> On Oct 23, 2015, at 5:31 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> Bruce wrote:
> 
>> Your statement about the PN of comparators conflicts with my measurements. The LTC6957 evaluation board had an 18dBc/Hz lower phase noise floor than a comparator circuit with 10MHz 15dBm inputs. However I only measured a single comparator circuit. The Holzworth sine to CMOS converter had a comparable PN to the LTC6957-4.
>> I haven't, as yet measured the PN of an optimised Wenzel circuit.My setup for this measurement had a PN floor of around -180dBc/Hz.
> 
> There are many, many ways of getting unnecessarily poor PN performance from comparators (including Wenzel-style squarers) -- one has to make sure not to make any of myriad mistakes in both design and execution.  You didn't say which comparator you tried, or in what circuit, so I'm not in a position to suggest things to check (or to confirm that the comparator you tried performs similarly poorly in my tests, if that is the case).
> 
> One sanity check you can try -- disable the filtering on your 6957 eval board.  According to the LT data presented in the chart I posted, which agrees very closely with my test results, at 10MHz/15dBm there should be essentially no change in the PN compared to the results you obtained with filtering enabled.  If you see a significant difference, then something is causing anomalous results.
> 
> Best regards,
> 
> Charles
> 
> 
> ps.  You often respond to one message by replying to a different message, as you did in this case.  It would be helpful for someone who just joins a thread, and for continuity in general, if you would reply to the message to which you are actually responding.  That way, readers who are new to the thread will have the context they need, and your interlocutor will have his or her previous message conveniently available to refer to in any further message.
> 
> 
> 
> 
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