[time-nuts] Unified VCXO Carrier Board

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Oct 23 19:16:24 EDT 2015

On Friday, October 23, 2015 05:31:46 AM Charles Steinmetz 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 If amplier PN 
I split the comparator output and feed it to 2 separa
> posted, which agrees very closely with my test results, at
> 10MHz/15dBm there should be essentially no change in the PN 
> to the results you obtained with filtering enabled.  If you see a
> significant difference, then something is causing anomalous results.
> Best regards,
> Charles

The comparator circuit measured was the front end of David Partridge's 
divider. I merely measured the 10MHz output.

I thought that I had made measurements for various filter settings and 
input levels. If I did, I don't appear to have saved them. I certainly named 
the various TIM files to indicate the filter settings.

I'll try and repeat the measurements for various input levels and filter 
One thing that I have found is that at low offset frequencies the measured 
PN is substantially reduced when air currents and other sources of thermal 
fluctuations are reduced. Even the effect of a thin piece of paper used as 
an air current shield can be easily seen.
With careful shielding from thermal fluctuations I measure the low 
frequency offset PN to be substantially lower than the datasheet values.
I've seen this effect with everything for which I've measured the PN.
I may machine a custom housing for the evaluation board rather than just 
using an oversize die cast box.
This may take a while as I'll need to check the compatibility of various 
hardware/software with Windows 10 that runs on my Quadcore laptop.
Failing that I do have a quad core PC with water cooled CPU that runs 
Windows 7.

One problem with comparators when attempting to measure their PN is 
that they don't have sufficient output to drive the TimePod input directly. 
An amplifier is required. To reduce the Amplifier PN contribution I split the 
comparator output and drive a separate amplifier from each splitter output 
and then use cross correlation. This makes the amplifier PN much less 
critical. Finding low PN amplifiers with relatively low gain ( ~ 10dB or so) 
with low distortion at 13dBm or so output is somewhat problematic. A low 
noise single transistor discrete amp with 30dB or more reverse isolation 
with a gain of 10dBm ought to be feasible at 10MHz.

> 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.

I dont always have convenient access to my email machine and sometimes 
resort to using a browser to compose a reply via my ISP's interface to my 
This apparently messes up the reply so it is associated with a different 
message to the one I believed I was replying to. This is a fairly recent 

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