[time-nuts] Unified VCXO Carrier Board

Charles Steinmetz csteinmetz at yandex.com
Tue Oct 27 08:50:25 EDT 2015

Gerhard wrote:

>Since the LM329 was mentioned above, I tried it also. Another surprise.
>0 dB = 1nV/sqrt Hz

The datasheet for both NS and LT LM329s shows broadband noise of 
<70nV/sqrtHz, with a corner at ~100Hz, rising to a touch over 
100nV/sqrtHz at 10Hz.  That is consistent with the LM329s I have used.

So, according to the datasheet, the 10Hz noise ought to be a touch 
above 40dB on your graph and the curve should level off above 100Hz 
at about 30dB -- if I have done my dBs right.  Still somewhat higher 
than most of the other diodes you tested, but not outlandishly so.

Your data seems to be *much* worse than this, with a 1/f noise corner 
at ~10kHz (!!), rising to what appears to be about 1mV/sqrtHz (!!) at 
10Hz.  Is it possible you got fakes?  (I know someone who bought a 
quantity of LTZ1000s that turned out to be fakes.  Not just abused 
parts dismounted with a blowtorch and with new leads welded on -- 
actual, never-were-real-LTZ1000s.  And they came from a second-tier 
distributor, not an ebay scammer, although further investigation 
revealed that the distributor had been dropped by LT some time 
prior.)  Or is the one you tried just broken?  In any case, the 
results you obtained are several orders of magnitude worse than the 
specs for the part.

>Are there differences between Fairchild and Motorola?  [MPSH81/MMBTH81]

Probably, although I don't know if any differences there are would 
have a noticeable effect on the performance of the circuit.  My 
current stock of H81s is all Fairchild.  I know I have used Motorolas 
in the past, and never had any reason to question whether there was a 
significant difference -- but I never compared them side-by-side, 
either.  But I don't think that Motorola/ONsemi has made them for a 
long time, anyway.  Indeed, I see now that even the Fairchild 
through-hole part (MPSH81) seems to be gone.  The SMD part (MMBTH81) 
seems to be available still.

Best regards,


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