[time-nuts] OCXO and fluctuations after EFC adjustment (Ben Bradley)

[Leon Pavlovic]

Speaking of high-performance crystal oscillators, EFC inputs and Tantalum
capacitors, how to select a good decoupling capacitor for its EFC input?

I'm attaching two PN measurements made yesterday on a HP3048A. There are
two 100MHz crystal oscillators in the setup, (1) a XCO made by myself,
which I know from previous measurements on a E5052 that has a stable! PN of
-130dBc/Hz at 100Hz and below (Vtune from HP3048A is connected to this
XCO). The other one (2) is a commercial Connor-Winfield OCXO, specified as
-125dBc/Hz at 100Hz. I'm not sure if this one has a stable PN. The EFC
input is decoupled by a 1uF ceramic in parallel to 33uF/16V Tantal and 10kR
to ground, and connected by another 10kR to its Vref output.

So where's the problem? The two measurements attached were taken only some
minutes apart. The first one is expected, the second one is a disaster
below 1kHz offset. Is it possible that the heated Connor-Winfield OCXO
affected the Tantal capacitor on its EFC node after some minutes (both are
in close proximity on a PCB), increasing the leakage or whatever phenomenon
that corrupted its PN (modulation of the EFC voltage)?

Possible reasons for a fluctuating/unstable PN to consider:
- one of the oscillators has unstable PN
- the Tantalum capacitor should be replaced by a film or a ceramic 10uF?
- broadcast FM stations influenced the second measurement
- ?

Thanks for any advice where to look at the problem ;)
Leon


---------- Forwarded message ----------
From: Ben Bradley <ben.pi.bradley at gmail.com>
To: Discussion of precise time and frequency measurement <
time-nuts at lists.febo.com>
Cc:
Bcc:
Date: Sat, 11 Apr 2020 23:22:29 -0400
Subject: Re: [time-nuts] OCXO and fluctuations after EFC adjustment
Despite my interest in the precision high-end of electronic design
(and thus being a subscriber to this list), I have very little
experience with such high-fallutin' designs. Still, I've done and seen
some "interesting" things in my career. It's amazing how the minutiae
of even jelly bean components can cause product failures.

A few decades ago (many of my jobs were engineering positions at
companies that made equipment for the POTS phone line), the company I
was with had some rather large surface-mount ceramic capacitors that
were failing short in a new product that used them across the phone
line (they were RATED for this application right there on the data
sheet).  The concern was our product would some day short out the
phone line and someone with another phone on the line would have an
emergency and wouldn't be able to call 911 to get help. The possible
legal liability to the company was obvious. In a discussion with other
engineers and managers, someone wondered if it was the way the board
was manufactured, maybe the solder process caused some caps to crack,
or whether the failures were strictly the fault of the capacitor
manufacturer. I volunteered to test the caps in an environmental
chamber that wasn't being used at the time, putting them near the edge
of, but still within, specs. I made a jig that put a bias voltage on
many in parallel (using a mechanical spring connection, not soldered)
and left them for a few days/weeks. Out of 100 to 150 devices, about 5
or 10 became unacceptably leaky. I wrote up my report and emailed it
to the other engineers and managers involved, and didn't hear back
anything more about it. The good news (!) is the product wasn't in
production for very long for several reasons, more recently of course
that the POTS phone line is no longer the most common form of
telecommunications. The bad news is it's no longer helpful to have
POTS product design on my resume.

More recently, I saw this Kemet presentation on Digikey about tantalum
capacitors. Certainly for aluminum electrolytic capacitors, the rated
voltage is "the rated voltage" and as long as the capacitor never goes
ABOVE that voltage (and has no overcurrent that would heat it up,
etc.), the cap is good for its combination of temperature and lifetime
rating. I (and as far as I know, everone I've known) assumed this was
the same for tantalums, but it appears that's not the case (this
presentation mentions several failure causes and shows how they are
multiplicative). As you go from 1/2 rated voltage to full rated
voltage, the chances of a tantalum failing goes up substantially. The
implied rule seems to be for maximum reliability, don't operate a
tantalum above HALF the rated voltage. I'd heard a lot of anecdotal
things about tantalums suddenly shorting out for this or that reason,
but hadn't heard of this, and here it is straight from the
manufacturer.
https://www.digikey.com/en/ptm/k/kemet/derating-guidelines-for-surface-mount-tantalum-capacitors/tutorial
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