Bruce
If one attenuates the amplifier output by a factor of 1000 then a 10811A,
with an aging rate of 5E-10/day, will require manual retuning every few
hours.
Typical aging may be better than this but this would still require manual
tuning every few days.
You are not taking into account that not all OXCO are as insensitive to
their EFC voltage.
On a 5Hz/V Tbolt OXCO, 10mv is 0.05Hz = 5e-9 and 100mv is ... (well, it is
ALMOST never going to drift that much once it is run in, unless you break
it).
AND
I do agree one should be aware of worse case errors especially if designing
a care free commercial product with NO access to adjustments,
BUT sometimes you seem to miss the practical simple solutions.
You don't always need a 24 bit DAC to make a great performing unit.
Sometimes a bit of original thinking can be just as useful.
For the home nut, real values of their actual unit is more important if they
are just building a one of a kind.
The EFC voltage ageing slope of My dual oven 10811 that is disciplined by a
tbolt is now a near linear function at 200 uv /day (-2e-11/ day)
(also has small temp dependence of a couple 1e-12 /C) (Dac gain =
0.909Hz/volt, More range than a single 10811 because there is no manual
adjustment)
and on another "standard" Tbolt unit the EFC voltage variation is a non
linear, non accumulative, 100uv max peak to peak variation over 3 days.
<<1e-11 /day ageing, It's EFC change is mostly due to temp variation of
about 1e-11 / C) (Dac gain = -2.85Hz/volt)
So having a max EFC range of 10 to 100mv would work OK on these two not so
typical OXCO.
ws
******************
----- Original Message -----
From: "Bruce Griffiths"
If one attenuates the amplifier output by a factor of 1000 (effective EFC
range is then ~1E-10) or so then a 10811A with an aging rate of 5E-10/day
will require manual retuning every few hours.
Typical aging may be better than this but this would still require manual
tuning every few days.
Bruce
******************
WarrenS wrote:
John ask
Translating nV/sqrt(Hz) to something
practical is basically the assistance I'm looking for here.
I would appreciate anyone being able to teach me a bit more about this.
If that is ALL you want to know, That's easy and quick.
For this application sounds like you already know ALL you need to know
about that, nothing.
Putting a 1 sec or so RC filter at the EFC input, takes care of all that
AND if you want it even better,
and to get the long Control loop time constants needed, JUST reduce the
(loop) gain, don't need no BIG caps.
That is attenuate the output of the control amp by typically a hundred to
a thousand instead of multiplying by 1.6 and add a fixed, adjustable,
stable, offset source. (electrical or mechanical)
The Buffer amp is not going to be your problem.
ws
************
[time-nuts] GPSDO Design
John Foege john.foege at gmail.com
Hi All,
Quick question for the more experienced members here with GPSDO
design/operation. Let's assume I'm using a 4096 phase comparator chip
followed by some kind of long time constant lowpass loop filter,
whether it be analog or digital, is not of concern for the following
question.
Obviously using a 74HCT4096 would mean that my EFC voltage range would
be approx. 0-5V. If I wanted to use an OCXO with say a 0-8V EFC
voltage range, then I would be inclined to simply use an op-amp
amplifier with a gain of 1.6 to scale the EFC voltage accordingly.
But not just any op-amp would do I take it? High-speed would of course
be of no concern. Also low-offset would be of little concern, as the
PLL would work to correct this, and it therefore seems to be
negligible. However, the part that's got me thinking is noise.
Obviously any noise at the ouput of the amp would adversely affect the
frequency stability of the OCXO.
I thought the best way to control this would be to use an extremely
low noise op-amp employing a rather large compensation cap to give me
a rather small bandwidth, perhaps only a few hundred hertz.
Anyone have experience with this? Assuming I have an OXCO with a max.
pulling range of 1ppm or 1e-6 over a 10V range, then I effectively can
pull 1e-7 per volt. This translates to 1e-10 per millivolt and 1e-13
per microvolt. Assuming that is a logical conclusion, then for a good
OCXO, in which I can at best hope for 5e-12 stability for tau=1s (e.g.
HP10811A), I would strive to to keep the noise at such a level that it
is an order of magnitude better than the best short term stability
figure. Accordingly, then I should shoot to keep any noise under 1
microvolt?
I don't have much experience with noise calculations. I know it is
specified in nV/sqrt(Hz) generally. Translating this to something
practical is basically the assistance I'm looking for here.
I would appreciate anyone being able to teach me a bit more about this.
Thank you in all in advance.
Sincerely,
John Foege
**************
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
