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Sun Nov 2 21:23:46 UTC 2008


used in a GPSDO the LESS there is the need to use a high resolution Dac.
The FS725 rubidium with it's 2e-9 external EFC range, would not seem=20
to need more than the most simple 12 to 16 bit EXTERNAL tracking Dac.=20
True that its internal Dac needs to have very high resolution if it is =
used=20
to lock an internal VCXO that has a range of say 1e-6.

Question:  Is the Aging rate of these low noise OCXO units poor enough =20
that you could not use a couple of fixed precision resistor and/or a pot =

for course adjustment and reduce the EFC range of the DAC by say 1/100 =
?.=20
Are there low noise OCXO (that are being used in GPSDO) whose long term=20
drift over say 6 months would need more than 1E-8 or so of  'automatic' =
turning?

And concerning the 10811A. Should one consider reducing its EFC=20
tuning range by say 2 to 20 to ease the requirements of the Dac?=20
Even if the EFC tuning range was reduced by just a factor of two,=20
It could then be done with just a standard 0 to 5 volt Dac =20
instead of the -5 to +5 that I saw suggested by its spec.
I would think that a well aged unit would be much better than its worse =
case=20
spec of 1E-7 per year, And even if not, I'm guessing  that many of the =
time-nuts,=20
would be very welling to trade off  it needing a have few extra manual =
adjustments=20
in order to get better performance.

WarrenS

*****************
----- Original Message -----=20
From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
To: "Discussion of precise time and frequency measurement" =
<time-nuts at febo.com>
Sent: Monday, December 01, 2008 3:38 AM
Subject: Re: [time-nuts] V standards


> Mike
>>   >Mike
>>
>>   > In testing  high resolution DACs (20+ bits) used in  GPSDOs  etc =
a
>>   > high resolution DVM (or equivalent) with low noise and  good =
short
>>   > term stability is useful if not essential.
>>
>>   > Monotonicity and  perhaps linearity together with good  short =
term
>>   > stability are generally more important than absolute accuracy.
>>
>>   > Higher resolution is usually accompanied with higher accuracy.
>>
>>   > DVMs like the 3457A. 3456A etc dont have sufficiently low noise =
or
>>   > good enough  short  term stability for testing  DACs  with  24 =
bit
>>   > resolution.
>>
>>   >Bruce
>>
>>   I wonder if 20+ bits is even realistic for a frequency reference.
>>
>>   That would  give a LSB of 1/2^20=3D9.53e-7, or 1ppm, and  none  of =
the
>>   voltage references  discussed will maintain this accuracy  over =
long
>>   periods.
>>
>>  =20
> 24 bit resolution isnt required when using a 10811A class OCXO with a
> total EFC adjustment range of about 1E-7.
> Even 20 bits is a bit more resolution than actually required in this =
case.
> However there are low noise OCXOs with EFC adjustment ranges of 1E-6 =
or
> more.
> In a GPSDO long term stability of the EFC DAC offset and gain isnt
> critical except when the GPSDO is in holdover.
>>   And what would a 24-bit DAC be used for? From  previous =
discussions,
>>   high accuracy sources, like H-Masers are not adjusted. And  it =
would
>>   seem silly to put such a high resolution DAC on a OCXO.  That =
leaves
>>   Cesium, which  I understand are used in GPS satellites  and  do =
need
>>   adjustment, but  I  don't   have   any   information  on  the =
tuning
>>   sensitivity to figure the effect 1 LSB would have on the frequency.
>>
>>   I haven't had much luck finding a true 24-bit DAC. There  are =
plenty
>>   of stereo dacs, but they can have gain drifts of 100ppm/C,  which =
is
>>   useless for a reference.
>>
>>  =20
> No you have to build your own.
> The trick is doing it without requiring impossibly accurate and stable
> resistors or resistor ratios etc.
> Since the DAC update rate is relatively slow in such applications
> (GPSDO) indirect techniques that are inherently monotonic can be used.
> The FS725 rubidium standard has an internal 22 bit DAC.
> Hydrogen masers usually include similarly high resolution DACs.
> Such DACs are usually constructed by combining the outputs of 2 lower
> resolution DACs with some overlap.
> The drawback being the relatively large differential nonlinearity when
> the MSDAC output changes.
> This increases the settling time of the discipling loop in the =
vicinity
> of such changes.
> However such MSDAC output changes don't occur very often.
> Another application for such high resolution DACs is in accelerator =
beam
> steering.
> In this case frequent calibration is used together with suitable
> software to avoid such large differential nonlinearities.
>=20
>>   With a  5V reference, a 24-bit DAC would give a LSB  of =
5e9/2^24=3D298
>>   nanovolts. If  I had to test one, and didn't have a  3458A,  I =
could
>>   use a 3456A. It has a resolution of 100nV on the 100mV range,  so =
it
>>   could verify the bottom portion of the DAC from zero to  100mV. =
Once
>>   the lower  8 bits are confirmed good, the rest of the  DAC  could =
be
>>   checked by  exercising each high-order bit singly,  then  in =
various
>>   combinations with the other bits.
>>
>>  =20
> I had such techniques in mind for checking the monotonicity.
> Could also check against a KVD (e.g. Fluke 720A).
> However the shipping charges are rather high.
> It is possible to achieve a readout resolution of 100nV using a 3457A =
on
> the 3V range.
> However this is only available via the GPIB.
> Similarly even a 34401A can achieve a resolution of 100nV on the 10V
> range but only via either the GPIB or its serial interface.
> However the 300nV LSB is uncomfortably close to the DVM noise level =
and
> DVM drift isn't insignificant.
> For this application the DAC transfer function only has to be =
monotonic
> so testing requirements are somewhat relaxed.
>>   Another method  would  be  to use two 24-bit DACs  and  a  AD8571 =
to
>>   measure the  difference between them. The AD8571 could be  set  to =
a
>>   gain of  100,  so  298nV becomes 29.8uV  which  is  well  within =
the
>>   capability of a 3456A.
>>
>>  =20
> The LTC1151 is perhaps a better choice in some respects in that it  =
can
> use -15V and +15V supplies allowing a greater input voltage range
> simplifying the input overvoltage protection.
> Although the AD8571 power supply could be bootstrapped to achieve a =
much
> larger input range.
> Even lower drift and noise is possible if one builds one's own chopper
> stabilised preamp.
>>   The test  would  be  to  set  both  DACs  to  zero  and  measure =
the
>>   difference in output voltage. It should be close to zero.
>>
>>   Next, set  the LSB of the reference DAC to 1. The 3456A  should =
read
>>   close to 29.8uV.
>>
>>   Then set  the LSB of the test DAC to 1. The 3456A should  read =
close
>>   to zero.
>>
>>   Follow this  procedure  with   each   bit   in  turn  to  verify =
the
>>   functionality, then  test various combinations to check  for  two =
or
>>   more bits  that  are  stuck  together.  The  LSB  could  be  used =
in
>>   conjunction with  the  bit being tested  to  prevent  saturating =
the
>>   AD8571.
>>
>>   The above tests are not as good as a dedicated test for  a =
precision
>>   DAC, but might serve in lieu of spending $4k to $7k for a 3568A.
>>
>>   Regards,
>>
>>   Mike Monett
>>
>>  =20
> Bruce
>=20
>=20
>



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