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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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