[time-nuts] GPS: ADEV or MDEV?
SAIDJACK at aol.com
SAIDJACK at aol.com
Sun Apr 22 04:52:21 EDT 2007
In a message dated 4/22/2007 01:11:30 Pacific Daylight Time,
hmurray at megapathdsl.net writes:
>Does anybody have a list of OCXOs with a separate ground pin for the oven?
>Some of the OCXO data sheets I've looked at have a Vref output. Seems like
>good idea. They already have good temperature control and that's most of
>trouble with getting a stable reverence voltage.
>Do any OCXOs also include the DAC internally? Isn't that the obvious next
* The Vref output of most OCXO's is from a Zener diode inside the can. These
typically have aging, thermal sensitivity and very poor voltage accuracy,
and there are much better monolithic high-precision, low-tempco voltage
reference available on the market now (Digikey etc). Depending on the internal
design of the OCXO, it may help the noise and stability to add an external 100uF
or larger tantalum cap to the Vref pin, since it's voltage is likely used as
the power supply for the Oscillator etc.
* Dithering the DAC to get better resolution is not a good idea, since it
will create spurs. If not through the low-pass filter into the EFC pin, then
through the power supply or ground rails into the PCB, or through all that
digital noise being generated by the constant updates of the DAC. It's better to
cascade two dacs through a matching network (coarse/fine DAC) and have as few
digital traces switch as possible. These DAC's as well as the matching
resistors, and especially the DAC voltage reference should be as low tempco as
possible. For the real time nut, the DAC's and Voltage reference will be
selected to have canceling Temperature Tempco's :)
One advantage of the cascaded DAC is that we can use 14, 16, or even 20 bit
DACS and their DNL/INL only needs to have 12+ bits of accuracy to make the
compound DAC linear enough for EFC control. For the dithered single DAC
approach the DNL/INL is totally critical (better than 1 LSB needed) to make the
dithering work well.
* Putting Digital circuits (read DAC) into the OCXO can is not a good idea,
we really don't want high edge rate (<10ns rise/fall time) inside the analog
OCXO can. It's gonna put noise on the sine wave. Plus it requires a lot of
additional pins (well, at least one).
* We investigated the effect of varying OCXO current on the
single-ground-pin Euro can (BTW: whoever designed that can without Kelvin Sensing was just
not thinking right at the time). One way around the problem is simply to solder
a thick ground wire to the can's case next to the VCC pin through which the
oven current will pass, and use the main ground pin (placed far away from the
VCC pin, another stupid idea) as the EFC Kelvin return. But keep the
soldering time very short since the oven may overheat otherwise.
As Bruce mentioned, measuring the oven current and compensating for it
electronically is another way to fix the problem. This is exactly what we do on
the Fury GPSDO, with a highly accurate 24 bit Sigma Delta instrumentation ADC.
unfortunately that part and the sensing element have their own very small
tempco's of course.
With careful layout, ensuring that the oven current does not pass in the
direction of the DAC/voltage reference etc it is possible to mitigate the Kelvin
sensing problem of Euro-can OCXO's to negligible levels.
* Bruce forgot to mention the use of smart algorithms like Kallmann
filtering to measure, and compensate for aging and temperature effects, this is
especially important for hold-over performance and also to minimize errors due to
diurnal temp changes etc. The algorithm should be as proactive as possible,
so that only small errors need to be corrected by GPS control (such as the rate
of change of the aging rate).
* maybe a bit above home-brew budgets, but being the lowest-cost
new-with-warranty GPSDO on the market as far as we know, the Jackson-Labs Fury GPSDO
almost fulfill's or exceeds all of Bruce's requirements :) I think our entry
level pricing is lower than most Z3801A's on Ebay these days *
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