[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  

Hi guys,
Some comments:
* 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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