[time-nuts] Fury PR in GPS World... Part 2

SAIDJACK at aol.com SAIDJACK at aol.com
Thu Jul 19 20:18:03 EDT 2007

In a message dated 7/19/2007 16:37:05 Pacific Daylight Time,  
pvince at theiet.org writes:

>Do I  understand that a version is available 
>that takes in an external 10MHz  clock and internally disciplines that 
>to GPS?  Perhaps by DDS  techniques?

>    Regards,

>        Peter

Hi Peter,
Thanks so much for your question! Let me try to explain how Fury can be  used:
We can steer an external 10MHz source with an analog voltage (0V to 5) with  
24+ bits of resolution, so the actual disciplining is happening in the 
external  steerable source via EFC voltage from the Fury.
There are two ways an external source can be used: the first is to remove  
the on-board GPS receiver, and feed an external 1PPS into the unit (say from a  
dirty Rubidium that needs to be cleaned up). This is often done where GPS  
reception could be denied (military apps for example). The Fury has a  1PPS input 
connector for this purpose.
In the second option we remove the on-board OCXO, and replace it with two  
SMA connectors, one is used to feed 10MHz into the Fury from the external OCXO,  
the other feeds the EFC control voltage (0V to 5V) to the OCXO for  steering.
The steering is done through a 24+ bits DAC into the OCXO, so there is no  
DDS on the Fury. The OCXO can be anything as long as it has 10MHz  output and is 
steerable. It can be a Rb or Cs, Cs Fountain, SAW  oscillator, etc. :) 
Actually, with a slight firmware change we can  discipline almost any oscillator 
from 1MHz to 15MHz+ if we had to.
It's a more traditional analog steering using the OCXO itself. I understand  
that the DDS method is patent-protected, and it also has the drawback of  
requiring a fast DDS running at some GPS-asynchronous frequency (say 400MHz, or  
1GHz) which could create noise and beat frequencies with the steered 10MHz  
output. The DDS implementation is also a sampled-data system,  so issues such as 
aliasing, Nyquist issues, Spurs, etc. have to be  carefully addressed not to 
show up in the -155dBc/Hz or better noise-floor we  are aiming for.
I personally like the simple, but elegant Varacter-Diode steered-Crystal  
solution better than having a DDS do the trick. 
That said, our FireFox DDS-based synthesizer can generate signals with 10  
microhertz resolution, and is controllable via SCPI commands over RS-232, so  
with a bit of software running on a PC (etc) and an external phase-comparator it 
 could be used to generate a software-steerable GPSDO.
Hope this answers your question,
thanks again,

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