[time-nuts] Austron PRR-10 GPS discliplined Rb...

David I. Emery die at dieconsulting.com
Sat Jan 27 22:27:31 UTC 2007


On Sat, Jan 27, 2007 at 01:32:40PM +0100, Ulrich Bangert wrote:
> Rob,
> 
> are you absolutely sure it works this way? I experimented a lot with a
> 48 bit dds chip from analog devices for a GPSDO just to learn that THIS
> way worked not good. What however works good is to imagine the
> combination of OCXO and dds as kind of 'pure digital efc'. That is: The
> output of the DDS (and not the RB's) is divided doen to a 1 pps which is
> phase compared to the gps receiver. This makes the system an overall PLL
> closed loop as seen with conventional efc circuits, however without the
> need for precise analogue circuitry, which is why i use it!
> 

	My understanding is that that is how the PRR-10s DO work. A DDS
is used to synthesize 20 MHz from the input reference 10 MHz and a 20
MHz VCXO is locked to this DDS output and divided down to produce 1 PPS
to compare with the UT+ or M12M 1 PPS.   And the phase error data from
that comparison is used to steer the DDS.   So yes, the 20 MHz is PLL
locked to the GPS by twiddling the DDS.

	I would know this much more precisely if more detailed
documentation on this now obsolete product became available (hint hint),
but since I have  several of the GPS locking boards now it may well
become worth it to get out the DMM on beep mode and start tracing etch -
and/or get out scope and logic analyzer and see what is going on...

	I do admit that without schematics or reverse engineering there
are some details that are a bit fuzzy - specifically what the relative
clock domains are for the two clocks (20 MHz from the VCXO and the 10
MHz from the Rb). I suppose one can handle this two ways - use a DDS
clocked with the 20 MHz VCXO to generate a 10 MHz signal and phase
compare (at 10 MHz) this with the input 10 MHz to steer the VCXO to lock
with the Rb 10 MHz input as adjusted by the DDS NCO.   This implies that
virtually all of the logic in the board is clocked at 20 MHz by the VCXO
output, and that another channel of the primary NCO chip or a second one
can be used to generate 1 PPS slewable in phase to acquire initial lock
with the GPS 1 PPS.   In this configuration errors in the 1 PPS phase
would be use to adjust the DDS ratio so as to make it synthesize the OFF
frequency Rb reference input 10 MHz.   

	And the other approach is to clock the DDS NCO chip with the 10
MHz Rb reference input and use it to generate a corrected 20 MHz which
can be used to phase lock the 20 MHz VCXO.   This implies a second 10
MHz clock domain for the DDS chip and related logic from the 20 MHz VCXO
world.    The first approach strikes me as cleaner, frankly, as the
board would have one clock rather than two... and if the input clock
disappeared (very possible in this application) there would still be
clock for everything if only as good as the VCXO and not reference
quality.





> Best regards
> Ulrich Bangert, DF6JB
> 
> > -----Ursprüngliche Nachricht-----
> > Von: time-nuts-bounces at febo.com 
> > [mailto:time-nuts-bounces at febo.com] Im Auftrag von Rob Kimberley
> > Gesendet: Samstag, 27. Januar 2007 09:39
> > An: 'Discussion of precise time and frequency measurement'
> > Betreff: Re: [time-nuts] Austron PRR-10 GPS discliplined Rb...
> > 
> > 
> > I remember them well...
> > 
> > Have sold them and installed them for Telco customers in 
> > Europe when I worked for Datum in the mid 90's. Novel design, 
> > and as you say they don't attempt to correct the Rubidium, 
> > but compare its output against GPS and use DDS to make the 
> > correction. In practise they worked very well and provided 
> > excellent Stratum 1 references.
> > 
> > Unfortunately I don't have any documentation, but have some 
> > of the blanking panels and other bits of hardware lurking in 
> > the store room here. Will have a hunt around, and see exactly 
> > what's available to anyone interested.
> > 
> > You got yourself an excellent bargain!
> > 
> > Rob Kimberley 
> > 
> > -----Original Message-----
> > From: time-nuts-bounces at febo.com 
> > [mailto:time-nuts-bounces at febo.com] On Behalf Of David I. Emery
> > Sent: 26 January 2007 22:23
> > To: time-nuts at febo.com
> > Subject: [time-nuts] Austron PRR-10 GPS discliplined Rb...
> > 
> > 	I snagged (for $150 BIN, which might have been too 
> > much) a Datum/Austron PRR-10 Stratum 1 Timing receiver 
> > complete with a LPRO Rb... and two channels of GPS 
> > receiver/timing board (redundant, hot swappable).
> > 
> > 	These things use the Motorola Oncore family timing 
> > receivers (the latest version (rev G firmware) can support 
> > the M12M+) and are primarily intended to supply precise 
> > timing for telco networks as DS1 or E1 output signals with 
> > all the right bits set for timing purposes.
> > 
> > 	There is a 4 channel "analog" output board available 
> > that can supply 10 mhz, 5 mhz and 1 mhz (I found one of those 
> > too), but the primary outputs in usual units one finds in the 
> > field/surplus are the DS1 or E1 variety.
> > 
> > 	The interesting thing about these units (which ceased 
> > production in July 2005 - possibly because of the abandonment 
> > of the Oncore receiver family by Motorola) is that they are 
> > the second kind of GPS disciplined clocks - namely phase 
> > microstepper based designs which accept a reference 10 mhz 
> > input and use a DDS chip to create a phase rotated and 
> > frequency corrected version which is used to phase lock a 20
> > mhz VCXO and from that generate a new 10 mhz and 1 PPS.    This is in
> > distinction to the Lucent RFTGs which adjust the C field of 
> > the LPRO RB to phase lock it to the 1 PPS input.
> > 
> > 	Apparently the firmware measures the frequency offset 
> > of the 10 Mhz reference input (in my unit generated by a LPRO 
> > 101) and its behavior over time and temperature and uses this 
> > to generate a phase step correction for the DDS which results 
> > in a precise 10 mhz output and 1 PPS used to compare with the 
> > GPS timing receiver 1 PPS and adjust the correction and its 
> > derivatives over time for optimum tracking.
> > 
> > 	This means they can take a slightly off frequency but 
> > stable 10 mhz and make a precisely on frequency and even more 
> > stable 10 mhz locked to GPS when GPS is available and open 
> > loop corrected  to the last GPS offset values when GPS is not 
> > using both  measured frequency offset and change of frequency 
> > offset with time (and I think temperature).
> > 
> > 	I have a users manual (circa 2001) in .pdf format, but 
> > would dearly love to find a source of more detailed 
> > documentation - the things are full of jumpers and 
> > stuffed/not stuffed options and it would be nice to know much 
> > more about them.
> > 
> > 	They do, however, provide some ADEV data on the quality 
> > of the input source as one of their data outputs available 
> > from the RS-232 port and as such are kind of neat...
> > 	
> > 	
> > -- 
> >    Dave Emery N1PRE,  die at dieconsulting.com  DIE Consulting, 
> > Weston, Mass 02493 "An empty zombie mind with a forlorn 
> > barely readable weatherbeaten 'For Rent' sign still vainly 
> > flapping outside on the weed encrusted pole - in celebration 
> > of what could have been, but wasn't and is not to be now either."
> > 
> > 
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> > 
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> > 
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-- 
   Dave Emery N1PRE,  die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."





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