[time-nuts] Trak 8821B ---- was Z3815A Internal SMA 10Mhz Output

GandalfG8 at aol.com GandalfG8 at aol.com
Wed Aug 22 14:21:03 EDT 2012


Hi Bjorn
 
Mine did come from a UK Ebay seller in Scotland, just under four years ago, 
 and he told me at that time he had two more and we'd originally discussed  
swapping those two for a radio, until the first one arrived and my  
enthusiasm tailed off a bit:-)
 
It's been a while since I've had it running but I don't recall  any GPS 
problems, I'll run it up again later and see how it behaves now.
I do remember it needs a negative 48 Volt supply and am pretty sure also  
that it feeds 12 Volts to the antenna.
 
Although the choice of oscillator shouldn't affect the IRIG B output it did 
 occur to me after posting earlier that it might have affected the 
programming  for the Low-Rate output, and if the 1MPPS had been generated from 10MHz 
by  a simple hardware divider, for example, then switching the oscillator 
to  16.384MHz might have been less than ideal for that function:-)
 
With regards to the Oncore VP carrier phase measurement option, I'm  not 
too sure that the 8821B does actually make use of that.
 
My 8821B, manufactured in 1999, does contain the 8 channel Oncore  VP 
B8121Z116 as I mentioned previously, and the 8821 data sheet from 2004  does 
refer to all 8821 models as using an 8 channel receiver, but the 8821A  manual 
from 1997 refers to that as using a 6 channel receiver.
Given that the only obvious difference between the 8821A and 8821B seems to 
 be the oscillator module I'm assuming the change to the 8 channel receiver 
 would have occured simultaneously for both versions, and am wondering if  
that might not just have been based on availability rather than to take  
advantage of any technical enhancements.
 
Regards
 
Nigel
GM8PZR
---------------------
 
 
 
In a message dated 22/08/2012 14:55:03 GMT Daylight Time, bg at lysator.liu.se 
 writes:

Hi  Nigel,

Yes, I have the same version. We might have got it from the same  UK epay
source. The GPS did not work reliably, and I did start testing that  module
outside the Trak unit, and also tried VP(8chan) receivers that I  had. Did
not really conclude why the GPS did not behave properly. I was,  like you,
disappointed by the lack of IRIG and other features that are in  the
standard units. I also drew the conclusion this was a "high  volume"
special model, where it was cost effective to eliminate all - for  the
specific application - unuseful outputs.

However it is a very  decent size OCXO inside.

The phase measurement Oncore model is  interesting, since I have seen very
few GPSDOs that use phase information.  It gives the possibility to reduce
the short term GPS noise to cm/s level.  However this is probably lost in
the PPS generation hardware, since the  Oncore does not lock its internal
crystal to the external  OCXO.

--

Björn

> Hi  Bjorn
>
> Is yours also the cut down 2.084MHz version, or do you  have a "proper"
> 8821B with a more useful GPSDO frequency and all the  outputs fitted?
> I think this one was originally used in a trunked  radio  system.
>
> I've just checked and the fitted VP Oncore  is  the B8121Z116, so perhaps
> the same module, but I'd forgotten  until I  looked again that the unit is
> without the IRIG-B output  as well  as the Low-Rate output that normally
> allows a
>  variety of pre-set pulse rates,  hence my "cut down"  comment.
>
> The Low-Rate ouput speciification suggests some  interesting  
possibilities
> for experiments in conditioning other  oscillators, as  amongst some very
> slow pulse rates it also  offered an option of 1  MPPS.
> Unfortunately, not only are the  connectors not fitted but quite a lot of
> presumably associated PCB  components in that area are also missing.
>
> Do you have a manual  for the 8821B?
> I've only ever found a manual for the 8821A, although I  suspect the  only
> significant difference may be the fitting of  the larger B7A OCXO  to the
> 8821B, which necessitated the height  increase to 2U, but it would be nice
> to be
> able to compare  them.
>
> regards
>
> Nigel
>  GM8PZR
>
> In a message dated 21/08/2012 21:05:02 GMT Daylight  Time,
> bg at lysator.liu.se
>  writes:
>
> Hi   Nigel,
>
> Have one of those Trak 8821B too. It is a bit  interesting since  the
> Motorola Oncore VP inside mine has the Z  option (phase  measurement).
>
> --
>
>  Björn
>
>
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