[Premium-Rx] Frequency Standards

[Charles Hutton]

Oddly enough, my experience is the exact opposite. A previous project used
GPS as the timebase for an OFDM based Wireless Local Loop system. OFDM is
very phase noise sensitive; that is one of its significant drawbacks. GPS
timebases served us well, although we weren't buying $200 units - more like
$1200 Trimble boards.

I would not give up on GPS at all, as long as you've climbed out of the
homeowner receiver level.


One man's experience -


Chuck
  -----Original Message-----
  From: premium-rx-bounces at ml.skirrow.org
[mailto:premium-rx-bounces at ml.skirrow.org]On Behalf Of Gary Geissinger
  Sent: Monday, July 21, 2003 10:56 AM
  To: premium-rx at ml.skirrow.org
  Subject: RE: [Premium-Rx] Frequency Standards


  Terry and Others,

  We have top of the line Datum lab GPS receivers at our ground stations
that are used as time servers.

  As they have a highly accurate reference frequency output, we used these
as the frequency reference for the LOs in our X-Band downconverters.

  In this configuration we observed that periodically the bit error rate in
our X-Band downlink would increase by two orders of magnitude.  After
considerable troubleshooting the cause was found to be the phase noise in
the GPS receiver reference output.

  Multiple high end GPS receivers were tried; no joy.  We then modified the
system to use double ovenized quartz reference oscillators instead.  The
system now has very repeatable (and very good) bit error rate performance.
(Of course, with the error coding on the link, there are essentially no bit
errors in the actual data.)  Naturally we have to peridically calibrate the
quartz oscillators, but the link performance is well worth it.

  I would be inclinded to hang onto an old reference oscillator standard for
those times when phase noise really, really matters.

  Gary WA0SPM
    -----Original Message-----
    From: Terry O'Laughlin [mailto:terryo at wort-fm.terracom.net]
    Sent: Monday, July 21, 2003 11:22 AM
    To: premium-rx at ml.skirrow.org
    Subject: Re: [Premium-Rx] Frequency Standards


    I have an HP Z3801 and Trimble Thunderbolt GPS standards running side by
side in my shop.  After a few weeks, I donated all my other standards to my
local technical college (where I teach).  Nothing came close to the accuracy
and ease of the Thunderbolt.  I think of the Z-3801 as my backup now.

    I found the Thunderbolt on eBay for $175 and bought two.  I donated the
other one to the college and it is our shop standard.  They are amazing
units.  The Thunderbolt is much smaller (about the size of two hard drives)
and uses 60% less power than the Z3801 (at 24VDC which is easier to supply).
It also receives more satellites (12) at lower signal levels and appears to
be more accurate, although it is beyond the limits of my equipment to
compare the accuracy with the already excellent Z3801.

    Terry O'


    At 02:55 PM 7/5/2003 -0400, you wrote:

      GandalfG8 at aol.com wrote:

      I used to use an ovened crystal standard with distribution amp, built
in-house by Siemens in the UK,  but there was always the niggling doubt as
to how well calibrated it actually was.

      That's a problem if you operate stand-alone. Hence the need for a
receiver.

        When the Z3801A GPS units became available that seemed like the
ideal answer, and probably still is, but I just couldn't resist the rubidium
unit. I've realised that it's as easy to get hooked on nice test gear as it
is on radios:-)
      Tell me about it !! Just as expensive too.

        This particular Racal standard is quoted as having an accuracy of
plus or minus 5 parts in 10 to the minus 11 per month, or 5 in 10 to the
minus 10 per year, and I doubt that it's even two years old yet.
      That's somewhat better than the specs on crystals, but in my
experience crystals are very often 10x better than spec if you let them run
for a long time (months or years). My crystals have been running 5 years
plus and are about as good as Rbs. BTW, Rbs are really crystals, locked to
the Rb cell.

        Other than having a desire for things being spot on, or as good as
possible anyway, I think it will take quite a while before that degree of
drift is going to matter much for me.
      True. Unless you are doing very special stuff (like physics research),
1 in 10 E 10 is just fine.

        What is the recommended calibration interval for something like
this?
      I don't know the 'official' schedule. I just tweak my unit if the
freq. goes off more than 1 in 10 E 10. The frequency always increases, so I
move it down 2 in 10 E 10. Easy.

      Take care,
      John


        regards
        Nigel

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