[Premium-Rx] Frequency Standards

[Gary Geissinger]

John,
 
You hit the nail right on the head.  
 
"Trimble characterizes the phase noise of the Thunderbolt's output at about
15 dB worse than an HP 10811E at 10 kHz from the carrier (-145 dBc/Hz versus
-160 dBc/Hz), and in my experience, you can take that to the bank."
 
Here are the phase noise specs for the crystal oscillator we are using:
 
			
  10 Hz        	-135 dBc/Hz

100 Hz        	-155 dBc/Hz

1 kHz          	-165 dBc/Hz

10 kHz        	-168 dBc/Hz

50 kHz        	-168 dBc/Hz	
 
It is a Vectron OCXO with the low phase noise option.  We couldn't find a
GPS receiver output that could touch this.  When you are running a link BER
of 1e-9 (without coding), low phase noise really matters.      
 
If you want to see what a low bit error rate can do, look at our web site:
www.digitalglobe.com <http://www.digitalglobe.com> .
 
Like I said, "When low phase noise really, really matters..."
 
Gary 

-----Original Message-----
From: John Miles [mailto:jmiles at pop.net]
Sent: Monday, July 21, 2003 10:43 PM
To: premium-rx at ml.skirrow.org
Subject: RE: [Premium-Rx] Frequency Standards


Agreed, Gary's observations don't make a whole lot of sense at first blush.
A GPS standard *is* a high-quality quartz standard, kept in constant
calibration against the GPS signal in a *very* low-bandwidth PLL.  The
effective loop time constant on the Thunderbolt is probably on the order of
a minute or two.  
 
Trimble characterizes the phase noise of the Thunderbolt's output at about
15 dB worse than an HP 10811E at 10 kHz from the carrier (-145 dBc/Hz versus
-160 dBc/Hz), and in my experience, you can take that to the bank.
 
Sounds like the Datum receivers had faulty internal standards.  It's really
inconceivable that GPS disciplining, by itself, could increase a standard's
phase noise.  
 
-- john KE5FX
 

-----Original Message-----
From: premium-rx-bounces at ml.skirrow.org
[mailto:premium-rx-bounces at ml.skirrow.org]On Behalf Of Charles Hutton
Sent: Monday, July 21, 2003 8:22 PM
To: premium-rx at ml.skirrow.org
Subject: RE: [Premium-Rx] Frequency Standards


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.

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