[hpsdr] Status of THOR - Gibralter?

[Chris Albertson]

--- michael taylor <mctylr at gmail.com> wrote:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> On 6/12/07, David Woodhead <dwoodhead at bms-inc.com> wrote:
> >
> > If you are looking for a relatively simple solution, look for a
> Rockwell
> > Jupiter GPS board (they show up on Ebay quite often - I paid about
> $30
> 
> A quick look at its datasheet suggests the Jupiter is accurate to
> within ±1 microsecond, whereas there are numerous cheap new GPS
> modules (SiRF StarIII ~$50-75 US) are accurate to within 100ns.

The problem I've seen with the low cost consumer GPS units is that
the output is serial NMEA.  The internal clock might be very good but
there is some unknown in the time through the NMEA serial 
communications path between the GPS and the computer.   At 4,800
bits per second the time between the bits is well greater then the
accuracy of the internal clock  I doubt the bit rate clock is
synchronized with the real-time GPS clock.  The bit rate clock is
likely a crystal.  So you can't know when the internal clock was
sampled
in relation to the start of a MNEA "sentence" to more then 1/4800
of a second.  

There are some GPSes that provide a TTL level pulse per second
This is what yo want.  But I don't think these sell for under $100.
(But I hope some one will correct me here.)

But do we care?  What really matters is how long one must integrate
the GPS time to get the desired accuracy goal of what? 10E-11 seconds?
The GPS signal is only used to control long term drift in the ovenized
crystal.

One other idea.  On one project we needed to control the temperature
of a small device and hold it very stable.  The solution used here
would 
be a double oven with resistive heating.  But they had another design.
The part was clamped to a small block of silver, a Peltier device (aka
thermo-electric cooler) was clamped to the other side of the block and
a temperature sensor was embedded in the silver block.  A computer ran
a servo loop to control current to the Peltier.  The entire stack was
placed in a bottle "filled" with vacuum.  Vacuum being the best
possible
thermal insulator.  No exotic tools were required just some high grade
plumbing parts.  Hand powered vacuum pumps are not all that expensive.
One advantage of the Peltier is that it can both heat and cool the part.

Chris Albertson
  Home:   310-376-1029  chrisalbertson90278 at yahoo.com
  Office: 310-336-5189  Christopher.J.Albertson at aero.org


       
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