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Have you
heard of a GPSDO before ? I hadn't until about 6 months ago and
wondered what the who-ha was all about... Why would you ever need
a GPSDO ?
Many people
wouldn't - but it's nice to know that they are available if you
have a use for them. But you may ask, what is their use ???
The GPSDO
is a very high stability oscillator (typically 10.000000000 MHz
+/- ) that uses data from the GPS satellite system to provide timing/locking/disciplining
information to control the internal oscillator within a couple of
parts in 1E-11 (i.e. 1 in 10000000000) once stabilised.
If you are
a serious microwave enthusiast, these types of accuracies are fast
becoming mandatory so that if you specify that your transmission
is on, for example, 10.368050000 GHz, then it really is on that
frequency - and when you get to techniques like Earth-Moon-Earth
(EME) then you need to make sure that you have the same frequency
selected as the other end - there isn't much room for inaccuracy.
To that end, serious types often feed the 10 MHz GPSDO reference
frequency to lock their other equipment to.
My needs
are simple at the moment although I may get closer to the "serious
enthusiast" point in the future : (1) I have a Marconi RF signal
generator that uses an external 10MHz reference to set the output
frequency; and (2) my 1GHz frequency counter can also utilise an
external 10MHz as an alternative to the internal 10MHz oscillator.
That means that I can very accurately measure frequencies on the
counter - and - generate them on the sig gen. Their accuracy - basically
the actual frequency divided by 10+/- the GPSDO error. If
the GPSDO output at 10 MHz is within 1 part per 10-9 (typical worst
case after warmed up) and the sig gen was set to 1.000 GHz (1000MHz),
the error is 1Hz. Given that most of the time, the accuracy is going
to be even better, that means that at 1 part per 1E-10, the error
would be 0.1Hz at 1GHz.
There are
a number of articles on the WWW that deal with GPSDO use and that
number will increase phenomenally over time as more people become
aware of what a GPSDO can do for them. This web page is more about
how I did it - rather than anything else. One of the continuing
things that was noted was the use of a Trimble ThunderBolt GPSDO
unit - and since these are now obsolete having been superseded by
newer devices using newer technologies, the older units are now
becoming available through the likes of eBay. Something to note
- many articles talk about using the Thunderbolt but very few document
what antenna is used or required.
The PDF of
the user manual on the basic device is available from here
on the main Trimble site. The other thing that is needed is their
"TBOLTMON" support software ( available here
) as it tells the user the exact status of what GPSs it is currently
"seeing" and the internal 10MHz oscillator error. The
final document is for their 'Bullet' antenna : here.
If these web references disappear, the same files can be downloaded
from this site : User manual : ThunderBoltBook2003.pdf
(1090KB) - TBOLTMON.EXE
(585KB) - Bullet
PDF (250KB)
The
following is a bit blog-ish but best describes the due process and
findings.
Mouse
over the images to see more detail....
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Trimble ThunderBolt GPSDO "Front" view |
"Side View" :
The important thing to note is the 6 pin 0.1" black power
connector
Pin order from LHS to RHS : -12V, Gnd, +5V, +5V, Gnd, +12V |
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After searching
eBay for "Trimble Thunderbolt GPSDO" devices over a number
of weeks, I chanced across one on a Sunday morning that had just
11 minutes to go in the auction. The highest bid was $US86 so far
so I took the plunge and put in $US100 - or about $AUS130 as a maximum
bid. My winning bid was $US96 - or about $AUS122 and the shipping
$US25 - so a total of $AUS155. The device was located in China and
shortly after winning the auction, I made the payment by PayPal.
Within about 30 minutes, I had an email back from the vendor - he
was shipping the goods by EMS courier - and even supplied a tracking
number. It took 2 days before the tracking number actually appeared
on the China EMS web site - the vendor was probably awaiting confirmation
from PayPal that payment had in fact been made. On the following
Friday, the box with the GPSDO arrived via Australia Post courier...
Wow, only about 5 days delivery after the auction finished - I was
impressed.
This particular
eBay vendor ( http://stores.shop.ebay.com/Rubidium-and-GPS-DO-From-China
in July 2009) offers an active antenna to go with the Thunderbolt
for $US20 and it is probably a good idea to take one simply because
the Trimble Thunderbolt receiver is DEAF. The original matching
Trimble "Bullet" antenna has an internal preamplifier
of 35dB (at 1575MHz) and a simple antenna just does not work. As
supplied, the Thunderbolt requires +12V @ 700mA, +5V @ 400mA and
-12V @ 10mA - and no power supply is included (although you can
get a suitable switch-mode supply from them too - although it probably
will not be approved in Australia). The quickest way to get it going
was to grab an old AT-style power supply that had recently been
removed from an older series PC, take the required voltages from
the yellow, red and blue colour coded leads respectively (with respect
to the black "common" lead) and use that to power up the
device.
{ Note: my Thunderbolt unit from this supplier included a 6 pin
power connector with fly-wires - other suppliers may not include
one }
The relevant
TBoltMon software was loaded onto the PC, the serial port cabled
across to the DB9 on the Thunderbolt and it was powered up. The
TBoltMon software asks which COM port is to use each and every time
but it simply pushing a radio-style button so is quick. It all seemed
ok except there was a series of yellow alarms down the middle of
screen - Antenna Open and Antenna Short - which was correct seeing
as how there was no antenna connected - plus others like Satellite
Tracking and Oscillator Disciplining - all of which I considered
normal. I won't go through what I did do exactly but I will say
that eventually I went into the Setup menu item, the Position sub-item
and set the approximate latitude and Longtitude here, remembering
to save it as an "Approximate Position" and to "Save
Segment" - this latter function is the most important. The
main reason for setting the approximate position is so that the
receiver knows which satellites are likely to be found and makes
the "lock-up" time shorter.
I tried building
a simple
patch antenna (per the ARRL info, and using double-sided PCB
material for the 2 plates) and even after I etched a few PCB tracks
and mounted an ERA3 MMIC preamplifier ( about 20dB gain ) on the
back of the backplane, the GPS signals were showing but not enough
for the Thunderbolt receiver to lock. Note - the centre pin of the
Thunderbolt's 'F' antenna connector has about +4.75V on it - to
supply power to the active antenna. I left the Thunderbolt powered
on so the internal TCXO would stabilise temperature - the normal
state for a device like this. A quick visit to the RF Shop (www.rfshop.com.au)
on the Monday and a 'GPS-325SMA'
active antenna ( 27dB preamp inbuilt ) was purchased (about
$AUD25). Using an adapter lead with a SMA female, it was connected
to the F-female on the Thunderbolt unit and, with the antenna placed
outside on the adjacent fence post and with the coax cable fed through
the window, up came the signals.
Within a
couple of minutes, the GPS had locked on 3 channels, the GPSDO status
was starting to synchronise. The mode changed from "starting
up" to "locked" and the displayed error at 10MHz
started to swing around "0.00 ppb" ( which I take is Parts
Per Billion ). Over the next 20 minutes, the status changed to the
maximum 8 GPS channels locked and the error swing around the '0.00
ppb' point started to decrease.
TBoltMon with
7 of 8 GPS channels locked
I took the
10 MHz output and fed it into the front of the frequency counter
- it was reading 10.0000 MHz on 1 second gate resolution, and flicked
between 9.99999 and 10.00000 MHz on 10 second gate resolution. It
appeared that my counter was going to be pretty close anyway - but
that was pure luck - coupled with the fact that the counter has
been powered on for some weeks and it's internal TCXO had stabilised
at the same value that I had calibrated it at some months ago using
WWV. I put the 10MHz GPSDO reference into the back of the Marconi
sig gen and set it to 1000.000MHz (1.0GHz) and fed the generator
output to the input of the counter (not sync-ed) : it displayed
999.99999 MHz (10 sec gating) and just stayed at that. The counter's
internal (free-running) TCXO was within 20Hz at 1GHz (ie +/- 10Hz)
- not bad for a $500 counter back some years ago when 1GHz was the
normal upper frequency limit for this type of device.
I was happy
- the Thunderbolt GPSDO was working as it should. For an investment
of under $AUS200 : a very stable reference oscillator for the workshop
/ radio shack. I would be able to turn off the counter and just
leave the GPSDO running- with a power consumption of about 10 watts
of power - and still know that when I powered up the counter or
sig gen, it would be on-frequency and stable. Once I extended that
frequency to be able to stabilise the other radio gear, I would
be able to turn them on for immediate use - knowing that they wouldn't
drift either. Apart from the 6 metre band (starting at 50.000 MHz)
, the direct harmonic radiation of the 10.000 MHz reference would
not be an issue as none of the other amateur bands include multiples
of 10 MHz at their main operating points. Note : main operation
on 6m is Dx calling 50.100 CW & 50.110 SSB, primarily beacons
between 50.000 and 50.100, national working 50.150 MHz and above.
One thing
that seems easy to overlook and may turn out to be important - once
the GPSDO is stabilised, go to the Options Menu again and "Save
All Segments". The other thing I noted was the DAC control
voltage value for the TCXO with "0.00 ppb" errors and
entered that in the Starting DAC Voltage in the relevant Options
Menu ( & "Save Segment" ) as that should help keep
the frequency error down if switched off/on at some later time.
The above
was all step ONE.... Now to move on to the subsequent plans....
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