[Elecraft] K3EXREF and Trimble Thunderbolts

[Paul Christensen]

Those of you considering a GPS-disciplined oscillator for use with the 
K3EXREF may be interested in this: I recently purchased two Trimble 
Thunderbolts on the surplus market to compare against my HP 58540A and 
Brandywine GPS4 units.  After several days of testing, I'm retiring the HP 
and Brandywine units.  All devices have exceptionally good stability, but 
the Trimble units consume far less power, run substantially cooler, and the 
monitoring interface provides much more status information than the prior 
units.

In recent discussions with John, KE5FX, he has performed phase noise 
measurements across several GPS-DO units and even various oscillator brands 
within the Trimble Thunderbolt model.

http://www.thegleam.com/ke5fx/tbolt.htm

Early Thunderbolts use a Piezo Corp. OCXO and when combined with GPS 
correction, its phase noise performance is good but not exceptional.  By 
contrast, the Trimble-branded OCXO offers phase noise performance several 
degrees better than most other GPS-DO units near Fc.  The area between 1 Hz 
and 100 Hz is usually a good indicator of the overall phase noise 
performance. It's not easy getting good numbers that distance from Fc. 
Although the phase noise performance will not carry over to the K3, it's an 
important parameter if the GPS-DO will be used in other applications or 
other transceivers that phase lock onto the 10 MHz external reference.

Some suggestions:

1) Look for Thunderbolts with a year 2004 Rev. E stamp.  These use the 
better quality OCXO units with the Trimble-branded label.  KE5FX has sampled 
several from this batch.  As noted, early units with the Piezo-branded OCXO 
are worse in terms of phase noise performance.  I do not know the OCXO 
quality of recent units;

2) You will see Thunderbolts in a high quality case where an internal DC-DC 
converter is used.  My recommendation is to avoid being tempted by the nice 
looks and what may be perceived as a "better" unit.  I can almost guarantee 
that the switch-mode converters will present noise problems with your K3 
receiver -- and seen on your panadapters.  I've been down that road with 
other GPS-DO units and ultimately, I scrapped the converters and fed them 
with linear supplies.  Stick to the basic ugly OEM Thunderbolt module and 
feed it from a triple-output linear supply;

3) Power supplies:  I like OEM/off-board supplies by PowerOne, Condor, and 
International Power.  They offer excellent performance and low noise.  I am 
using an International Power IHBAA-40W.  Also look for HBAA-40W.  The user 
must add a fuse, power cord and wiring harness.  It's a little more work, 
but you get a lot of performance for the money;

4) Look  for sellers who will accept offers.  Both my Thunderbolts were 
purchased for USD $70 ea and a small shipping change.

Even if you have no plans to use K3EXREF, get one anyway as a precision 
frequency reference for your station.

Paul, W9AC