GPS freq lock- Trimble Thunderbolt?

[VK2GOM]

Have been reading on the web with interest about GPS-derived frequency locking. A unit of choice appears to be the Trimble Thunderbolt GPS disciplined clock, to generate a 10MHz standard output.

For my particular rig (IC746) I would need to feed that into something like a G4HUP DFS30 tripler/filter to generate 30MHz to then couple that to the rigs existing local oscillator coil with a couple of turns of thin wire.

Just wondering if anyone has tried the GPS locking route, especially with the Trimble Thunderbolt, to any success? Any thoughts on receiver performance related to the spectral purity of the reference oscillator output? Also - any sources in Australia for the same?

73 - Rob VK2GOM / G0MOH

[VK3PF]

Rob,

Have a look at the VK3HZ website under "RigLock" and/or the 2008 GippsTech Proceedings.

Good luck,

Peter VK3PF

[VK3HZ]

Hi Rob,

I have a number of GPSDO (it can become an obsession ...) including a Trimble Thunderbolt.

For locking your IC-746, there may be an easier way. If you square up the output of the GPSDO by passing it through a logic gate, then it becomes rich in odd harmonics - the first being at 30MHz. If you then couple this square wave into the reference oscillator coil, it will discipline the oscillator to 30 MHz. Peter VK3SO did exactly this to lock his IC-706. I'm not sure what, if any, degradation would occur - you're still using the original reference oscillator.

The Wenzel web site is a good source of circuits for waveform conversion etc. : http://www.wenzel.com/documents/circuits1.htm

Regards,
Dave.

[VK4ADC]

Rob

I have started along the GPSDO path ( http://www.vk4adc.com/gpsdo.php ) using the Trimble Thunderbolt and have it locking the sig gen and frequency counter at the moment. The GPSDO has been powered continuously here for a few months so has made it very easy to work on the 23cm transverter as I know the sig gen freq is absolutely correct.

I bought some TBB206G PLL divider chips ( plus a few TBB202G prescaler dividers) a few weeks ago (from Rockby in Vic) to build up my own LO boards. My plan is to have 2 versions - with VCO's at either around 32MHz or around 96/102MHz - with the actual frequency set by a PICAXE controller - probably an 18X so that I can bring out extra I/O pins to a DIP switch to select the desired output frequency from those pre-programmed into the PICAXE code. The TBB206G's will work with a 10MHz ref ( GPSDO sourced ) and can operate at up to 90MHz directly without a prescaler so mainly need a VCO added plus the microcontroller (/PICAXE) to load the divisor values. I have already calculated the divider N values for a 10MHz reference and my list of output frequencies but have had too many things running in parallel time-wise to even start working out a starting-point schematic or PCB layout.

The IC-7400 ( /746 Pro) I have requires 32.0000 MHz in lieu of 30.0000 MHz - as does my IC-718, both of which list a CR293 Hi Stab option. They must have changed the injection frequency from the earlier IC-746.

My HF (/30MHz) version LO will be made to feed into the IC-7400 etc while the 100MHz version LO's will be replacement LO's for the transverters ( eg 96.000 for 1296 with 144 IF or 95.91666 for a 145 IF / 95.6666 for 432 with a 145 IF or 96.000 for 144 IF / 101.000 for 432 with a 28MHz IF / etc...). If I need an oddball LO frequency in the future and the divider and ref frequency numbers will work out properly then it will just be a matter of loading a different set of numbers into the PICAXE.

I also have a stand-alone 10.0000MHz TCXO in a small box that I can use when the GPSDO is not available - eg on field days - and I have already calibrated that TCXO back to the GPSDO frequency.

I don't plan to make it a publicly-available physical project other than documenting it on my website so others can use my outcomes/processes to build their own versions. At this stage the projects may not materialise until 2010 but I have the starting-point hardware and just need the time and motivation.

Doug VK4ADC

[VK2GOM]

I now have my Trimble Thunderbolt 10MHz GPS-DO running happily on the bench. I'm currently using an el-cheapo $5 Chinese GPS antenna which so far has managed to lock onto a distinctly unimpressive 4 satellites. Still, it has enabled a 3D fix, which has informed me the antenna was 62.8m above sea level, and more importantly has allowed disciplining to occur. The frequency counter agrees. After a couple of hours warm up I am seeing 10MHz +/- 0.01 parts per billion.

I have also built the tiny SMT G4HUP DFS30 tripler/filter, which once the SMA cable assemblies arrive from China (Ebay) it'll be ready to tweak and verify/measure the 30MHz output. I may yet need to build in a Pi attenuator on the input to it, as I have no idea what dBm level the GPS-DO spits out. The specs say 12.5dBm +/- 2.5dBM. The DFS30 expects 10dBm on the input. The local club has a spectrum analyser that I have kindly been offered access to for setting up purposes.

I will also be fitting an SMA bulkhead connector to some spare metalwork on the back of the rig, and wiring two coupling turns from that socket to the LO coil.

Since the current GPS antenna is a) poor, and b) wired to the GPS-DO with bare cables pushed into the F socket (can't obtain an antenna with an F plug), I am looking at a much higher gain 55dB antenna from the USA which is terminated in a BNC plug. It's then a simple matter to use an adaptor from F male to BNC female.

Should all then be good to go for drift-free WSPR operation on 30m, and in the future hopefully some WSJT operation on the VHF/UHF bands once I am in a QTH I can actually put an antenna up!

73 - Rob VK2GOM / G0MOH

[VK3OE]

Hi Rob,
Good progress on your GPS rig lock.. Just some further thoughts.

I also have a Trimble Thunderbolt that I use to lock an IC 910H. There were two interesting effects that I noted when using the Trimble. Firstly on 1296MHz the sensitivity was degraded by about 7dB when using the Trimble due to its higher phase noise. On 2m there was also about 10mHz random drift. To check this out Rex VK7MO kindly lent me a HP GPS unit and I was able to verify that the 1296MHz sensitivity was not degraded and that the random wader on 2m was no longer evident when using the HP GPS. Mind you, this is on the extreme end of performance but may indicate that the Trimble may not be good enough to lock rigs above 432MHz because of high phase noise. Bryan Ackerly also recently posed a message on VK VHF saying that he had measured a high phase noise on the Trimble Thunderbolt, the result may be some small wander at VHF and microwave and reduced sensitivity on microwave bands.

Andrew
VK3OE

[VK2GOM]

Hi Andrew

Thanks for your comments - interesting reading.

At the moment, I don't plan to go much higher than 2m with GPS locking my IC746. I have never ventured higher than 70cm in my amateur radio career to date

Out of interest, I was looking at the Kuhne 23cm transverter, for which the latest version of the ready-made module accepts a 10MHz input from a GPS-DO or similar source. I wonder how that would perform with a Thunderbolt? I will have to have a Google around and see if anyone has had this combination running.

I believe a good 'brick' OCXO should be almost as good as the Thunderbolt, and of course without the noise. The only advantage with the GPS is you can arrive at 10MHz standard fairly quickly and not have to wait a good few days for the OCXO to stabilise.

73 - Rob VK2GOM / G0MOH

[VK3OE]

Hello Rob,

Given current knowledge, there is a high probability that the 1296MHz receiver sensitivity will be compromised if a Trimble Thunderbolt is used as a 10MHz reference.

It seems that 432 and 144MHZ receiver sensitivity would not be measurably compromised.

Andrew
VK3OE

[VK2GOM]

Hi Andrew

It seems there are a few versions of the Trimble Thunderbolt, some with better OXCO's than others, and correspondingly differing phase noise measurements.

This site gives a bit of background:

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

and some practical measurements of various GPS-DO's here:

http://www.leapsecond.com/pages/fury/phase.htm

73 - Rob VK2GOM

[VK3OE]

Hello Scott,

Most interesting.. Thanks.

Do you have any idea of how good the phase noise is for your Thunderbolt, or if there are particular OXCO's we should look out for???

Andrew
VK3OE

[VK2GOM]

Scott?

I can't measure phase noise here. I don't think it'd be evident on an oscilloscope.

There is one particular 10MHz OCXO that everyone raves about, of which there are usually a few on Ebay. I can't remember the name off the top of my head, but it was certainly a consideration before I went the Thunderbolt route. The clincher for me was a fast lock.

73 - Rob VK2GOM / G0MOH

[VK4APN]

Isotemp OCXO 134-10

[VK2GOM]

Isotemp OCXO 134-10

That's the one

I noticed today, during bench tests, since it was nearly 40degC in the shack, the Thunderbolt kept moving to the next setpoint temperature in a bid to keep the OCXO stable. In the end it was running at 48degC. I found a remarkable bit of software called 'Lady Heather' that gives graphical representation of all important parameters for testing. A quick Google will find it. The icon for the program is quite amusing; a leather clad dominatrix with a whip

Also - for anyone needing a good GPS antenna... genuine high quality Trimble GPS waterproof active antennas, with TNC connector from VK6, advertised in back pages of December's AR for $10.50 including post. I have ordered one. I think Brian the seller has a few. Say VK2GOM sent you!

73 - Rob VK2GOM / G0MOH

[VK3HZ]

There are at least two different voltage supply levels for GPS antennas that I've come across - 5V and 12V.
The TBolt puts out 5V. Make sure your antenna doesn't require 12V to operate properly.
I think most modern mouse-type antennas will work on either voltage.

BTW, is the antenna outside and in relative clear? The GPS signals are fairly weak and I get no signal inside (tin roof).

Regards,
Dave.

[VK2GOM]

Hi Matt,

Good news.

You might want to try this software once you know the thing works:

http://www.thegleam.com/ke5fx/heather/readme.htm

It's more comprehensive than TBoltMon, and shows all parameters graphically so you can keep an eye on it, it's history, and any trends. A bit fiddly to use, but you get used to it. Basically hit the space bar when running, to bring up the menu and all the options/controls.

73 - Rob VK2GOM /G0MOH

[VK2GG]

Well done Matt!
That Heather proggie does look pretty fiddly to set up Rob!
Think I'll use the standard. I've had a Trimbole running on my HP freq counter for months.
Dan VK2GG

[VK3YFL]

My Trimble Thunderbolt arrived today. I plan to use it to lock my Kuhne 10GHz transverter. What are others using as a +5V and +/-12V power supply for Field Day use where a car battery is the primary source of power? I see Jaycar have a "12 Volt ATX Computer Power Supply for Cars", but a 205W PSU seems a bit of overkill!

Thanks,

Bryon
VK3YFL

[VK2GOM]

Hi Bryon,

I am using a $10 5V switching reg in a hybrid TO-type package for the 5V off eBay. 12V is self explanatory, and the -V comes from an NE555 Voltage inverter circuit which gives about -11V which is fine. Anything more negative than -8V is OK. The current requirement for the -Volts is only a couple of milliamps, so no great power required there. My PSU for my Trimble that runs off 12V all fits into a small grey Jaycar project box.

I originally ran a normal 5V reg but it was getting too hot. A heatsink helped but it still ran hot after a while. Then it needed a fan - but then I discovered the switching regulators in the USA on eBay. Problem solved

73 - Rob VK2GOM / G0MOH

[VK3YFL]

Rob, thanks for the pointer regarding a suitable PSU. All built now and I can see via Tboltmon that the Trimble Thunderbolt is basically functioning, but still awaiting the arrival of my GPS antenna from RFSHOP to confirm it's fully functional.

Now I'm about to conect the Trimble Thunderbolt 10MHz output to a new Kuhne PLL stabilized crystal oscillator, however the Kuhne spec sheet says 3 - 10dBm input while the Trimble Thunderbolt specs show 12.5 +/- 2.5dBm output.

Can others please share their experience with this configuration in that do I need to pad the Trimble Thunderbolt 10MHz output down by at 5 - 7dB?

Thanks,

Bryon
VK3YFL

[VK2GOM]

Hi Bryon,

I feed my Trimble output into a home-built 106.5MHz Synth as I have the kit built Kuhne transverter, and the synth is designed to accept a nominal 10dBm 10MHz input. I measured my Trimble on my oscilloscope and I had a little over the Trimble spec 12.5dBm figure; about 13.2dBm from memory. It's worth measuring yours to see exactly what it spits out, and maybe write it in indelible pen on the casing somewhere for future reference.

You can easily make yourself a 50R ports resistive Pi attenuator if needed - plenty of online calculators for such things. But for a couple of dBm difference, I would hazard a guess that it's really not going to be too critical... I didn't bother attenuating mine and it works fine in my application.

On the other hand you might measure yours and find it's at the low end of the spec at a nice 10dBm or there abouts in which case just feed it right in there with confidence

You could drop Michael DB6NT a quick email once you know the figure and hear it from the horse's mouth, so to speak.

73 - Rob VK2GOM / G0MOH