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Old 21st Feb 2021, 11:50 am   #1
mole42uk
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Default Homebrew GPSDO

Here's some pictures of my GPSDO. I used the design form James Miller G3RUH (http://www.jrmiller.online/projects/ministd/frqstd0.htm) and modified it to use the Jupiter GPS receiver and Bliley OCXO that I had. One extra modification (not pictured) is to add a 50Ω 1MHz output on the rear panel to lock my Marconi 2955 radio test set to an external reference.

Generally speaking, the GPSDO works as intended, the Marconi 2955 measures the output at exactly 10MHz and the GPS receiver responds well to NMEA commands and provides reassuring output messages that the receiver is tracking 10 satellites. Next I will borrow another GPSDO to attempt triple-redundancy - the OCXO is quoted at 10.000000MHz, the Marconi 2955 measures the output to 10.0000MHz so if I use another 10MHz reference and lissajous the two outputs I should have confidence in my new reference! I intend to make a polystyrene box for the OCXO to help it remain at a stable temperature, but the oscillator will be running 24x7 as soon as I''ve proved the whole.

In the images below, the front panel "Locked (1 pps)" lamp is simply a 'heartbeat' - if the system is running correctly that lamp flashes at 1Hz. The front panel 10MHz output socket is a buffered 50Ω feed to any equipment that needs it. The Jupiter GPS receiver is ex-military equipment and is the TU30-D400 version which uses 3.3V power but outputs directly to the 5V TTL on the Veroboard. The OCXO is another ex-military pull that has a pretty good specification, and the manufacturers in the USA still answer email enquiries about it!
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Old 22nd Feb 2021, 5:46 pm   #2
Al (astral highway)
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Default Re: Homebrew GPSDO

Quote:
Originally Posted by mole42uk View Post
Here's some pictures of my GPSDO.

That's a great project, Richard. I've been following this with interest as I find this kind of thing fascinating.

Quote:
Originally Posted by mole42uk View Post
In the images below, the front panel "Locked (1 pps)" lamp is simply a 'heartbeat' - if the system is running correctly that lamp flashes at 1Hz. The front panel 10MHz output socket is a buffered 50Ω feed to any equipment that needs it. The Jupiter GPS receiver is ex-military equipment and is the TU30-D400 version which uses 3.3V power but outputs directly to the 5V TTL on the Veroboard. The OCXO is another ex-military pull that has a pretty good specification, and the manufacturers in the USA still answer email enquiries about it!

That's pretty ingenious and it looks great, too. Thanks for sharing!
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Old 28th Feb 2021, 11:05 am   #3
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Default Re: Homebrew GPSDO

Nice project.

Last year I purchased one of the BG7TBL GPSDOs from China, I really didn't know much about them but I wanted something to check/calibrate a frequency counter, this turned out to be bang on to the last digit.
New products of this kind are very expensive.
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Old 28th Feb 2021, 12:33 pm   #4
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Default Re: Homebrew GPSDO

Today’s tests have put me into a loop.....comparing my GPSDO against another one that’s been running on my bench for a week, they are not the same frequency!

Comparing the two outputs on my ‘scope and triggering off one of them, the other trace drifts ever so slowly. A lissajous image presents a wobbling oval :-(

Now I have to find another 10MHz reference to find out which of the two is adrift!
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Old 28th Feb 2021, 2:16 pm   #5
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Default Re: Homebrew GPSDO

Can recommend the bodnar ones. Dirt cheap and they work! http://www.leobodnar.com/shop/index....roducts_id=301
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Old 28th Feb 2021, 4:45 pm   #6
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Default Re: Homebrew GPSDO

Further work, including measuring the frequency with my HP 5302A counter, concludes that the two 10MHz oscillators are only 6Hz apart so maybe I don't need to worry too much.
I'll check them again after 24 hours and see....
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Old 1st Mar 2021, 6:46 am   #7
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Default Re: Homebrew GPSDO

Confucious he say " man with one watch know what time is, man with two watches not sure"
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Old 2nd Mar 2021, 11:32 am   #8
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Default Re: Homebrew GPSDO

I have a Quartzlock 2A off air standard to which I have incorporated a GPSDO based on a Jupiter GPS Rx. The GPSDO uses the existing Quartzlock's circuitry from the phase locked loop onwards. I have provided a switch so that I can flip between off air or GPS. I keep meaning to post some details of it on the forum with some photos. The point I was leading up to was that comparing the two reference sources on a Racal 9839 (last digit 0.1Hz) I have never seen any difference between the two.
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Old 2nd Mar 2021, 9:51 pm   #9
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Default Re: Homebrew GPSDO

A really cheap way to check the 10MHz would be one of the GPS modules using the u-blox NEO-7M (well under £10).
These can be programmed to give an output frequency up to 10MHz in 1Hz steps.
The catch is that there will be considerable jitter on output but averaged over a period of a few seconds or more, using a counter or by eye using an oscilloscope, sub Hz accuracy at 10MHz is easily attainable.
Programming can be done via 'u-center' software on a PC (via a USB-TTL serial converter) or using an arduino or similar.

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Old 3rd Mar 2021, 5:52 pm   #10
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Default Re: Homebrew GPSDO

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A really cheap way to check the 10MHz would be one of the GPS modules using the u-blox NEO-7M
Jim
Of course, that has set me off on a new quest - designing a GPSDO using one of the NEO-7M devices, setting the output to 10MHz, then using that and the output of the 10MHz OXCO into a frequency comparator to drive the adjustment on the OXCO..... with a suitable low-pass filter it should ignore the jitter on the NEO-7M and, by using a very stable power supply, give a much more accurate output than using the 10kHz comparator that I have now.....but should I build the whole into a constant-temperature enclosure?
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Old 5th Mar 2021, 7:00 am   #11
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Default Re: Homebrew GPSDO

The "constant-temperature enclosure" is probably not required.
Particularly if the oscillator is already in a metal case with internal thermal insulation.

I am no expert, not even close on this subject, but after the latest post, I was quite curious about my own DIY GPSDO, and ran it overnight for some observations, which may be of interest.

My device uses a uBlox Lea5T RX and an NDK EG041 10MHz OXCO (cannot find a datasheet for this XO).
The whole device is in a 200x125x50mm plastic case.

So, starting from cold at 25 deg C, the case temperature of the XO runs up to about 50 deg, and stays there +\- 2 deg during stabilisation.
By this time the device has GPS stabilised with no alarms, and a temperature of 49 deg.
Most OXCOs have a temperature range over which they will work, typically -10 to +80 deg C.

I used a K1 type probe and digital meter for temp measurement directly on the XO metal case.

As the OXCO has its own temperature control, I don't see what an extra thermally insulated box will achieve, provided that the OXCO is within the manufacturer's operating range.


Testing my GPSDO
This is a literal dilemma, complete with garden paths, wormholes, and measurement uncertainties.
You get to a stage where you have to initially distrust everything.

I compared mine with two separate other working GPSDOs (permanently ON) at a mate's place while we had coffee and reminisced about radio for eight hours.
We used the CRO Lissajous comparison method.
After two hours, we had a stable circle, and it stayed like that until I went home six hours later.
If there is any doubt, if there is any slow circle wander, even very slow, there is a problem, and you have to rethink the process.
A GPSDO with an output frequency within a few Hz of nominal is not operating correctly.


My OXCO
After five minutes from ON, my OXCO seems to start about two hertz low, and drifts up to close to 10MHz, ie within 1 Hz, even without GPS action.
So some of these oscillators are already very accurate to start with, and quite possibly all that is required for most radio workshop use.
I say "close to 10MHz" because here in my workshop I cannot absolutely measure the OXCO frequency to say 0.1 Hz, without another separate GPSDO feeding 10MHz as EXT REF for my Philips Freq Counter.

I can live with this, as I can now use my GPSDO as EXT REF for my Freq Counter when measuring frequency of all other devices.

Very useful device to give measurement certainty.
Very worthwhile project.

Good luck with your projects.

Last edited by Radio1950; 5th Mar 2021 at 7:18 am.
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Old 5th Mar 2021, 9:57 am   #12
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Default Re: Homebrew GPSDO

Only 2Hz low 5 mins after cold turn on for you OCXO is rather good. The HP 10544B can be around 1kHz low stating at 22C room temp. The 10811A SC cut one is similar. These are at 10MHz so that's an error of -100ppm!

The reason is that quartz intended for 80C oven use is cut to give the flattest frequency/temperature slope at that temperature. So the curve is parabolic, centred on 80C and room temp is a long way down the cold side slope.

These oscillators, at switch-on, are a lot further off-song than TCXOs. But people do tend to assume they're always better and consider frequency error inconsequential in anything with the OCXO option, right from pressing the 'on' button.

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Old 5th Mar 2021, 10:50 pm   #13
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Default Re: Homebrew GPSDO

I bought some CTI 10MHz OCXO's they turned up a few days ago

If you monitor the frequency, you can see them shift frequency from around 9.somethingMHz at switch on to 10MHz (+ about 100Hz) after a couple of minutes.

It is a very sharp jump, not a gradual move up in frequency.

I haven't run them any longer to see where they settle, I just wanted to confirm they were actually working.
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Old 6th Mar 2021, 1:50 am   #14
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Default Re: Homebrew GPSDO

If you want to really compare the agreement between two 10MHz oscillators and you don't want to wait for ages you could try and borrow an old HP8405A vector voltmeter.

In my old youtube video below I tried to adjust two Agilent ESGD signal generators to be as close as possible using the calibration interface for the internal 1E5 OCXO. One OCXO is fed to channel A and one is fed to channel B of the old HP8405A VVM.

In the video you can see I managed to get 1 degree of phase slip in about 8 seconds. This is a difference of just 0.00035Hz between the two oscillators.

If it slipped 360 degrees in 1 second then that would be a 1Hz error as it slips a whole cycle in 1 second.

Therefore a 1 degree slip in 1 second would mean 1/360 Hz difference = 0.00278Hz.

1 degree in 8 seconds is 8 times better than this at 0.00035Hz.

https://www.youtube.com/watch?v=Eo2k6mWZ2VM

This is much faster than waiting for a long time to look for a cycle slip on a scope for example. It would also be possible to hook the phase recorder output at the back to a DVM so the error could be logged over time to look at the short term drift characteristic.
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Old 6th Mar 2021, 7:21 am   #15
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Default Re: Homebrew GPSDO

Quote:
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If there is any doubt, if there is any slow circle wander, even very slow, there is a problem, and you have to rethink the process.
A GPSDO with an output frequency within a few Hz of nominal is not operating correctly.
.
This is my current dilemma. The lissajous test gives me a wandering circle so I know that one of the references is not accurate. The one I borrowed is a commercial unit based on a Trimble core so Iím inclined to trust that.

Investigating my DIY version, I have no confidence that the GPS 10kHz is having any effect on the OCXO. Fiddling with the resistors in the feedback circuit gets me to about 2 phases per second but it not locked, removing the antenna from the GPS (which should disturb it enough to be measurable) doesnít make any difference.

I believe that I should be able to get two GPSDO to give me a perfect lissajous circle after they have settled - but these have both been running in the bench now for a week.
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Old 6th Mar 2021, 10:31 am   #16
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Default Re: Homebrew GPSDO

As I mentioned in my post #8 regarding my Quartzlock 2A modified with a Jupiter GPS to give a GPSDO function but still allowing switching back to off air (198KHz) operation, I have never seen any difference between the two methods of obtaining the frequency reference. One thing I noticed during the development of this dual mode standard was that the Jupiter provided the 10KHz output even without having acquired any satellites. It is close enough to the precise frequency to allow the 2A phase locked loop to lock onto it thus giving the illusion that it is providing a GPS accuracy output. To get around this a wrote a PIC program to monitor the NMEA output of the Jupiter to show (via an LED) whether there was a true GPS source providing the 10KHz signal. After the Jupiter has acquired satellite lock and the 10KHz is GPS referenced I noticed that if you remove the GPS antenna the internally generated 10KHz initially remains at the locked correct frequency and only slowly drifts back to the internal reference frequency. There must be some sort of phase locked loop running in the Jupiter Rx. This means that for short interruptions in the GPS signal the 10KHz output remains steady.
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Old 6th Mar 2021, 11:26 am   #17
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Default Re: Homebrew GPSDO

For those not familiar with the Quatrzlock 2A off air frequency standard I have attached a photo of my modified one. The meter shows the PLL control voltage and is thus a lock indicator. As mentioned in my previous post this is not a reliable indicator when used with the Jupiter GPS and you will see the LED labelled GPS OK which is driven by a PIC which is monitoring the GPS NMEA data. The switch selects between off air (198KHz) and GPS operation. Reassuringly I have never seen any difference in frequency between the two sources.
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Old 13th Mar 2021, 4:28 pm   #18
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Default Re: Homebrew GPSDO

An update today. I was concerned that there was uncertainty about the signals from the OCXO and the ability of the PLL to lock. Turns out that a 14-pin DIL socket had two faulty connections, the 10MHz input and the ground pins were, at best, intermittent.... I'm now planning to remove the main board and rebuild it with new turned pin sockets.

Moral - don't just grab IC sockets out of the junk box.
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