Antenna for GPS frequency standard?

[Gerhard_dk4xp]

Unfortunately, as mentioned in an earlier post Allan Deviation over a longer time scale tends to show a good result from any GPSDO. I am more interested in the short term stability over perhaps no more than 10 seconds as the GPSDO is used to calibrate test equipment and to hold UHF radio equipment on frequency.

I would call that "fortunately", because at least in the long run you get the exact
frequency from the Cs units in the GPS network with a cheap receiver.
In the short run, you are at the mercy of your local crystal oscillator. GPS works
only by averaging over a long time.

I use a Lucent GPS receiver, built by HP. It uses two MTI-260 double ovens,
one of them (in the lower plug-in) as a hot spare. The MTI-260 has options
that reach into Rb territory for long term stability. Unfortunately, MTI does
not tell us what HP was willing to pay for.

The row of 5 SMA connectors on the upper unit is my own invention.
(doubler from 5 to 10 MHz and CMOS level 1PPS)

Gerhard

[Johnny B Good]

@Solder_Junkie,

 I saw those first two graphs yesterday morning but couldn't spare any time to answer on account of a social engagement (my son's MIL's 90th birthday celebration party).

 The first graph neatly demonstrates the benefit of a timing receiver module over a navigation module (fake or not). The spiky plot of the M8T is simply the effect of rapid ionospheric disturbances ("Space Weather") which the correction data packets are unable to compensate for.

 The second graph clearly demonstrates the effect of trying to discipline a TCXO instead of an OCXO. A choice Leo had made to reduce not only cost and bulk but mainly the power requirements to allow it to be powered from a USB port.

 As you've observed, the long term stability averaged over hours to days long periods is more or less the same for all gpsdos (assuming no interruptions to the GPS signals). Unfortunately this on its own is of little use when using it to lock a microwave transverter's LO on frequency or allow you to syntonise a Rubidium oscillator in a relatively short time (a mere hour or so versus several days).

 Obviously, how far you go in ensuring the best possible performance depends on your requirements and how much cash you can bear to part with in order to achieve your goals. At a professional level, the specification can be clearly defined and a budget set. At the hobby level, this tends to be a "movable feast" leading you into "going down the rabbit hole" of which the time nuts so often allude to.

[edpalmer42]

What does the Allan Deviation of those fake module units look like?  That usually shows how good or bad a GPSDO is.

See attached. Unfortunately, as mentioned in an earlier post Allan Deviation over a longer time scale tends to show a good result from any GPSDO. I am more interested in the short term stability over perhaps no more than 10 seconds as the GPSDO is used to calibrate test equipment and to hold UHF radio equipment on frequency.

The setup for the Simple GPSDO by James Miller (G3RUH) is with an external "mushroom" antenna with a fairly clear sky view, Toyocom TCO-6703N oven osc, cheap eBay (£7.90) GPS module with a fake Ublox Neo-6M module. The reference is a rubidium oscillator which is off frequency by 2 parts in 10^11, as shown in the long term averaged image. A TinyPFA was used as the method of connecting the 10 MHz reference signals to TimeLab.

SJ

The ADEV graph confirms that everything is working as it should, both in the short and long terms.  Is this graph of a unit with the fake GPS module?  I was wondering if it would show an obvious degradation due to the use of a navigation receiver, but if it's there, I'm not seeing it.

The graph shows that the GPSDO shouldn't be used for short-term measurements that have a required resolution of lower than about 1e-10.  This is determined mostly by the noise/jitter of the oscillator and covers a time range up to about 200 sec.  If this wasn't acceptable, you'd have to decrease the noise level by replacing the oscillator with a better quality unit and/or otherwise revising the design.

At longer time intervals, the graph turns and starts to drop.  You'd need more data to be sure, but this looks like the point where GPS starts to show an effect.  This confirms that the GPSDO is actually doing what it should.  A simple GPS receiver with no disciplined oscillator can manage about 1e-10 @ 100 sec.  Performance is worse at shorter time values and better at longer ones.  If you followed this line further to the right, it would drop and then start to level out.  That indicates the ultimate noise floor of the GPSDO which varies from type to type.  If the noise level is too high, either due to design limitations or failing components, it starts to affect the GPSDO's ability to lock to GPS and shows up as a line that fades away to the right instead of dropping lower and lower.

Here are some graphs of high quality GPSDOs, including the James Miller unit which holds its own against the much higher price commercial units.

http://www.leapsecond.com/pages/gpsdo/

Ed

[Solder_Junkie]

Ed, yes the Allan Deviation plot was using a fake Ublox Neo-6M eBay module at that time.

SJ

[Johnny B Good]

Ed, yes the Allan Deviation plot was using a fake Ublox Neo-6M eBay module at that time.

SJ

 It's been bugging me for a while now and I've been meaning to ask whether that "Neo-6M" was supposed to be the same NEO-6M (genuine - the on board flash actually worked!) that I'd had problems trying to lock the ocxo at 1KHz by setting the PPS to its 1ms fastest setting with a 50% mark/space ratio to fake a 1KHz square wave as I'd described here:- 
https://www.eevblog.com/forum/projects/ublox-neo-m8n-gps-navigation-signal-amplify-module-for-arduino-rasppery-pi/msg2803298/#msg2803298

?

 Thinking about it, now that I've typed this far, ISTR you mentioning that this was reported to be a neo M-7 model by u-centre which was lucky for you if you had been trying to get it to work with the RUH based design like I had, were I'd been left with the feeling that I'd been  all along.

[Solder_Junkie]

I had two GPS boards from Chinese sources on eBay, at the price they aren’t genuine. The one marked M8N reported as an M7N in U-Center and the M6 doesn’t behave as it should. Still the boards are useful to swap out the fakes for real ones and have USB sockets for configuring.

For the budget minded they do work to some extent and can give reasonable results.

I have used both in a G3RUH simple GPSDO at 10 KHz, plus in a Lars/Murray Arduino controlled GPSDO with 1 pps.

SJ