GPS References

[Ray Cannon - W7GLF]

I am posting this as it probably is of general interest.  I got a mail from Nick with this question:

Hi Ray,

Finished both of the FM mods on the 1st. 122 GHz unit. 

I was wondering, if you know of a "good low phase noise" GPSDO I can order so I can use it with the 122 GHz. My unit is the same as Rick's and does not work right at these frequencies due to too much phase noise.

What phase noise level @ 1 hz should I be looking for in the specs?  I have seen some that have a -85 dB for 1 Hz. Most seem to have worse than that @ - 60 dB.

I have seen units from Leo Bodner, Symetricom, and others but I am not sure what to order that will work so I can align my 122 GHz feedhorn.

I am not much of an expert in this field but here are my observations so far:

I know the Leo Bodner unit has a good reputation but I have never tested it myself so I cannot speak to its stbility.  Is that the one Frank has?  I think he mentioned it had a problem with harmonics which presumably a filter could handle.

I have tested the larger BG7TBL unit (the date on mine is 2017-03-29) and it hiccups about once a second.  You can hear the hiccup on the attached file.  Other than that it works fine.

I have also tested my Thunderbolt, Z3801A and an Jupiter GPS unit I built years ago from a board from pyrojoe. 

I would comment it really depends on what you want to do. 

If all you are interested in stability and a clean signal it is hard to beat the dual oven ocxo that beatjhunky is selling on EBay.  It takes 8 minutes to warm up but once it does it is extremely stable.  It has become my standard reference to compare everything else to.  It is not GPS locked but I find it very close to being on frequency and if you are really a perfectionist it has a reference voltage output that you can tie a multi-turn pot to allow tweaking its frequency on its vco input.  I did not try that since it is close enough without going to that much trouble.

As far as phase noise (which I believe is determined by the reference OCXO):

      The EBay dual oven OCXO quotes a phase noise of -90 dBc @ 1 Hz (-130 @ 10 Hz)

      My PyroJoe model uses an Isotemp OCXO model 134-10 which says -105 @ 10 Hz

      The Thunderbolt uses a Trimble OCXO -120 @ 10 Hz (http://www.ke5fx.com/tbolt.htm).  A disadvantage of this unit is that it requires +5 and +/- 12 volts.

      The Symmetericom Z3801 uses an HP 10811 double oven spec'ed at -100 @ 1 Hz -130 @ 10 Hz (see https://www.febo.com/pages/gpsdo_comparison/)

      The oscillator in my BG7TBL is a CTS model 1960017.  It is spec'ed to have phase noise of -90 @ 1Hz.

I am thinking of tearing apart my BG7TBL and putting in a better oven - perhaps try the double oven and see if that solves the hiccup problem.  The low pass filter may need to be fiddled with as well?

I have W7RNB's tiny GPS that does not work with 122 GHz here and opened it up.  It uses a CTS 970-2178-46 oscillator.  There is a discussion of this particular unit at:

https://www.eevblog.com/forum/testgear/new-gpsdo-bg7tbl/25/.

Note: I am not sure W7RNB's is really from BG7TBL because there is no identification on the PCB board or the unit however it clearly is a copy of a BG7TBL design.  The board looks identical to the one with a very interesting discussion at:

https://www.vklogger.com/viewtopic.php?f=40&t=13917.  The conclusion there seem to be a problem with the circuit board design? 

Hmm it might be interesting to replace OCXO with another oven as well..

 

I am not sure just going by the phase noise will be sufficient but I note all of the ones that I have had good luck with are -90 dBc or less at 1Hz.

[Nick Krachmalnick]

Thanks very much Ray,

I need to do some reading on this from your email. 

I also found an earlier project using a Trimble board. I have it running and it does put out 10 MHz on the scope.(2-3V sinewave). 

I have one of the bestjhunky dbl oven oscillator units that I was planning using at the tuning source 122 GHz unit.

Thanks again,

Nick

--
You received this message because you are subscribed to the Google Groups "PNW Microwave" group.
To unsubscribe from this group and stop receiving emails from it, send an email to PNW-Microwav...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/PNW-Microwave/84273def-87e7-484e-9f04-2f7fad736e2bo%40googlegroups.com.