[time-nuts] Using GPSDO as a Refrence for Protable Amateur Radio Microwave Operations

Bob Camp kb8tq at n1k.org
Wed Dec 21 19:44:40 EST 2016


The missing element in your evaluation is phase noise. If you directly multiply *any* 10 MHz source up to 
10 GHz, you have created a noise monster. The only rational way to do it is with a cleanup oscillator (or two)
somewhere between 10 MHz and 10 GHz. There are a variety of reasons why you get the noise increase and
multiple noise floors involved. 

If you pick something like 100 MHz as your (first?) cleanup, a bandwidth in the 10 Hz region is not at all impractical 
with a narrowband OCXO as the 100 MHz source. For many decades microwave sources have been built this way.
There are *lots* of designs out there to look at. The power and complexity impact is minimal. 

Once you do that,  the only thing that matters on your 10 MHz is it’s long term stability. If you have the power budget,
a Rb is the obvious choice. If not, a DOCXO would be the right way to go. In either case, calibrate them before you 
take off on your “adventure”.


> On Dec 21, 2016, at 2:06 PM, Eric Haskell <eric_haskell at hotmail.com> wrote:
> Hello Time Nuts,  I have been on the group and have promoted it to other folks for a while but this may be my first post here.
> I am microwave amateur radio operator and I have question to pose relating to the use of GPSDO's with amateur radio for microwave communication.
> First, the more generic question.  A friend was discussing using a eBay purchased Trimble 57963-D for providing a 10 MHz refrence for his portable microwave station  (primarly at 10GHz).  He wants a clean high stability 10 MHz refrence mainly to lock the station LO. First I think a GPSDO is overkill for this application and I am thinking that a good surplus ovenized crystal oscillator should get him to within a few Hz after warm up and a Rb could do better but may have short term stability that may degrade phase noise of the LO.  I am concerned that a GPSDO is not designed for portable operations.  Moving it should probably force a new site  survey which may take a day or more  to complete before it goes into disciplining mode so you would loose any potential benefit of a GPSDO by moving around frequently.  If he wants to do this I think he should leave it connected at his home location for an extended time (several days at least), then when he want to go portable (roving), he should
>  disconnect the GPS antenna entirely to force the unit into holdover mode maintain continuous power with battery backup which should maintain the internal OCXO very close to the target frequence and allow the holdover algorithm to compensate for OCXO for aging and best it can.  I would guess that if he chooses to used the GPSDO with the antenna connected it would probably never exit the site survey mode and you would have the output default to the last known good DAC value when it was been disciplined so it would be operating as a OCXO only (although potentially starting from a very accurate starting point, if it had been in use at a fixed location for a good while) before going portable.  Is this a correct view of the situation?  Any recommendations?
> I also know of a fellow who has developed some excellent open source Linux software to drive an Ettus Research USRP microwave SDR transceiver for amateur radio microwave applications.  His code also has features to calculate antenna baring and with other available code compensates for satellite Doppler shift and/or synchronize digital communication modes using the GPS coordinates and timing data.  He has a built in interface for a Trimble Thunderbolt for this purpose.  I think it also might be a better solution to use a OCXO for 10 MHz and a cheap USB GPS sensor for location?  Is there a cheep USB GPS that provides PPS?  Any recommendations?
> I have seen simpler GPS controlled 10 MHz sources like the Miller design that divides down a 10 MHz ref and compares it to a 10 KHz output from a Jupiter T GPS to tweak the ref freq that may or may not be better suited to this application as it may add phase noise to the LO but would be more real time in it's GPS correction to the reference frequency.
> Regards,
> Norman Eric Haskell
> Keller, TX USA
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.

More information about the time-nuts mailing list