[time-nuts] UBlox GPSDO

Matthias Brändli matthias at mpb.li
Fri Apr 17 08:03:46 EDT 2015


Hi all,

some additional info about the generation of the timepulse: in u-blox 7
and 8, the timepulse is generated from the internal 48MHz clock. So you
will see a lot of jitter if you configure it to 10MHz.

A 8MHz it's better, but since the internal 48MHz clock is not
disciplined (It's derived from the PLL that uses the 26MHz reference
crystal), you will also see jitter when the timepulse output gets
corrected. The LEA-M8F is different: the internal 30.72MHz VCTCXO gets
steered by the GNSS timing. It's the only module you can call a GPSDO,
the others don't have the "DO" part.

For the curious, I have designed a PCB that fits the USRP B200
software-defined radio transmitter (design available under free
licence), and we're using the M8F to synchronise and syntonise DAB
transmissions in a single-frequency network.

http://www.opendigitalradio.org/lea-m8f-gpsdo

Cheers,
mpb

PS:
it's my first post on this mailing-list. It's very interesting to read
you guys !


On 17. 04. 15 00:53, Bob Camp wrote:
> Hi
> 
> How to phrase this … hmmm….
> 
> The 10 MHz you get off of these modules is “dithered” off of the 26 MHz TCXO. That’s not
> really the best way to generate a clean signal. You can help that with some RF filtering. 
> 
> You also miss out on the long time constant narrow bandwidth filter that most GPSDO’s 
> have in them. Without it you will pass a lot of very hard to filter noise. 
> 
> Since the 10MHz is not sawtooth corrected, it will have the same hanging bridge issues
> as any uncorrected output. 
> 
> Low cost - certainly. High performance … not so much.
> 
> Bob
> 
>> On Apr 16, 2015, at 12:24 PM, Wayne Holder <wayne.holder at gmail.com> wrote:
>>
>> According to page 51 of the protocol manual for the UBlox NEO-M* series GPS
>> modules, the timepulse can be programmed to output a 10 MHz signal rather
>> than a 1 PPS signal:
>>
>>
>> http://www.u-blox.com/images/downloads/Product_Docs/u-bloxM8_ReceiverDescriptionProtocolSpec_(UBX-13003221)_Public.pdf
>>
>> I'm curious if anyone has tried this.  Seems like a possible way to get an
>> inexpensive GPSDO, as these modules are very inexpensive.
>>
>> Wayne
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