[time-nuts] Ublox neo-7M GPS

SAIDJACK at aol.com SAIDJACK at aol.com
Tue Aug 19 13:41:52 EDT 2014


Hi Graham,
 
its not a GPSDO though, not even a simple one :)
 
It does not discipline an oscillator. It generates the output by  
mathematically calculating how many phases it has to add/drop in a second, then  
digitally adds/drops/extends/retards the phase of the output clock to achieve an 
 average of number of desired clock cycles. This causes huge cycle-to-cycle 
phase  jumps. One cycle maybe 100ns long, and the next adjacent cycle could 
only  be 87ns long!
 
Without filtering, I doubt the output is useful for much because it has  
phase jumps from cycle to cycle of 10's of nanoseconds or more. A true GPSDO  
(even the cheapest one) has cycle to cycle phase "jumps" of femtoseconds 
only  due to oscillator jitter.
 
You can easily make a GPSDO out of it though through a simple EXOR gate  
(74AC86), feeding a TCXO/VCXO through a low-pass filter, and designing a phase 
 loop low-pass filter with less than say 10Hz bandwidth.. That approach has 
been  discussed here in the past a couple of times and is very 
cost-effective.
 
That is essentially what the Conner Winfield units do. The drawback is that 
 you have very large phase and frequency jumps when going into and coming 
out of  holdover on these units, because the unit does not have a holdover 
oscillator  with any type of reasonable stability, and whatever high ADEV  
stability your filter oscillator has is lost due to the analog loop  bandwidth 
of typically >10Hz meaning the internal $1 crystal of the GPS  receiver 
itself determines ADEV.
 
Bye,
Said
 
 
In a message dated 8/19/2014 10:03:27 Pacific Daylight Time,  
CollinG at navcanada.ca writes:

Said,

Agreed, hence my reference as "a very simple  self-contained GPSDO".

Even after a very quick first glance at the  documentation it didn't seem 
like it would be much of threat to more  traditional GPSDO's.  It will be 
interesting to play around with and see  what it can do.

Cheers, Graham ve3gtc


-----Original  Message-----
From: time-nuts-bounces at febo.com  [mailto:time-nuts-bounces at febo.com] On 
Behalf Of Said Jackson
Sent:  August-19-14 12:44 PM
To: Discussion of precise time and frequency  measurement
Subject: Re: [time-nuts] Ublox neo-7M  GPS

Graham,

Those are not GPSDO's by definition.

They are  based on NCO technology.

The difference being many orders of magnitude  higher phase noise and ADEV 
noise.

We tried to measure their phase  noise and our TSC5125A could not even lock 
on  to the 10MHz - they were  so noisy.

You can make a GPSDO out of them if you use a post filter  oscillator loop 
locked with sufficient time constant (>>10s)..  

Sent From iPhone

> On Aug 19, 2014, at 9:20, "Collins,  Graham" <CollinG at navcanada.ca> wrote:
> 
> Good day  all,
> 
> On another list to which I subscribe, there has been  chatter about the 
Ublox neo-7M GPS receiver. It seems that the device's  configurable timepulse 
output is configurable from 0.25hz to 10 MHz as well as  it's duty cycle 
and can also be set to be one condition when the GPS is not  locked and a 
different condition when locked (i.e. 1 PPS if not locked, 10KHz  when locked).
> 
> This seems all too good to be true. Sounds like  a very simple 
self-contained GPSDO.
> 
> I don't know anything  more about the device. I have just downloaded the 
documents and will be  spending some time reading them.
> 
> I am curious if any other  list members were aware of this feature of 
this device and have had any first  hand experiences with it.
> 
> There is another model, the 7N. the  7M uses a simple crystal clock, the 
7N a TXCO.
> 
> Cheers, Graham  ve3gtc
> 
> 
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