[time-nuts] Ublox neo-7M GPS
SAIDJACK at aol.com
SAIDJACK at aol.com
Tue Aug 19 17:41:52 UTC 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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