[time-nuts] Ublox M8N - have I a XO or TCXO ?
kb8tq at n1k.org
Wed Dec 21 12:34:19 EST 2016
Given the short term stability limits on the frequency source (TCXO or XO) it’s a pretty
good bet that the crystal in both oscillators is very similar. There’s not a lot of difference
between a “normal” crystal and a “TCXO” crystal other than the angle of cut tolerance.
Since the XO has frequency rate of change specs on it for the GPS to work, it’s got
some angle limits that get it pretty close to TCXO land even in that regard.
> On Dec 21, 2016, at 8:43 AM, Peter Reilley <preilley_454 at comcast.net> wrote:
> You seem to be assuming that the crystal in the TCXO is the same as the crystal in the XO.
> Wouldn't it be likely that the crystal would be higher quality in the more expensive
> product; the TCXO? How would a cheap crystal vs an expensive crystal appear different
> in the GPS data presented (ignoring the TC part)?
> On 12/21/2016 7:37 AM, Bob Camp wrote:
>>> On Dec 21, 2016, at 4:20 AM, Mike Cook <michael.cook at sfr.fr> wrote:
>>>> Le 21 déc. 2016 à 00:08, Kiwi Geoff <geoff36 at gmail.com> a écrit :
>>>> Hello All - and Seasons Greetings,
>>>> One of the advantages of the recent hobby drone phenomena - it has
>>>> brought to the market a lot of low cost GNSS modules that are
>>>> lightweight for drone flight control systems. Those of us with other
>>>> hobbies, like "Time Nuts" and RTK - these low cost modules can be a
>>>> little goldmine for cost effective toys.
>>>> However the dark side is that some vendors are re-badging lower cost
>>>> modules, printing their own labels and marking as "Ublox M8N" for
>>> It wouldn't surprise me, but you have a reference for this?
>>>> Can anyone comment on the following data, and whether they think the
>>>> oscillator in "my" M8N is a XO or a TCXO ?
>>> I don’t thing that you can get find out from that data. Ublox indicate in the product info sheets that the TCXO option is used to get to a first fix quicker in weak signal conditions. It is not specified and I think that it is logical, that there is an improved timing solution. I would expect that both XO and TCXO versions are the same frequency and the better long term stability of a TCXO not be an influence on the single shot quantization error of the 1PPS.
>> The TCXO and XO both have similar short term stability at short tau. The only
>> advantage to the TCXO is faster time to first fix. If anything the XO will “spread”
>> the quantization error (sawtooth error) better than the TCXO. The TCXO has
>> more inflections in it’s frequency vs temperature curve. Thus there are more
>> opportunities for hanging bridges.
>> In both the TCXO and XO case, the PPS out of the module is based on the
>> clock edge closest to the PPS estimate. If the clock involved has a period of
>> 20 ns, the error will distribute over ~ +/- 10 ns. The process is identical regardless
>> of the oscillator. The distribution will be the same with both oscillators. As long as the
>> phase noise and short tau ADEV are in spec, the PPS estimate will be the same
>> in both cases.
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>>> "The power of accurate observation is commonly called cynicism by those who have not got it. »
>>> George Bernard Shaw
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