[time-nuts] ublox NEO-M8T improved by insulated chamber?
kb8tq at n1k.org
Sat Nov 4 08:31:52 EDT 2017
There always is a …. but ….
When you slow down the temperature change process, you increase the
length of hanging bridges on the pps output. That might not be a big deal
with NTP or in an application that uses the sawtooth correction message.
In an application where a post filter process is expected to average out the
sawtooth, it can be an issue.
> On Nov 4, 2017, at 6:16 AM, Leo Bodnar <leo at leobodnar.com> wrote:
>> From: MLewis
>> Is this coincidence or can reception improve with:
>> - a higher temperature module?
>> - a more stable module temperature?
> Short answer:
> Long answer:
> Ublox firmware tracks gradual shifts in its reference frequency (XO, TCXO or external input) and adjusts LO base offset to compensate.
> However, the time constant of this correction tracking loop is quite high (and can be adjusted depending on the reference oscillator type.)
> Sudden changes in temperature and, as a result, in reference frequency result in correlation level drop (seen as sat signal level level drop) or in total loss of tracking and return to acquisition.
> In other words, absolute reference frequency offset (i.e. its temperature) is not a problem - it is gradually compensated for, but sudden shifts in frequency are.
> If your design does (hopefully) does not rely on convection for getting rid of heat, try filling internal voids with cotton wool. This will stop turbulent (naturally random) and laminar (usually caused by external events) airflows from affecting the reference. Compartmentalising the design is just another way of separating the airflows but it does not stop them within the compartments.
> There are subtler cases where reference frequency source has a sweet spot where its stability is greatest (like OCXO) and absolute temperature matters as well but this is the first order effect.
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