[time-nuts] The difficulty of low noise measurements

[Magnus Danielson]

Hi,

On 2020-01-11 21:53, Poul-Henning Kamp wrote:
> --------
> In message <643136da-9599-18ca-be85-ffa62ab04aea at rubidium.se>, Magnus Danielson writes:
>
>>> I have on my ever-growing TODO list to test if serial-BLE adapters
>>> are any good.  Has anybody tried that yet ?
>> My recommendation is to build on the ESP32, then you can get serial port
>> to WiFi directly and the entry level price is small enough and there
>> exist a community of things. It is competent enough that you may let it
>> do some of the processing for you.
> The reason I noted "BLE" and not "WIFI" is the lower power:  Less RF
> can never be a bad thing in these circumstances.
With ESP32 you can do both Bluetooth and BLE besides WiFi and Ethernet.
Cost is low, so it is worth considering.
>
>> The EFOS10 maser has UPS and batteries, sure they are not prime but I
>> can for shorter measurements just go and unplugg power if needed.
> Have you tried it ?
Yes.
>
> How is the electrical grid configured on your rocks with respect to PEN ?
>
> Do you have a ground electrode at each house or does the utility
> provide the PEN potential via their cable ?

PEN provided with the three-phase as per standard delivery here.

Then from the central PE and N is separated.

> In the former case:  How old are the one at your house ?  If more
> than few decades old, it may not provide good contact any more.
I have not had reason to consider such problems.
>
> In the latter case, your PEN will be very noisy, and if you think
> you can hammer a 2.5m hard copper "electrode" into some kind of wet
> underground, a galvanic trafo on the mains might be relevant.

Actually, it's not my absolute noise which would eat into my
measurements, it's the relative differences between different parts of
the installation. In practice the lab is a relatively dense connection
on a separate branch of the rest of the house, with only a weak link to
the other part.

My main problem is in 50 Hz and overtones from what I so far have been
able to derive from noise. I did sniff one of the common mode
surpressions and it was very quiet in that range, so I saw some noise
that is more due to switcher supplies but not that strong.

One needs to think about isolation between sources, but actual isolation
may be the wrong method to go about it. The correct method can be to tie
things tighter together at low frequency. Mesh-bonding networks is
easier to maintain and has EMI benefits.

>
>> I have a ham friend that is fairly well into batteries as he needs to
>> for his telecom installations.
> LVDC people are much better at this than UPS people, the latter often
> think that batteries only need to last "until the diesel kicks in"
> and that is a very different application.

If you have a diesel, then the UPS is just a bridge. If you don't it is
the bridge until the power grid is back. Yet again, I have analyzed
spurious 8,5 kHz noise from a large UPS (it was running a load of 400
kW) and let me tell you, they did care about this spurious even if it
took them 18 months to replicate. With an older type of the filter, the
switching transistors could miss-fire out of phase such that
cancellation was not as intended. There is UPS folks that *DO* care,
because with their massive power the spurious can start to have
unpleasent consequences such as florescent lights singing and making
people go nuts.

Having a background with high dynamic audio systems, sniffing noise and
low signals is natural, I will follow the many threads there is.

Cheers,
Magnus