[time-nuts] Re: Dual Supplies for Low Offset Phase Noise Analyzer

[Magnus Danielson]

Erik,

On 7/14/22 16:46, Erik Kaashoek wrote:
> Magnus,
> See below
>
> On 14-7-2022 14:50, Magnus Danielson via time-nuts wrote:
>> I am in full agreement. While there is a certain pain in doing it, 
>> but it removes a long set of issues relating to not having it. I've 
>> had that standpoint from the start, but felt that Erik may benefit 
>> from getting a lot of the other things going first and learn from it, 
>> as he seemed so negative about it. It is only when one sees the 
>> alternative cost that one can make the correct decisions. However, 
>> when trying to fix many things, it can take time to spend on all the 
>> things, so spreading things out a bit in time can serve it's 
>> purposes. This not to be disrespectful to Erik, rather the opposite. 
>> Then again, sometimes there is a different balance that pans out for 
>> it's purpose.
> No offense taken!!
Great.
> Unfortunately I'm the type of person that learns most by making 
> mistakes. So I first wanted to try to get the single supply working as 
> the PNA is not getting its own permanent supply. Performance at 10kHz 
> offset is now at the -155dBc/Hz noise level, although still with a lot 
> of mains harmonics but these could also come from the not so good 
> audio cable connecting the PNA and the Audio input or a ground loop.
> Replacing the single supply with a triple supply is still to be tested.

OK, great. Then I correctly assumed your modus operandi that I was 
trying to respect and support. Also great that you consider testing it.

As for ground-loop, that is somewhat misunderstood, as most often the 
ground-loop is not driven by magnetic fields, but different 
ground-currents driving up potential differences, and when 
cross-connecting causes a current to handle the difference in such 
voltage. The two main solutions are: Break away supply currents from 
reference voltage/grounding and make the ground-reference using bigger 
wires (both to the common point and between the different parts). I've 
found that considering it all as an operation of isolation helps to move 
away from incorrect analysis.

>>
>> I'm setting up measurement rig myself and it starts of with +5V and ± 
>> 15V. Luckily I have a spare supply delivering exactly that, will just 
>> check that it is reasonably in good condition.
>>
>> There will be a challenge of getting all the pieces fit together, but 
>> I'll slam a RPi4 to it and see how that works out.
>>
>> I've realized that I should be doing a input level measure and 
>> programmable damper. Wonder if someone already have nifty small 
>> boards for that
> As long as you want check for any level above +7dBm a simple rectifier 
> will do in combination with an analog input of the RPi

There is a whole board inbetween with lots of goodies, including RPD-1 
mixers. I was considering the input into the RPD-1 mixer from various 
sources which comes from completely different boxes in the labs. Some of 
them is wheel-around level of boxes.

Cheers,
Magnus