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

[Bob kb8tq]

Hi

Die cast boxes are your friend when building this sort of thing. Hammond 
makes them and so do a lot of other folks. They are reasonably cheap and
pretty easy to work with. I have accumulated a pretty good pile of them “in
inventory” in a dusty box in a back corner of the basement :). 

One nice thing about a fairly heavy metal enclosure is that you can tie things
like BNC’s straight to the case. They give you a pretty low resistance “bonding”
without a lot of crazy effort. There are good reasons to do this and equally
good reasons to isolate depending on what you are doing. Blocking the RF
ground with a 0.1 uf cap is one example of isolation that folks do use.

My vote normally is to tie everything tightly together and then isolate the 
“far end” of the sources. This may involve grounding on supplies ( = use
a battery  maybe ) or other “interesting” approaches. Running everything 
involved off of a single *good* isolation transformer is not all that crazy.
(hint: You want the toroidal one with the better magnetic characteristics …). 

The other nice thing about the metal box is that when you knock the device
off on the floor, it’s less likely to re-wire itself. They tend to last longer that way :)

Bob

> On Jul 14, 2022, at 7:01 AM, Magnus Danielson via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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
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