Low-noise power for LNAs

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Marcus D. Leech

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Sep 15, 2023, 3:14:27 PM9/15/23
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I just ordered a couple of LTC3045 modules from Amazon and they'll be
here in a few days.

I'm going to re-organize the power distribution to our H1 feed system at
the observatory, and these LTC3045
  low-dropout regulators have very very low output noise and very very
good (70dB) PSRR.

Currently, we have a linear 15.4V supply "down below" which sends power
up the coaxes, and then there's
  a two-stage regulator, producing about 8.5V with a switching buck
converter, and then a linear output stage
  at 5V, based on LM7805.   My suspicion has been that our failure to
find the RRLs in M17 last summer was
  due, in part, to residual power-supply noise.

We likely won't get to that project for a few weeks yet (for one, the
scaffold we use to service the feed is currently
  tied-up in structural-repair work).

I'll report here on any significant results.

I wonder what some others are using in order to reduce power-supply
noise.  I'm sure some beginners are just using
  "whatever that USB port supplies", and getting
adequate-for-beginning-amateur results.

Even with the existing power supply chain, we get about 50-70:1 SNR on
the H1 response, with very-short (a few seconds)
  integration times.  I want to drive that down a little further.


Anthony

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Sep 15, 2023, 4:46:13 PM9/15/23
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Marcus,

I’m looking into Bypass capacitors for filtering, using them to prevent noise from entering the system and passing it to the ground? I’ve been doing research and reading up on removing excessive noise from linear power supplies. I haven’t dealt with two-stage regulators, that may be a bit more complex for what I’m doing.

The Ferrite Beads & chokes tried but didn’t see changes using them. I’m working on redoing my grounding as well, using the star grounding, a common method (all devices connected to a single earth point) to help with preventing grounding loops, known to introduce noise into the system.

This may not be practical Marcus, but have you thought about placing your Linear power supplies and regulators in a well shielded metal container to reduce noise but that probably depends on your environment, temperature, pressure, and humidity probably. Maybe consider moving power sources as far away as possible from the system and shielding everything well, which I'm sure you've thought of already or have maybe restrictions in distance?

I’m overhauling my entire interferometer, from the feedhorns, power sources, LNA & BPF to even the cabling. I have been looking at Heliax low signal loss coax cable as compared to LMR low loss coax cabling for noise reduction. 


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Alex P

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Sep 15, 2023, 5:47:19 PM9/15/23
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....  a switching buck converter ...  

Stay with Linear Voltage Regulators ... such as
LVR_5v.jpg

Marcus D. Leech

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Sep 15, 2023, 5:51:52 PM9/15/23
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On 15/09/2023 17:47, 'Alex P' via Society of Amateur Radio Astronomers wrote:
....  a switching buck converter ...  

In general, yes, particularly if the output is going to be directly connected to the LNA power input.  But it is known that noise
  can be reduced by using a linear output stage in a two-stage approach, which is why we tried that, and it seemed to
  improve things considerably, but not *quite* enough.


b alex pettit jr

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Sep 15, 2023, 6:05:41 PM9/15/23
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Try adding a Balanced Line Filter ( or two ) ... Separate the Ground between In/Out of this

Inline image


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Anthony

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Sep 15, 2023, 6:42:46 PM9/15/23
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Alex, can that balanced line filter be used for the Linear power supply, regulated or both?

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b alex pettit jr

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Sep 15, 2023, 7:00:26 PM9/15/23
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Hi Anthony,

1) use NO switching supplies, buck/boost regulators etc. Only Linear
2) use the filter near the device being powered.




Anthony

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Sep 15, 2023, 7:03:56 PM9/15/23
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Thanks, Alex!

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b alex pettit jr

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Sep 15, 2023, 7:05:43 PM9/15/23
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Alex P

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Sep 16, 2023, 6:59:18 AM9/16/23
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DC Power  Line Filters 

Anthony

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Sep 16, 2023, 7:20:32 AM9/16/23
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Amazon, pretty much has almost everything. 😉 

I do like the NXTop or FTVogue models.

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b alex pettit jr

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Sep 16, 2023, 7:43:37 AM9/16/23
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Yes, they are hermetically sealed and fully shielded.

I'm using a linear PS , but have never tested adding an LNA power filter ..  will do that .

On Saturday, September 16, 2023 at 07:20:33 AM EDT, Anthony <itpart...@gmail.com> wrote:


Amazon, pretty much has almost everything. 😉 

I do like the NXTop or FTVogue models.

On Sat, Sep 16, 2023, 6:59 AM 'Alex P' via Society of Amateur Radio Astronomers <sara...@googlegroups.com> wrote:
-

Marcus D. Leech

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Sep 16, 2023, 9:42:47 AM9/16/23
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The problem with all of these is that for maximum effectiveness, you need to isolate the grounds of the power-supply and LNA.  In
  some deployments, that's not practical.


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b alex pettit jr

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Sep 16, 2023, 9:51:25 AM9/16/23
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A Linear Power Supply is pretty clean w/o added filtering  
( 12 V dc   w/ 1mv PkPk Noise)

Inline image


b alex pettit jr

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Sep 16, 2023, 10:05:45 AM9/16/23
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You could float the PS such that the output has only the feed/LNA as the true Ground point.
==============================================================================


The problem with all of these is that for maximum effectiveness, you need to isolate the grounds of the power-supply and LNA.  In
  some deployments, that's not practical.



Inline image


Marcus D. Leech

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Sep 16, 2023, 10:26:05 AM9/16/23
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On 16/09/2023 09:51, 'b alex pettit jr' via Society of Amateur Radio Astronomers wrote:
A Linear Power Supply is pretty clean w/o added filtering  
( 12 V dc   w/ 1mv PkPk Noise)

Yes, that's not bad.  Indeed, your pics suggest that extra external filtering does almost nothing
  to the residual noise...

For most LNAs, the DC is introduced at the output pin of the LNA, so whatever noise is present there,
  is also "seen" by the down-stream device--attenuated considerably both by the typical L-C input
  filter in the LNA bias circuit, and by the output capacitor of the LNA.


Inline image


On Saturday, September 16, 2023 at 09:42:49 AM EDT, Marcus D. Leech <patchv...@gmail.com> wrote:


On 16/09/2023 06:59, 'Alex P' via Society of Amateur Radio Astronomers wrote:
DC Power  Line Filters 
The problem with all of these is that for maximum effectiveness, you need to isolate the grounds of the power-supply and LNA.  In
  some deployments, that's not practical.

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Anthony

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Sep 16, 2023, 10:42:40 AM9/16/23
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What do you mean, isolate the grounds, separate them?

Lester Veenstra

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Sep 16, 2023, 10:45:57 AM9/16/23
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If you can float your power supply, ie no output ground connection for (+/-) leads this will work well with the added feature of avoiding ground loop induced noise.  Single point ground at the LNA.

 

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les...@veenstras.com

 

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b alex pettit jr

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Sep 16, 2023, 10:53:21 AM9/16/23
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Yes.  That was the idea  :)

Thanks, Alex
========================================

b alex pettit jr

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Sep 16, 2023, 11:04:18 AM9/16/23
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This was the idea.....
Lots of this is going to be cut&try....
Inline image


Anthony

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Sep 16, 2023, 11:56:57 AM9/16/23
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Ah!

Anthony

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Sep 16, 2023, 11:57:30 AM9/16/23
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Appreciate it!

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Anthony

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Sep 16, 2023, 5:19:55 PM9/16/23
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Hey Alex,

Did I miss the email, did you test the LNA filter and grounding from the LNA, instead of the linear power supply?

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b alex pettit jr

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Sep 16, 2023, 5:36:06 PM9/16/23
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Hi Anthony,

No. That is something that needs to be done with the system fully operational
and outside looking at the Cold Sky to be able to see a change in baseline noise level.
In the coming weeks when it gets cooler..

Alex

Anthony

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Sep 16, 2023, 5:45:06 PM9/16/23
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Ok,  that's what I thought, you hadn't tested yet. Tonight or tomorrow I'll make a choice on Amazon.  The Kuhne LNAs I'll test in the coming week or two.




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b alex pettit jr

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Sep 16, 2023, 8:10:45 PM9/16/23
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You might consider making up an AA battery pack to power the LNA and located near/at the LNA as a "noise free" reference.
Use that as a baseline to compare the performance of your actual LNA power supply..



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