Measuring noise temperature of LNA

[Fong Zi Hao]

Hello all,

I came across this article from WVURAIL(pdf attached below), page 3 regarding a method to measure the noise temperature of LNA. The hot load used will be a SMA termination load, 290K. For cold load with no input, it is assumed to be 0K.

So I tried this method and I got 160K for the Nooelec H1 LNA. From the datasheet, the typical noise temperature is 59K. What causes the difference and is this the expected result? If you are using the LNA from Nooelec as well (the one with casing) or any other LNAs, please share your measured noise temperature with this method for comparison. Thanks!

[Dave Typinski]

Is it wise to assume 0 K for an unterminated LNA input?
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[Alex P]

I made some Open/50ohm measurements of the SawBird H1 and the WD5AGO set several months ago.

Using the suggested method ( attached )

SawBirdH1 :  Tsys = 89K     ( 50-Open =  6.3 dB )

WD5AGO   :    Tsys = 28K    ( 50-Open = 11.5 dB )

But, I question this as there may be issues with changing the input Z into the LNA and Open is probably not really  0 K .

Alex KK4VB

[Marko Cebokli]

Hello!

Using open as a cold reference makes no sense. Such a measurement gives
no useful data about the amplifier's noise level.
Noise figure is very sensitive regarding the input reflection. For a
real measurement, you must make sure, that the impedance which the
amplifier "sees", does not change between hot and cold. Open is a
maximal possible reflections, so the result is complete nonsense.
No amount of fancy huge ubercomplicated software can't change that.

Search the net for "ground/sky moise measurement" for a hugely better
method.

Marko Cebokli



On 17.07.2022 13:03, 'Alex P' via Society of Amateur Radio Astronomers
wrote:

> I made some Open/50ohm measurements of the SawBird H1 and the WD5AGO
> set several months ago.
> Using the suggested method ( attached )
>

> SAWBIRDH1 :  TSYS = 89K  ( 50-Open = 6.3 dB )
>
> WD5AGO   :    TSYS = 28K  ( 50-Open = 11.5 dB )

>
> But, I question this as there may be issues with changing the input Z
> into the LNA and Open is probably not really 0 K .
>
> Alex KK4VB
>
>>
>

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[b alex pettit jr]

Hello Marko,

Thanks for confirming my questions.

FYI : a full system Tsys Ground/Sky measurement gave 77K  with the WD5AGO LNA

Alex

===================================================================

[Fong Zi Hao]

Thanks for the clarification. There's this article from them that sums up the noise temperature of different LNA using the open/50ohm method, the numbers obtained seem to be reasonable. But from what Marko has mentioned, I will try the ground/cold sky method tomorrow.
Few questions: 
1. Is 10K of the brightness temperature of the cold sky away from the plane of the milky way a good approximation? Because from my location(equator), it is difficult to observe at the celestial pole, so I wonder if there is a list of regions of sky that is suitable for cold sky calibration.
2. For ground, is it ok to assume 290K if the dish is pointing at the wall since there is no clear view of trees at my location. I read that we can use RF absorber material to cover the opening of the cantenna but I think this measurement will remove any sidelobe response which will be present in the cold sky measurement.

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[Lester Veenstra]

Incorrect cold load

 

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[fasleitung3]

Hi,

We measured three Noolec Sawbird H1 with a noise figure meter and they all were well within spec. Typical values were 0.56 dB (40 K).

We use 10 K as the brightness temperature at the celestial pole. I am not sure where that number came from, probably from the survey by Reich & Reich. Since this location is difficult for you, you might want to try a location well outside the galactic plane. Check with the 1420 MHz continuum survey by Reich et.al. that there are no significant source where you are pointing. This should also give you something in the order of 10 K.

The best approach is to use an absorber as hot reference and use your ambient temperature then. You actually want to have the emission from that material entering through sidelobes as well. A wall is not ideal, and its brightness temperature will certainly depend on the type of material. We found in tests that this is not very reliable. Trees/bushes are much better but I understand this is not an option for you.

I do agree with the opinion that an open input is not a useful cold reference.

Best regards,

Wolfgang

[Marko Cebokli]

Hello!

The "sky" temperature at 21 cm in most cases will be more due to the
sidelobes of your antenna picking up ground noise, especially if you use
a wide beam antenna, like a dish feed.
For the extraterrestrial noise, you can use the "radio eyes" (if I
remember right) software, to tell you approximately what the sky
contribution will be, when pointing at a particular point in the sky,
with a given antenna beamwidth. With a wide beam antenna, it's best to
point at zenith. You can check at what time there are no big sources
there at your location - but with a wide beam dish feed, that is not
very important.
Realistic values for total "sky" measurement will be between 15 and 50K,
with a wide beam dish feed more towards 50.

For the "ground" measurement, point at something absorptive, like a
dense bush (vegetation). Try a few things, to see which gives the
highest noise. Do not put the antenna too close (<1m) to the stuff, to
avoid changing it's impedance.
Pointing at a shallow angle at a flat ground will give you too low noise
(sky reflection).

If you have some software, that can display the spectrum within your
bandwidth, check that you have no interference (sharp deviation from
flat noise, level jumping up and down, etc).

Use a total power measurement in your (as wide as you can manage without
interference) bandwidth, not a marker on a spectrum display, which will
be noisy.

Marko Cebokli

>> SUCH A MEASUREMENT GIVES NO USEFUL DATA ABOUT THE AMPLIFIER&#39;S
>> NOISE LEVEL.

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[Fong Zi Hao]

Thanks for the input.
So I believe the idea is to use a good absorber for hot load, cold sky with no big source for cold load while maintaining similar sidelobe response for both measurements to ensure we are only changing one variable which is the brightness temperature of the load. I remember there is another way to calculate system temperature as well using RMS of spectrum*sqrt(FFT resolution*integration time), so this can be used to verify the result obtained from ground/cold sky method.

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[Marcus D. Leech]

For measurements involving the sky as a "cold" source, it's probably
best to use a largish horn antenna, since they have lower side-lobes.  
I had a local HVAC shop make me
  a conical horn that is about 66cm at the aperture. It wasn't cheap,
but it wasn't horrifically expensive either--about C$200.00.

[Fong Zi Hao]

Correction:

Misread somewhere that the celestial pole is used for the cold sky measurement but actually it is the galactic pole, no problem observing at that region. Sorry for the confusion.

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[Monroe Pattillo]

I've tried on two separate days to use the SARA site to renew my now expired membership. Both times after putting it in my shopping cart and entering my contact information the site indicated it can't currently do it and returns an HTTP 500 error.

Monroe Pattillo
monroe_...@bellsouth.net

[Mike Stewart]

I experienced similar issues…never could get it submitted. I’ll mail a renewal when I can find some checks. Used to have a bunch of those once upon a time. 

Mike

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[James H Van Prooyen]

Hi All

I am trying to register for the conference,

it send me to this web page:

https://radio-astronomy.org/store/

But there is no entry for the eastern conference.

Any one have any idea where it is?

LOL...

Jim Van Prooyen

gr...@sbcglobal.net

https://groups.google.com/d/msgid/sara-list/PWPGKSVCgbXsRqV1AIVPStL7NJ5uQPzGk5mPBmGu94tYQGGxN-MVCtPcgxXSXrxYWSOicHiRisoruQ3ncPkAHNuBaVG8dbBXVKwEOhB3Lyw%3D%40protonmail.com

.

[duncan campbell-wilson]

Suggestion:

This is not a new technique for a simple cold load, the technique having  been developed in the 1970's.

(1) Find an LNA (cold reference)  which has a known noise temperature much less than room temperature at the system characteristic impedance.

(2) Terminated the Cold reference LNA output in  its characteristic output impedance (50 ohms).

(3) Connect the input of the Cold reference LNA  to the input of the device under test (DUT) and power the device.

(4) It is important to know the loss in the input connectors as they contribute noise at room temperature.

(5) There will be  errors in the measurement from mismatching but this is much better than an open circuit.

(6) Make sure any digital devices in the vicinity are switched off.

(7) Use the Y factor technique to assess the noise factor ratio.

(8) My experience is that below  0.3 dB noise, the measurement uncertainty is a difficult parameter to assess.

Rgds Duncan

 

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[Thomas D Jacobs]

Monroe:

It is nice to know that the SARA store is not just picking on me. I too tried to renew my SARA membership using the "store" pages. I don't receive an HTTP500 error. When clicking on the "PayPal" link to enter the credit card information, my browser just hangs (current version of Firefox). When I try clicking some other link on the page I get a "Firefox dialog" warning me to stop and go back; the cert and the domain do not match or can't be verified.

To circumvent this problem I have decided to renew using "old school" methods. I'm going to mail the treasurer a paper check.

Be safe and stay healthy,
tdj

-- 
Take care,
Tom Jacobs

Wilmington, North Carolina
mailto:cretaceou...@gmail.com
mailto:td...@bellsouth.net
KD4BFQ
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+------------------------------------------------------+

[Fong Zi Hao]

Hello all,

So I tried to determine the system temperature of the setup by pointing the dish at a wall, 300K and cold sky(galactic pole), 10K. The system temperature is around 200K. Tried adjusting the position of the cantenna but there are no significant changes, only a difference of 20K or so. Can the probe length affect the system temperature? I am using a length of 5.25cm, no access to a nanovna so not sure if it is in resonance.

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[duncan campbell-wilson]

Hi Fong,

What is the expected spillover for your antenna system?

These are tricky measurements.

Rgds Duncan

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[Alex P]

Hello Fong,

Try this Calculator

https://www.janbob.com/electron/Calc_QE.html

Alex KK4VB

[Marcus D. Leech]

On 2022-07-21 07:34, duncan campbell-wilson wrote:

Hi Fong,

What is the expected spillover for your antenna system?

These are tricky measurements.

Rgds Duncan

The "seat of the pants" number range I usually use for guessing at Tsys for an amateur system is between 90K and about 120K.  30 years ago, it would have been
  200K to 300K.

I'd agree with "tricky".   Without knowing the spillover profile of the antenna, that 10K "cold sky" could easily be more like 30-40K.

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[Fong Zi Hao]

Actually I have no idea regarding the spillover profile of the antenna. I found this calculator that plots the efficiency and antenna pattern with spillover: https://portia.astrophysik.uni-kiel.de/~koeppen/JS/AntennaFeeds.html#ANTPAT . Not really how to interpret these results to the system temperature. But I guess the tricky part is in getting the correct brightness temperature for the objects the antenna is pointed to.

Using the calculator from Alex, it shows 120K, which is just in the range.

Dish diameter: 1.4m. Collar height: 0.46m. Feed diameter: 0.145. F/d: 0.345. 

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[b alex pettit jr]

Since the definition of "ColdSky" is in the almost-too-difficult-to-determine category, what about

 comparing  the delta dB between Earth&Sky as a performance characterization value ?

Alex

[Marcus D. Leech]

On 2022-07-21 08:50, 'b alex pettit jr' via Society of Amateur Radio Astronomers wrote:

Since the definition of "ColdSky" is in the almost-too-difficult-to-determine category, what about

 comparing  the delta dB between Earth&Sky as a performance characterization value ?

Alex

As a rough "is my LNA working", getting some amount of difference between pointing at the ground, and pointing at a galactic-plane-free sky works just fine.

The ratio as seen at the output of the detector will be:

(Tsys + 297) / (Tsky + Tsys + Tspill)

But you can see that without knowing fairly-precisely the value for Tsky where you're pointing, and Tspill from your antenna, it's hard to get anything other than
  "is my LNA working" numbers.

[Jeff Kruth]

OK, very tough measurement with many variables, that if unknown cause great uncertainty.

However, one of the biggest contributors to system noise temp can be any loss between the Antenna and the LNA. Cable, bad (cheap) connector, etc. can contribute. Even mismatch on the probe VSWR can cause insertion loss ripples which if a peak loss ripple happens to fall at your frequency, it can worsen things. A .1 dB loss is 7K or so, if you have 1 dB loss then you get another 70K noise temp (approximately).

Just a thought.

Jeff Kruth

 

[B & MR Randall]

The CFL Light in the picture puts out a strong noise signal at 12 GHz.  IBT could easily see it at 3 meters away on bore sight.  Probing around the edge of the dish can give a feel for where spill over is.  Tried this same light at 1420 MHz and it is not detectable.   A similar light with a plastic housing is easily detectable at 1420.  The metal grid is apparently close enough spaced to be a shield at 1420?  Note that CFL bulbs are fragile and if you bump the light the glass can break.

 

CFL bulbs are less common than LED's now so I keep a few spare CFLs around as an uncalibrated signal source.  LEDs do NOT make any noise at 1420 or 12 GHZ.   LED's power converter can make a lot of noise at 20 MHz.

 

Crude test equipment can't make measurements but can often give insight into what is happening.  I always have the CFL lamp along at IBT demonstrations.

 

Sorry about shoe in picture.  I am not a very good camera operator!

 

Bruce Randall

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[Marko Cebokli]

Hello!

I would first try to find a way to check the feed impedance. Also of any
stuff between feed ad LNA.
Maybe some nearby HAM can help you? If you plan to experiment more with
antennas, a small sub 100$ pocket VNA would be a very good investment.
The original nanoVNA is only good to 300MHz, but there are new versions
(SAA-2 etc), that go beyond L band, still for <100$.

Marko Cebokli

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[fasleitung3]

Hi,

A few comments on "Measuring the noise temperature of LNA" and " system temperature".

As discussed, measuring the noise temperature of the LNA requires a calibrated noise source and a careful measurement setup.

Trying to determine the LNA noise temperature from a systen temperature measurement seems to me almost impossible as the system temperature contains many contributions. Only one of these contributions is the LNA itself, but there are many others like spillover and all the others which have been mentioned in this thread. These contributions are not known and also almost impossible to assess on a theoretical basis.

So I think the best you can do is to take the specification of the LNA as provided by the vendor for granted. As mentioned, we have checked three of these devices with a noise figure meter and they all performed well within specification. So this would give you something anywhere between 40K (our measurement) and 60K (vendor specification).

As you have measured a system temperature of 200K this means that you have significant noise contributions from other effects. If you would like to improve your sensitivity you may want to look at the other contributions rather than the LNA. My gut feeling would be that spillover might be the #1 offender. For a small dish like yours (1.4-m I believe), the numbers mentioned by Marcus seem a bit optimistic for me. But in any case there should be some room for improvment.

Good luck,

Wolfgang

[duncan campbell-wilson]

Hi Fong,

My exploration of a similar problem started with excessive noise temperature in two antenna systems. One a 6m parabolic antenna  the second a 13m wide  cylindrical antenna. Both systems  showed excess system noise 100 K above the expected.

Both antennas have an f/D of 0.4.

My strategy for  isolating the issue is to determine or confirm the operation of individual elements  and then integrate the units and test.

(1) I am lucky to have an old HP 8970 noise figure meter and noise head, so the confirmation of 22 +/-5 K LNA operation was relatively easy once all that calibration and noise drifts  from the measuring  system were included.

(2) I constructed a metal test box where I could place my antenna relatively free from external  noise and RFI. The aim was to understand the material  loss in the antenna structure. The box  represents a short circuit / electromagnetic cap and has TEM modes so the antenna loss has some large error bars as I haven't worked through the TEM details. The antenna contribution looks like  10 +/- 5 K.

(3) As designed, the  antenna/ LNA design doesn't have an in-built coupler for calibrated noise injection. To this end  I manufactured a noise source based on a resistor, MAR6 amp and a poorly matched printed spiral antenna. The spiral antenna is   positioned directly above the LNA MMCX input connector pads. With a bit of trial and error an increase in noise temperature of 10 K was measured and confirmed by the noise figure meter.    

(4) The goal of (3)  was to put a known  modulated noise source as close to prime focus as possible to measure the actual system Tsys.

(5) This work (4) has been curtailed by weather for the present.

(6) Where is the collector sweet spot for maximum signal coupling from the collector on the Z axis?  A journey still in progress in both antenna cases.

(7 ) My take on spillover.

This is the rudimentary way I view the issue. Consider two unit spheres, one sphere  represents the polar response of the illuminating antenna (dipole) at prime focus, the second is a polar map of the thermal environment as viewed from prime focus. A cautionary note is that there are a high fraction of steradians in the antenna  side and back lobes. Similarly  small errors  in antenna sidelobe performance,  especially from the collector open angle to horizon have profound effects on the ground noise contribution Tsys.

I hope this helps.

Rgds Duncan

      

     

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