scope in a a box question

[Conrad Cardano]

Hi

What is the purpose of the 50ohm resistor?

Thanks

Conrad

[Alex P]

It is used to create the Background Correction  ( bandpass filter gain )  File for the

Kaminski IF_average Plugin of  AirSpy SDR# Studio  

( and that file set then processed by Pettit HLine3D and Rinearn Graphics )

https://github.com/AP-HLine-3D/HLine3D

[b alex pettit jr]

AND, the   Background Correction process creates nice data like This

[b alex pettit jr]

Blue  Background Corrected        Orange UnCorrected

[JERRY TAYLOR]

I recently upgraded my SDR to the air spy mini.  My IF Average window looks like that below.  My RTL SDR always filled the IF Average display and was a flat line except for the detected Hydrogen Line.  Why does my Aitspy Mini look like this?

Jerry Taylor 

 On Wed, Dec 24, 2025 at 3:33 PM 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com> wrote:

Blue  Background Corrected        Orange UnCorrected

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[Conrad Cardano]

Hi

I used the 50ohm resistor to remove the background, but still had a noisy image.

Alex,  I cannot get the nice smooth curve that you have.

Conrad

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[b alex pettit jr]

You Want the Flat Line 

Are you acquiring Several Minutes of Data ?

[b alex pettit jr]

Jerry, try whatever sample rate and decimation that creates a bandwidth of 1.2 MHz

( in my case 3MSPS & Decimation Factor of 2 )

[Sweet Thing]

Ambient temperature termination to show the noise floor of the LNA without any sky noise and without any R.F I

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[JERRY TAYLOR]

Hi Alex.  Yes I am acquiring 3-4 minutes of data.  Perhaps my calibration with the 50 ohm load was bad?  I switched to 3 msps and decimation of 2 thinking it would decrease the noise floor for calibration.

Jerry

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[b alex pettit jr]

 The 50 ohm termination creates a near 290K simulation ..

Its well above the LNA  Noise Floor &  higher than H Line and cold sky.

[b alex pettit jr]

Another combination : 6MSPS & Decimation of 4   .. I found if the bandwidth is too large the correction has 'tails' but 1.2MHz works

&  : WhatEver Condition you use for Calibration you Must Also Use for Data Acq

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

[JERRY TAYLOR]

Thanks Alex.  I will try the 6msps and decimation of 4.

Jerry

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