What are we actually doing when increasing the gain?

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Github
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What are we actually doing when increasing the gain?

Post by Github » Sun Jan 28, 2018 6:44 pm

I'm totally confused by signal amplifiers. When I search around, I know that amplifiers inside or outside an rtl-sdr amplify both noise and signals. But when you decrease the RF gain to zero, you get nothing, when you increase the gain, you begin to see something after a certain level of gain. But again, signal and noise are both being amplified, why you be able to see a signal after a certain level of gain?

Github
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Re: What are we actually doing when increasing the gain?

Post by Github » Sun Jan 28, 2018 6:56 pm

One hypothesis I can come up with is that an amplifier only amplify natural/human noise (universe noise, sun noise, out of band noise etc.), it doesn't amplify thermal noise. So the signal strength after an amplifier should be input signal strength + amplifer gain - amplifier noise figure - all noises except thermal noise. Is this right?

rtlsdrblog
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Re: What are we actually doing when increasing the gain?

Post by rtlsdrblog » Mon Jan 29, 2018 3:34 am

The ADC needs to see a high enough signal level to be able to work.

Also increasing the gain decreases the amplifier noise figure which increases SNR. This is a pretty good video that explains it https://www.rtl-sdr.com/an-video-explai ... culations/.

Github
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Re: What are we actually doing when increasing the gain?

Post by Github » Mon Jan 29, 2018 8:02 pm

Ok know I understand. From my understanding, every receiver have a noise figure (rtl sdr is really high!), when an input signal enter receiver, it needs to be amplified to overcome noise figure, because noise figure doesn't change when gain changes. The guy in video used a differnt way to approach thermal and all other noises involved, actaully I don't quiet understand this, but looks like thermal noise keeps the same after amplification, because noise at antenna decrease when frequency increase, that mathches what I've learned before, thermal noise keeps the same at all frequencies, but pulse/universe/sun noise decrease when frequency increases, and MW/SW subjects more interference compare to vhf/uhf. But there is another question, is it possible to retrieve a signal below thermal noise at antenna end by only using LNA?

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Re: What are we actually doing when increasing the gain?

Post by Github » Mon Jan 29, 2018 8:26 pm

I did some calculations by myself, I realized when reciever has the same noise figure as an LNA, or antenna noise temperature is too high, add a LNA is meanless. So when a signal is really really weak at antenna end, it is equivalent to high antenna noise temperature, am I correct?

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Re: What are we actually doing when increasing the gain?

Post by rtlsdrblog » Tue Jan 30, 2018 5:22 am

Even if the NF of the receiver and LNA are the same, you can still benefit from an LNA. It can help mostly if are big losses in the coax cables, or maybe any filters/switches etc along the signal path.

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Re: What are we actually doing when increasing the gain?

Post by Github » Sat Feb 03, 2018 11:51 am

I have a new understanding to the RF gain, don't know if it is correct or not. A LNA amplify everything include thermal noise by 30 db, then decrease 1 db to the SNR. And the reason we need high gain low noise figure LNA is because ADC needs more power to work better as long as the signal doen't overload the ADC.

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