Noice measument in S.A noob problem

Fovakis said:

Hi !

On the analyzer i think i see the - 174dBm + 10log(RBW) + NF of S.A

for RBW=1KHz and NF of S.A = 30dB ..Noice Input (analyzer with input at

50Ohm)=-114dBm

I think this is what i see in the display screen. Maye there is some also -( 2-3 dB) of noise that the Sample dETector adds..

Also i think the real display BW is 1.2*RBW ,because of gauss IF filter's shape

is this right?

So today i did a simple experiment with an LNA . I saw that the output noice power was not input +GAIN + NF .... i don't know why this...

Check the attached please https://i1284.photobucket.com/albums/a578/fovos1/question_zps45a75a76.png

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jiripolivka said:

In a low-noise receiver, the INPUT noise power is defined by your formula:
Pn in = -174 +10 log BW + NF, in dBm

After a low-noise amplifier with 30 dB gain, Pn out = Pn in +30 dB

What you see on spectrum analyzer is often a bit different. The reason is that its IF filter(s) bandwidth is often not exactly defined, and many spectrum analyzers have a high NF which seriously degrades the small noise contribution of the LNA.

Always make sure that in addition to the LNA you use another RF amplifier, and the gains of LNA and added RF amplifier exceed 40 dB. The real receiver noise figure can be estimated from Friis' formula:

NFr = NFlna + [NFamp-1]/ Glna,

where CAUTION the NF is not in dB but in "times" (anti-log)

- - - Updated - - -

I would never advise to use a spectrum analyzer directly to measure noise power. Its high NF degrades anything.
Spectrum analyzers are designed to compare SIGNAL power over a noise floor while the noise floor is not well defined. Many S.A.s use input harmonic mixers with quite undefined noise features.

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jiripolivka said:

First you need to have a modern CALIBRATED S.A. to be able to measure noise power reliably. A good S.A. I used was from HP, with switchable vertical resolution, 10, 5, 2 and 1 dB/div. It also had a memory to record the maxima of noise peaks.

I used this S.A. with an advantage to adjust a low-noise down converter with 0.1 to 10 MHz where RF power meters are not calibrated. I measured the Y-factor by using the input temperature ratio, "hot" the ambient, ~300 K, and "cold", liquid-nitrogen cooled load at ~77 K.

The best way to measure noise power is to use a good calibrated wideband RF power meter.

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Fovakis said:

Unfortunately i don't have a power meter. I have a transmitter sending 16QAM at 2465MHz BW=20KHz. The receiver some meters near take these data. So..

** to measure the real noise power input, i must put the S.A RBW equal to BW of my signal right? To detect how much noise is in the region of my signal?

So i will have: -174dBm+10logRBW=20KHz +NFs.a.

The real noise power input is the above without the NFs.a .i am right?

[URL=https://s1284.photobucket.com/user/fovos1/media/P6131098_zps37d09fcd.jpg.html] [/URL]

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