From: [email protected] [mailto:[email protected]] On Behalf Of erik@...
Sent: Saturday, February 8, 2020 12:32 PM
To: [email protected]
Subject: Re: [HBTE] TinySA construction

?

Jerry,

Noise level is bandwidth dependent.
Noise represents a certain energy and the amount of energy you get into your energy level detector depends on the bandwidth of the signal allowed to enter the energy detector.
This is different from a normal signal that has limited bandwidth, these signals are either in or out the bandwidth of the detector and thus either seen or not seen.

The noise energy formula is:

Pn (average level) = 10 log10(k * T * BW) + NF
Where:
k is Boltzman constant.
T is temperature in Kelvin
B is bandwith in Hertz.
NF is the Noise Factor (the internally generated noise level)

So you with a noise source of a certain temperature (e.g. certain amount of total energy that is leaving the source over all frequencies) the amount of energy you measure goes with the log of the bandwidth.
The PGA changes the overall level and if the noise input is more then the PGA itself produces, the PGA will amplify the noise. If the noise is lower you will only see the internally generated noise from the PGA.
AGC sets the PGA

The NF of the SI4432 is not specified but it is possible to detect a signal at -121dBm when using a bandwidth of 2.6kHz

So lets rewrite the formula:
Noise floor = 10 * log(k * T * 1000) + NF + 10 * low BW, T = 290K
Noise floor = -174 + NF + 10*log (BW)?
BW = 2.6kHz
Noise floor = -174 + NF + 10 * 3.4
-121 = -174 + NF + 34
So we find:?NF = 19
The LNA is not really low noise. Using a good low noise LNA (with a NF of 1) you can gain 18dB noise floor, of course the dynamic range will not change so you will lose at the high side.
.
Hope this explains what is happening.




--
HBTE Files section:?/g/HBTE/files
Erik, PD0EK