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Ultimately I'm generating some spectrum plots in L-Band (1200-1800 MHz) and I need to get calibrated flux units (dBm/Hz) for my y-axis. I need a little assistance on creating this memo.

I've been tasked to use simply a 50 ohm matched load followed by the 2 LNAs and RTL-SDR v3 and use the difference between the front end ON vs front end OFF to arrive at a value I can use for my reference. I realize there are a few assumptions going on here (input impedance vs. gain, RTL tuner noise lower than thermal noise, etc.), but they can be ignored for this (unless of course the tuner noise is indeed higher than thermal noise).

I'm having trouble putting the equations together to arrive at what I ultimately want (the ratio of the two?). I know Sin = kT (assume B = 1 Hz?). So Sin = 4.045 x 10^-21 W/Hz or -173.9 dBm/Hz. I will use GQRX for observing the noise floor in dBf. Do I solve for Teq2 which I can use kTB to give me the total estimated noise floor?

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    \$\begingroup\$ I'm not 100% sure, but I think a few RTL-SDR dongles have an AGC that can't be fully turned off, which would render your calibration useless. Make double sure this is not the case for your device! \$\endgroup\$
    – Marcus Müller
    Commented Feb 25, 2021 at 21:54
  • \$\begingroup\$ @MarcusMüller You can turn off hardware AGC in GQRX. For the rtl_sdr command, the AGC is turned off by default when you specify a gain. \$\endgroup\$
    – Macuser
    Commented Feb 25, 2021 at 21:59
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    \$\begingroup\$ again, there's different analog frontends for RTL2832U, be sure your actually has a disableable AGC. Just because you can set a checkmark in software doesn't mean the hardware does something. \$\endgroup\$
    – Marcus Müller
    Commented Feb 25, 2021 at 22:43
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    \$\begingroup\$ By the way, it's easy to build a noise source for that frequency range, with an ordinary small glass zener diode. I think about 12 V is the noisiest. Pass a small current through it. Build it right on an SMA connector, maybe with a 1 K smd resistor and couple the noise out with an smd capacitor. This will save you one LNA in your noise chain. The noise output will be up to 12 dB ENR, but it is not very flat. \$\endgroup\$
    – tomnexus
    Commented Mar 2, 2021 at 7:07

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