[hpsdr] NOISE FLOOR

[Scott Traurig]

Tony,

Common rule of thumb: attenuator noise figure = attenuation. When the
attenuator comes before the first amplifier, as it does in most radios, its
noise figure will dominate the noise figure cascade analysis.

You can easily demonstrate this by first applying enough attenuation to see
the noise floor rise a little (you have to make noise figure bad enough so
it becomes a factor). Then increase the attenuator by 10dB and you will see
the noise floor jump up approx. 10dB.

See here for more on noise figure and noise figure cascade analysis. There
is a link there to a handy calculator that works a three stage Friis
equation:

https://www.microwaves101.com/encyclopedias/noise-figure

The preamp (the only amp!) gain in the 7000 and 8000 is 14dB. In the other
radios it is 20dB. In all cases the preamp noise figure is about 6dB.

Lots of GREAT stuff at the Microwaves101 site, BTW.

Also somewhat related:

https://apache-labs.com/community/viewtopic.php?f=6&t=2463

73,

Scott/w-u-2-o
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