Wow George, you'll need to lose Google or soon no one will want to talk to you. :-)
It was quite enlightening to me when I realized that impedance matching and reflection are *not* exclusively RF concepts. They are valid from DC to daylight and beyond.
Jeroen Belleman
Levers and gears were invented before transformers.
Pity there's no decent mechanical equivalent to a buck-boost switcher. Cars still have to use bunches of gears.
wrote: [...] >> >> By the way, putting attenuators on a noise source does not necessarily >> reduce the noise level. I have on e where it would actually *increase* >> the noise. I use it for amplifiers with noise figures well below 3dB. > > Well OK if you've got big value resi stors. > (Just give me enough power and these 1% milli-ohm resistors and I' ll > have that excess noise down to almost nothing. :^) > > George H. Mmmh. Attenuators are usually impedance matched. What you are actually doing by choosing very high or very low resistor values is to intentionally mis-matc h the corresponding noise sources, so that their available noise power is m ostly reflected and doesn't end up in your signal.
ere I can buy an opamp and lower the source impedance. (Given opamp noise and all the other caveat?s.)
via the new groops... which looks like $H!t. Sorry.)
Yeah I feel like I've sold a bit of my soul to google. Well google chrome now.. grumble. George H.
Oh, that's interesting I never thought about power reflections at/near DC. Well, there's the battery power matching physics problem... I've never used it in practice. Mostly at DC I want no source impedance. :^) George H.
No small part of that is the properties of mechanical energy storage; springs and moving masses.
?-)
------------- You do. I mean, you do attenuate it as far as the noise that was *inputted*. But the "attenuator" creates its own -174 dBm/Hz noise. Because of that, a 3 dB attenuator has a 3 dB noise figure.
You can't distinguish between the white noise that was inputted and the white noise that is internally generated in the "attenuator," which is how the mixer behaves. They are random and uncorrelated.
ding ding ding...that was correct
however of course you can cool the attenuator (or mixer) and then it will attenuate without adding so much of it's own noise...
now for extra credit
how can an active LNA stage that is physically at 290K look like a good 50 Ohm load and at the same time have a noise figure better than 3 dB?
Mark
Because it isn't in thermodynamic equilibrium, due to the applied power. Just the same way that your fridge can cool beer below 300K even on a hot day.
Cheers
Phil Hobbs
---------------- Use "lossless" feedback.
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