Electronics > RF, Microwave, Ham Radio

Is it possible to build a ~unity gain RF preamp with extremely low noise figure?

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cvanc:
Well, it's not really a preamp, as such, but more of a buffer.  Main goal is NF reduction.

Design ideas:
75 ohm in/out
88-108MHz; filtered beyond that
Very low NF (1dB max)
Unity gain is fine, a small amount of gain is fine, but not too much (8-10dB would be too much)
High headroom to pass hot signals without trouble

Purpose:
Provide a back-of-the-tuner, easily installed improvement in reception, by reducing the NF of the existing tuner (which can be 5 or even 7 dB).

Possible?  Thanks.

Andy Watson:

--- Quote from: cvanc on March 03, 2016, 12:00:06 am ---75 ohm in/out
Unity gain is fine,

--- End quote ---
If you keep the same input and output impedance, unity gain is not going to improve your noise figure. The receiver input will still generate the same noise, for the same signal level at the same impedance - plus any additional noise generated in your new buffer.
Assuming you want to keep the same input/output impedance you will need to add some gain to increase the signal level. Also, if possible, reduce the bandwidth.

T3sl4co1l:
Yes, a piece of wire will do nicely.

...

Well, you can't get a useful noise factor without gain at all, you can only add noise in that case.  So it's certainly not going to be a "buffer".  10dB would be a good figure for a LNA at the beginning of a carefully designed signal path; 15-20dB might be more desirable for a poor one (which I guess is the case here?).

Tim

Z80:
Hi,  you can't improve the NF of your receiver without performing surgery on it and giving it a better front end, but you can provide it with a signal that has a better S/N ratio.  The tried and trusted method to do this is to use a good outdoor antenna with some gain and directivity pointed at the weak station and away from the strong.  Adding a preamp at the top of the mast right next to the antenna improves the performance of the coax downlead.  If you have a strong local station causing problems, you may also need to add a tunable notch filter to stop it overloading the system.

cvanc:
(Sighs, looks down at mug full of decaf)
Looks like I picked a bad week to give up caffeine  |O

Yeah it's totally obvious my idea won't work, now that you've explained it.  All right, with the goal still in mind of improving my system performance, let me ask another question:

Existing tuner has NF of 7dB.  Goal is to reduce this dramatically, by 5 or even 6dB.  What is the minimum gain a preamp would need to accomplish this?  (Assume preamp NF of roughly 1dB.)

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