Is it possible to build a ~unity gain RF preamp with extremely low noise figure? - Page 1
Forum section: electronics / rf-microwave

author: cvanc
author_rank: Frequent Contributor
timestamp: March 03, 2016, 12:00:06 am
content: Well, it's not really a preamp, as such, but more of a buffer. Main goal is NF reduction.Design ideas:75 ohm in/out88-108MHz; filtered beyond thatVery 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 troublePurpose: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.

author: Andy Watson
author_rank: Super Contributor
timestamp: March 03, 2016, 12:17:46 am
content: Quote from: cvanc on March 03, 2016, 12:00:06 amIf 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2016, 12:18:20 am
content: Yes, a piece of wire will do nicely....Well, you can't get a useful noise factor without gainat 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

author: Unknown
author_rank: Unknown
timestamp: March 03, 2016, 10:48:32 am
content: 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.

author: cvanc
author_rank: Frequent Contributor
timestamp: March 04, 2016, 02:02:38 pm
content: (Sighs, looks down at mug full of decaf)Looks like I picked a bad week to give up caffeineYeah 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.)

author: KJDS
author_rank: Super Contributor
timestamp: March 04, 2016, 02:35:23 pm
content: You need to convert from dB to linear gains and put them into the Friis equationhttp://www.microwaves101.com/encyclopedias/noise-figureThe maths is fairly straightforwards, it's worth putting it into a spreadsheet and working the numbers yourself to get a better feel for it.If you're feeling lazy then there are an assortment of noise figure cascade calculators available online.

author: rjeberhardt
author_rank: Regular Contributor
timestamp: March 05, 2016, 02:17:23 pm
content: There is a reason why FM tuners typically have noise figures of 5 to 7 dB. In a typical urban environment the background noise level can easily swamp the thermal noise in the receiver so there is nothing to be gained with a better noise figure. Dynamic range is more important.If your requirement is for a single low level station your best bet is a high gain, narrow beamwidth, antenna.Russell.

author: Kelbit
author_rank: Regular Contributor
timestamp: March 11, 2016, 09:05:32 am
content: Quote from: cvanc on March 04, 2016, 02:02:38 pmApplyFriis' Equation:F_total = F1 + (F2-1)/G1 + (F3-1)/(G1*G2) + ...In your case, you only have a two-stage system, so this simplifies to:F_total = F1 + (F2-1)/G1Converting to linear ratio, 1 dB NF is equal to a noise factor of 1.26 and 7 dB is equal to a noise factor of 5.01. Your target NF is 7-5 = 2 dB which works out to a noise factor of 1.58.We can then substitute:1.58 = 1.26 + (5.01-1)/G1Solving for G1 yields a linear gain of 12.5, or approximately11 dBfor an LNA with a noise figure of 1 dB.Designing an LNA with this kind of performance should be achievable. However, your P1dB will be pretty bad unless you bias your transistor with a *lot* of current. For a good introductory reference on RF design, I recommend "RF Circuit Design" by Chris Bowick.

author: retrolefty
author_rank: Super Contributor
timestamp: March 11, 2016, 05:05:56 pm
content: Quote from: rjeberhardt on March 05, 2016, 02:17:23 pmHe will get there eventually if he is serious about getting a specific station. Otherwise he will have to researchantenna rotation and all the various directional beam antenna designs. When it comes down to improving the S/N ratio there is no substitute for antenna improvements. Antennas don't get the respect they once did, but they are fundamental.

author: Mosaic
author_rank: Regular Contributor
timestamp: March 14, 2016, 05:12:39 pm
content: S/N improvement by filtering/lowering the bandwidth (reduced Gaussian noise) and also directly attenuating high signals you don't want (also considered focused non gaussian noise in this case)http://www.rtl-sdr.com/rtl-sdr-frontend-bandpass-filter/