400 MHz (ISM+HAM) LNA build + measurement - Page 1
Forum section: electronics / projects

author: hpux735
author_rank: Contributor
timestamp: January 16, 2014, 02:23:10 am
content: Hi all, I just finished a blog post about a LNA module I designed around a Skyworks LNA chip. I had to do some interesting deciphering of the data sheet and a nonsense matching circuit.http://www.housedillon.com/?p=1923Any thoughts? Does anyone know what the precise relationship is between sensitivity and noise figure?

author: Rerouter
author_rank: Super Contributor
timestamp: January 16, 2014, 02:37:53 am
content: not something i would know sorry :/just have to ask why you went for a trace on the ground plane as opposed to using the passive to jump the track?

author: hpux735
author_rank: Contributor
timestamp: January 16, 2014, 03:31:54 am
content: Yah, good point. That would have been a good idea.

author: Richard Head
author_rank: Frequent Contributor
timestamp: January 16, 2014, 07:06:08 am
content: Hi hpux735I read your trials and tribulations on the link you provided. It was interesting.All I can add is that when it comes to low noise amplifiers the input circuitry is key.Every fraction of a dB that is lost in the input circuitry/connector adds directly to the noise figure.That means that if your input matching network has a 0.5dB inserion loss then that must be added to the noise figure of the device to estimate the actual noise figure.Unfortunately a simple L network or similar generally doesn't provide decent out of band signal rejection. That's why all very low noise figure RF amplifiers (radio telescopes etc) don't have high Q narrowband tuned circuits up front. The narrow bandwidth (if desired) can be obtained by placing a bandpass filter in the drain, after the noise figure has been established.Good luckRich

author: hpux735
author_rank: Contributor
timestamp: January 16, 2014, 05:32:06 pm
content: Quote from: Richard Head on January 16, 2014, 07:06:08 amHi RichThis module is mostly intended to live up on the antenna mast in the hope of reducing input losses, including coax loss. Because I finally got system gain to be within 0.3dB of published specs, I hope that I was successful in minimizing losses from the input matching circuit.I'm definitely considering building a noisy diode ENR source to make better measurements of NF, but I'm not sure how well I would be able to get calibrated noise power for the normal and "hot" settings needed for the Y-method.- Will

author: Richard Head
author_rank: Frequent Contributor
timestamp: January 31, 2014, 09:36:28 am
content: WillI have a pretty comprehensive RF lab but still find it difficult to justify the expense of an HP noise figure measurement setup. I feel you get more bang-for-your-buck buying more directional couplers/circulators etc.So far I have got away with careful design and lived with the resulting NF. As RF semiconductor devices become better (lower noise) it's becoming more difficult to measure their noise figure using low cost equipment. That's my take on it anyway.Rich

author: rfbroadband
author_rank: Unknown
timestamp: February 05, 2014, 04:15:14 am
content: I am not sure what the exact overall objective is but here are a few things you could/need to consider.If you are antenna has an impedance other 50Ohm, doing a receiver NF characterization referenced to a 50 Ohm source is very inaccurate. (that raises the question on how would someone do NF measurements referenced to a non 50Ohm impedance...topic for a different post).You did not mentioned whether blocking is important in your case. You have a broadband LNA in front of your receiver and all receivers have to deal with strong unwanted signals. If that becomes important to your setup you will have to introduce filters and start considering linearity of your receiver together with NF.I personally use noise sources and a spectrum analyzer for NF measurements.

author: hpux735
author_rank: Contributor
timestamp: February 05, 2014, 02:26:09 pm
content: The same basic design serves two objectives. The skyworks LNA is usable across a pretty large band, and I have uses in mind for 400MHz and 1700MHz.The 400MHz case is to help pull in weak signals with my USB transciever (which I've written abouthttp://www.housedillon.com/?page_id=1494andhttp://www.housedillon.com/?p=1533). I'm planning on using a PIN diode RF switch to switch in-and-out the LNA and a skyworks PA, which I've written abouthttp://www.housedillon.com/?p=1925). In this case, I'm expecting to use a 50 Ohm antenna system, tuned against 50 ohms with a bridge.The 1700MHz case is a HRPT receiving station (http://en.wikipedia.org/wiki/High_resolution_picture_transmission). This example is more noise sensitive, and unfortunately, my SA only goes to 1.5GHz, so I'll have to assume that its performance mostly matches data sheet values. I'm more comfortable doing that now that I have experience with the PCB and the IC.In both cases, I'm considering a passive band pass front-end. I don't know a lot about how they affect NF, other than that putting in any resistors is strictly verboten.