RF LNA design with MGA71543 - Page 1
Forum section: electronics / projects

author: LukeW
author_rank: Frequent Contributor
timestamp: October 02, 2015, 08:51:53 am
content: For reference: Avago AN5273 reference design for 3.5 GHz.http://www.avagotech.com/docs/AV01-0255ENI'd like to put together a MGA71543-based LNA, primarily aimed at use around 5.8 GHz.But it would be nice, ideally, to have a "one size fits all" design that can be useful across a broad part of the 100 MHz-6 GHz band that the MGA71543 supports. As much as is practical.http://www.china001.com/ifile.php?xname=PPDDMV0&fname=2_2_1202621822.jpgHow much are those reference component values, particularly the matching component values, going to change as the frequency of interest changes?I really don't want to play empirically with hours of VNA work trying different design values - and I don't have access to one for frequencies that high anyway.If I go with the reference design matching components for 3.5 GHz, is it going to have any chance of reasonable performance at other frequencies, say 5.8 GHz?How broadband is it going to be?How practical is it going to be to just wing it, and choose baseline design values that are probably going to be ok for an initial build?I'd like to use CPWG on cheap 60 thou 2-layer FR4 - at least for a try. So, looking at those three microstrip sections in the reference design, their width (22 thou) is 50 ohms for their specified Rogers board. (10 thou Rogers 4350 on a FR4 mechanical substrate, and I'm assuming the FR4 can be ignored in the calculations.) So I can just change that to keep the width of the stub at 50 ohms for the new stackup.And then there's the length of those stubs. Can I just calculate their electrical length for the given length, frequency and stackup in the reference design, and keep the electrical length the same for the new frequency and stackup? (Obviously this is frequency-dependent, so it won't be a one-size-fits-many broadband design as I'd hoped.)For example, the 160 thou microstrip stub in the original design - on 10 thou Rogers, at 3.50 GHz - works out to be about 27.8 degrees. If I change those parameters for 60 thou FR-4 at 5.8 GHz, I end up with a length of about 94 thou, roughly, to stay at 27.8 degrees. Is this the right approach?Any general design advice? Thanks

author: rfeecs
author_rank: Frequent Contributor
timestamp: October 02, 2015, 05:18:01 pm
content: The reference design for 3.5GHz won't work well for other frequencies. It is narrow band.It's not at all easy to get something to work well from 100 MHz to 6 GHz. It all depends on the performance you want.The way people would normally approach this is first play around with circuit simulation. The datasheet gives the S-parameters and noise parameters. So you can simulate gain, noise figure, and return loss.If you want to do everything empirically, first just bias the part up unmatched with a short length of 50 ohm microstrip (or CPWG) on input and output. For that you need just bias chokes and blocking/decoupling capacitors. You should get some gain (assuming you can tame any oscillations), even out to 6GHz. But you will have terrible return loss and lousy noise figure. Then you can add some shunt caps to ground on the input and output for tuning. You tune by varying the position and values of the caps.

author: ConKbot
author_rank: Super Contributor
timestamp: October 02, 2015, 07:59:02 pm
content: I dont know what your critical parameters are, but if you want it broadband, its hard to beat internally input/output matched amplifiers, something like the HMC476MP86 may be up your alley, though the noise figure is ~2.5 instead of 1.1 You can see from the images of the eval kit, its a transmission line, input/output coupling, and DC bias. Plus if youre going to be using a big, huge wide transmission line (even a GCPW is wide on 62 mil boards) that formed lead package will be a bit more friendly too. At 6 GHz the amount of wiggle room you have for impedance mismatches without disturbing things too bad gets pretty small,

author: LukeW
author_rank: Frequent Contributor
timestamp: October 03, 2015, 03:23:51 am
content: I'd really like something that's a low-noise amp, rather than a general purpose gain block. For arguments' sake, let's define "low noise" as <= 1.1dB or so.The MGA71543 is attractive not only because of its wideband 100MHz-6GHz spec but also because it's cheap - AUD$1.86 at element14 AU. So it's a tenth of the price of something like a HMC717, even with a few passives for matching. That price and that wide bandwidth is highly tempting.For 4.9-5.9 GHz operation there are other candidates available such as the SKY65404 which seem much simpler and more forgiving in the external matching networks, while also being pretty cheap. You've also got other LNA options for lower frequencies, such as the Mini-Circuits PSA4-5043+ which goes up to 4 GHz, and that seems to be pretty easy to set up in terms of the external components - but it's also more expensive.The MGA71543 seems a nice holy grail for many general-purpose LNA applications, at a very low price across a wide frequency range. That is, if it is actually possible to set up the external network matching to get useable performance over a good range.I suppose it's just easier to stick with the SKY65404 in the 5GHz spectrum; it can be spun up into a working layout with less uncertainty.

author: LukeW
author_rank: Frequent Contributor
timestamp: October 03, 2015, 03:32:16 am
content: Quote from: rfeecs on October 02, 2015, 05:18:01 pmOK, suppose we start with this schematic:http://www.china001.com/ifile.php?xname=PPDDMV0&fname=2_2_1202621822.jpgSuppose we remove the lumped inductor at the input port, and remove the microstrip stub at the IC input.Remove the series inductor on the MMIC output, remove the series 8.2R resistor, and remove the shunt capacitor.Keep the bias tee inductor, keep the the 47pF coupling capacitor on the output port after the bias tee, keep the 10 ohm resistor (?), and keep the existing biasing and decoupling as-is.Keep the two 50-ohm sections (with no soldermask) at the input and output, and start putting matching back in on the input and output with shunt components, and tweak the matching from there. Does that sound sensible?

author: KJDS
author_rank: Super Contributor
timestamp: October 03, 2015, 06:13:47 am
content: If you want to achieve broadband match and low noise figure, it sounds about as sensible as making a tyre by gluing blocks of rubber to a wheel.One of the mini-circuits amps, this was the first I looked athttp://www.minicircuits.com/pdfs/CMA-545+.pdfwould be much better for a broadband approach. Either that or learn how to create a broadband matching network then be prepared to spend a week playing with an RF simulator and another with a VNA and noise figure set-up tweaking the design to get it to work in practice.

author: LukeW
author_rank: Frequent Contributor
timestamp: October 03, 2015, 09:18:40 am
content: Good old hermetically sealed nitrogen purged, terrorist-killing, gold-plated nuclear bomb proof Space Shuttle grade MiniCircuits chip.I'm sure it's a very, very nice amplifier, but I doubt it's $1.80 a pop.

author: Howardlong
author_rank: Super Contributor
timestamp: October 03, 2015, 11:35:38 am
content: Generally speaking, broadband isn't compatible with low noise figure (using the <1.1dB NF quoted) across the band. And certainly not at $1.80.As with most things engineering, you're probably going to want to compromise somewhere.Also you might want to consider whether it would be a good idea to have such a broadband LNA ahead of your relatively narrowband application: a cacophony of strong out of band signals, while they might not drive a decent LNA into non-linearity, the same might not be the case for the application's front end.

author: rfeecs
author_rank: Frequent Contributor
timestamp: October 03, 2015, 06:20:27 pm
content: Quote from: LukeW on October 03, 2015, 03:32:16 amYes.Keep in mind that this part is really just a transistor with a bias network and bypass switch in a package. It looks like the 50 ohm noise figure is not bad. Depending on losses, you could get less than 2dB up to 6GHz. But you will have bad return loss. This might not be a problem depending on your application.Just noticed that you need a negative voltage for the input bias circuit! You might be better off just using a super-low noise pHEMT like NE3512S02:http://www.cel.com/pdf/datasheets/NE3512S02.pdfThe broadband gain block parts will be easier to use, have better match but worse noise figure.Why not get them all and try them out? The shipping will cost more than the parts.