Author Topic: Low Noise Amp for wideband SDR?  (Read 6206 times)

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Offline rwgast_lowlevellogicdesin

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Low Noise Amp for wideband SDR?
« on: June 07, 2014, 06:06:18 pm »
I recently ordered one of those Realtek USB radio dongles, and I would like to make an preamp for the antenna that will work up to 2.5/2.6ghz. I know absolutely nothing about RF though. I found reviews on this LNA4ALL amp, http://lna4all.blogspot.ie/, for 25 bucks which is not a bad deal, but I cant find a place to ac actually buy it. I figured maybe id just try to make my own on vero board, cant be to hard?

So I strated looking at how the LNA4ALL works, and it uses some special chip, which I also can not find supplier for!! What the hell is the deal, cant find the LNA4ALL nor can you find its main components! Well do I acually need some special broadband low noise amp chip?? What about a Low noise Instrumentation amp? Im not sure how opamp bandwidth play in to RF. I have a few AD8428, http://www.analog.com/en/specialty-amplifiers/instrumentation-amplifiers/ad8428/products/product.html, these are SUPER low noise and have 7ghz of gain bandwidth, but the gain is fixed at 2000, is this going to be to hi? I also have a few LT1167s around, these guys are a little nosier and not as fast, but they have programmable gain.

So I guess the question is will an opamp do the job here, or does the LNA4ALL use a fancy chip becuase an opamp isnt very good for this application?

Offline Marco

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Re: Low Noise Amp for wideband SDR?
« Reply #1 on: June 07, 2014, 07:09:25 pm »
That opamp has an actual bandwidth of a couple MHz. Nothing with GHz is easy.

I think you simply have to email him to buy it. These wideband amplifiers use MMIC's, I think internally they are darlington common emitter amplifiers. There are some similar kits on ebay but they require a little more DIY ...
« Last Edit: June 07, 2014, 07:20:36 pm by Marco »
 

Offline gaijin

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Re: Low Noise Amp for wideband SDR?
« Reply #2 on: June 07, 2014, 07:50:04 pm »
The amplifier they use is a minicircuits PSA4-5043+
http://www.minicircuits.com/products/amplifiers_smt_low_noise.shtml

A while ago I modified a copy of this design:
https://github.com/loxodes/rtl-sdr-lna
that uses the Avago MGA-68563
http://www.avagotech.com/pages/en/rf_microwave/amplifiers/low_noise_amplifiers/mga-68563/

It was fun soldering the 0402 passives by hand.
 
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Offline larry42

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Re: Low Noise Amp for wideband SDR?
« Reply #3 on: June 07, 2014, 10:47:10 pm »
To the OP- I think you should buy a suitable LNA  - there are ~no suitable opamps that work at 3GHz and have low noise… Verobaord is not suitable for 3GHz.

RF design has become easier with off the shelf packaged MMICs, but its not something for completely new ppl to start out with (unless you know how to debug a circuit oscillating at 8GHz)

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Offline Zad

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Re: Low Noise Amp for wideband SDR?
« Reply #4 on: June 08, 2014, 01:01:36 am »
Veroboard tracks have a track to track capacitance of approx 3pF per inch. A mental calculation gives an impedance at 2.5 GHz that is around 20 Ohms. Excluding any losses in the dielectric.

How about something like: http://www.minikits.com.au/electronic-kits/rf-amplifiers/rf-wideband/Gali-39-Amplifier

If not that, then look around for amateur radio TV (ATV) kits and designs for the 13cm band (2.4GHz).

Offline rwgast_lowlevellogicdesin

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Re: Low Noise Amp for wideband SDR?
« Reply #5 on: June 08, 2014, 10:22:35 pm »
.I wouldn't say I am a "new person" to electronics. I have built a quite a few things from power suppplys to robots



I mostly use protoboard but I am always very meticulous doing layout. Most electronics I have worked with are digital though, I've just recently started learning the analog side of things in the last 6 months, right now I am working on a digital/analog dummy load that can run in 1mA divisions up to 10 amps and am using a 16bit DAC and 24 bit ADC, and trying to tune a control loop using dithering. I would love to to learn more about RF I just have no idea where to start I figured this may be a good place. I dont have to use potoboard, I could use copper clad and scrape pieces out like a I see a lot of RF designers do, using deadbug or manhattan i think its called.

What I would really like to do is build a preselector for the dongle (which would also include up/down converters and LNA), where it uses a bunch of filters and a hardware knob to select the range, from what I understand filtering out everything but what you want to listen to will make a huge difference in reception. The insperation comes from this amazing project . I just dont even really know where to start besides just jumping in and failing until I get it right. Any advice for learning RF would be nice.

Would building amplifiers for different signal ranges, be easier and better than building/buying one amplifier to cover the whole spectrum im looking to listen to?
 
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Offline Marco

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Re: Low Noise Amp for wideband SDR?
« Reply #6 on: June 09, 2014, 03:33:34 pm »
Would building amplifiers for different signal ranges, be easier and better than building/buying one amplifier to cover the whole spectrum im looking to listen to?
No, it would be multiple times harder ... you could build it out of discretes though, so it would teach you more. You're never going to get circuits which are as incredibly basic as you can make them with these MMIC's.

http://www.rfbayinc.com/upload/files/data_sheet/sch/amp86sch.pdf

PS. gaijin, since you seem to have a little more knowledge on the topic ... can the SDR handle the signal strength from these wideband amplifiers? Some SDRs use a selectable low/band/high pass filter to at least keep some of the energy away from the front end. Or is the antenna selective enough for that in this case?
 

Offline LukeW

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Re: Low Noise Amp for wideband SDR?
« Reply #7 on: June 10, 2014, 07:17:52 am »
We can do a quick parametric search of Digi-Key for some RF MMIC amplifiers, looking for ones that will do most of the 0-2.4GHz frequency range you want, and with a noise figure of, let's say, <= 1dB.

Here are a couple of candidates, but there are dozens of others you could use.

http://www.digikey.com/product-detail/en/BGB%20741L7ESD%20E6327/BGB%20741L7ESD%20E6327CT-ND/2410163

http://www.digikey.com/product-detail/en/SPF5043Z/689-1101-1-ND/2708621

To get started, basically just read the data sheets and copy the reference schematics as closely as possible. And do some homework and learn about how to make a PCB layout for a 50-ohm transmission line. :)

Another option might be to get a pre-built block in a can with SMA connectors, such as Mini-Circuits ZX60-P103LN+, which will be much easier but a little more expensive, but more likely to work with known performance characteristics.
 

Offline cdev

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Re: Low Noise Amp for wideband SDR?
« Reply #8 on: April 17, 2021, 02:32:46 pm »
You can buy PSA4-5043+ receive LNA chips in as little as 20 quantity from Mini-Circuits.  They cost around $2.50 each at that quantity. Which is a great deal because they also have built in basic ESD protection. And the quality of the LNA they make its quite decent. Typically the NF is around 0.6 or 0.7 db. Which is low enough to use them for satellite reception. No, they wont cause problems with your receiver, usually, but lets face it, RTL dongles are not the most selective receivers out there. So attenuation is very useful with them. As is a preselector.
In building an LNA the most important things are impedance matching and especially proper grounding. (right underneath the chip)  If you use the PSA4-5043+ keep in mind that the current draw is always around the same. around 53ma. Any more than a small bit lower or higher and something is definitely wrong. You should also be able to see any problems with self-oscillation immediately it will show in the current draw as a larger random variation in the current draw than usual. Also it will show up in the rtlsdr's not working properly and possibly producing noise that can be picked up in an rtlsdr or other receiver nearby. All of my recommendations here need to be followed to ensure its not acting up. Yes, you can add filters to roll off unwanted inputs (thats preferable to output filtration)

You can buy an LNA4ALL directly from Adam, by mail. His address is on his web site. $25 is a good buy from a very nice and smart RF designer who deserves your business. Here I am giving you a good idea of whats required to make your own.

If you look for an LNA dev board on ebay you can find PCBs suitable for use with the Mini Circuits  devices. You can also make your own and if you have it professionally done, many difficulties can be done for you by the PCB firm.. Its a PITA making PCBs that perform well over this wide range. When I was just starting out I carved one with an x-acto knife. it did not perform well. i used my hand drill (I did not have an electric drill) to laboriously drill holes and fill them with wire for vias. (Its not easy to make vias that way, you have to drill holes and fill them with 1mm wire and then solder both sides to link them, and then chop off the extra right at the surface) Don't skimp on the vias! (or it will self-oscillate) Put them right under the ground pins. literally.
 You could also dead bug them. That will work up to around 500 MHz. Its not rocket science. The usual LNA chips you should use are microscopically tiny. They are receive amps, not transmit amps. Big difference!  Lots of devices have a similar form factor, so can use a similar pattern.

Proper Grounding of the MMIC and input-output stage isolation and impedance are the most important things. Use a length of good coax of the right impedance on your inputs and outputs and that will make a good design perform at its best. Also pay attention to the DC blocking input capacitor's self resonance and the same with DC isolating inductor . Read up on bias-tees and design so the self resonances are damped. . thats the same set of issues.

A flat copper sheet has lower inductance than most wires. So if you dead bug it then the two legs must connect directly to your ground plane Then use coax (short) for the output and a DC block right there . Use SMD caps (0402 is good) and connect the input directly to the input pin through a capacitor thats literally soldered to the center pin of the input RF connector. So as short as it could possibly be. Leading the RF right into the MMIC's input pin. You can dead bug it but use SMD parts. Just as is, connect them directly to each other with minimal wires. UGLY construction this is called and they are ugly. But it works well. Its a fun project especially when your final result is an LNA that dramatically improves some very weak signal.

Have fun!
« Last Edit: April 17, 2021, 04:01:55 pm by cdev »
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