Basic FM RF LNA: how to? - Page 1
Forum section: electronics / rf-microwave

author: technix
author_rank: Super Contributor
timestamp: November 28, 2018, 09:44:12 am
content: I need to run my FM receiver antenna through two stretches of 3 meter coaxial cables as an extension, but that deteriorated the reception. I wonder if adding two LNA at the antenna end and the middle of the extensions would help.Questions:1. Will those simple single-NPN circuits work across the entire FM radio band?2. Can I send 5V DC power down the coax cable from the receiver end to the antenna end against the flow of the RF signal, without interfereing with the RF?3. For individual parts, is MMBT9018 in SOT-23 and 0603 components a good match for that frequency?4. For the PCB, can I do it using regular 1.6mm thick double layer FR4, or do I need to run microstrips or striplines on a 4-layer board?

author: vk6zgo
author_rank: Super Contributor
timestamp: November 28, 2018, 10:55:08 am
content: I'm surprised that 6m of coax is causing that much signal loss at around 100MHz (I assume you are referring to broadcast FM)You should not need a LNA, but, in the worst case, you should only need one.(1)Not sure, haven't seen them-- many older "antenna preamps" used dual gate FETs, I think modern ones use MMICs, or dedicated ICs.(2) Yes, it is very commonly done, all you need to do is prevent the DC affecting the signal circuitry, & , stopThe DC supply from looking like a short circuit to earth for RF.(3)Probably, it is used for amplifiers & local oscillators in that band.The passives should be good.(4) Plain old FR4 double sided will work OK.Micro strips etc, while nice, are overkill at 100MHz, bulk L&C are still quite achievable, & were certainly used in the ones I've seen.Going back to the signal loss, is the second 3m making all the difference?If so, you may have a faulty cable, connectors, or both.How good is your antenna?I would certainly look at it, & the coax cable before spending time & money on an amplifier.

author: OwO
author_rank: Super Contributor
timestamp: November 28, 2018, 12:14:27 pm
content: Linearity of the LNA is going to be extremely important because you have very strong signals in the FM broadcast band along with weak signals you might want to pick up. I would use something like the 2sc3357 transistor at a high bias point (~50mA or more) since it has an OIP3 of around 30dBm.Avoid mosfet based designs since those tend to have horrible IP3 (on the order of 0dBm). Using a dual gate mosfet as a variable gain amplifier is previous century design practice and not seen anymore (other than in vintage/"ham" designs on the internet). If you need adjustable gain the cookie-cutter solution is a ultra-linear LNA followed by a step attenuator (e.g. PE4312), or the other way around if noise figure isn't important (step attenuator will have about 1-2dB loss).

author: technix
author_rank: Super Contributor
timestamp: November 28, 2018, 12:53:10 pm
content: Quote from: vk6zgo on November 28, 2018, 10:55:08 amIt is broadcast FM. Will the impedance of the coax make a difference?Quote from: vk6zgo on November 28, 2018, 10:55:08 amWhere should I go to look for those IC's?Quote from: vk6zgo on November 28, 2018, 10:55:08 amWill 30pF in series + 120 ohm ferrite bead to ground as signal isolation and 4.7uH in series + 10uF to ground as power filter work? If so I would use those types of LC filters to combine or split the signals.Quote from: vk6zgo on November 28, 2018, 10:55:08 amThis confirmation cuts my cost significantly really.Quote from: vk6zgo on November 28, 2018, 10:55:08 amThis would also make my life easier.Quote from: vk6zgo on November 28, 2018, 10:55:08 amI know my cables are good, but I guess I might have an impedance mismatch. Can LNA also double as impedance matching circuitry?Quote from: vk6zgo on November 28, 2018, 10:55:08 amNot so good really, but since it is integral to the whole home decor I can't really do much with it.Quote from: OwO on November 28, 2018, 12:14:27 pmIf you are suggesting LNA chips, any suggestion or pointers I should look in to? 我是中国人，你可以发中文链接给我。

author: cvanc
author_rank: Frequent Contributor
timestamp: November 28, 2018, 02:54:54 pm
content: I suggest you try a different cable. Six meters of good coax should not be terribly lossy in the FM broadcast band - one or two dB, maybe, but not much more.Good luck getting it sorted out.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 28, 2018, 05:27:46 pm
content: Quote from: technix on November 28, 2018, 12:53:10 pmsome chinese crap coax are not real 50 ohm characteristic cable. i realized this when i got my VNA and tested my hunglow cable. there are all sort of bump and down but that on higher BW more than a GHz or so, below that esp FM i think its no problem. but then assuming your coax is good at the operating freq, your antenna source (or load?) and the receiver input impedance must also same as coax characteristic impedance otherwise there will be all sort of capacitive or resonance effect with differing length of cable. i assume receiver is good 50 ohm input, how about your antenna have you checked? thats what i suspect because recently i tried to make dipole antenna both arms stretched in opposite direction 180°, it turned out my VNA didnt register 50 ohm antenna impedance at operating freq, only when i change both arm to some angle 120° then it becomes 50 ohm. but dont take my word too seriously as i'm just a perpetual learner, i dont know all this stuffs, but my door bell now works flawlessly like earth and heaven before and after i made the correct antenna with a VNA.

author: technix
author_rank: Super Contributor
timestamp: November 28, 2018, 06:42:59 pm
content: Quote from: Mechatrommer on November 28, 2018, 05:27:46 pmThat might be a problem then... The antenna is some sort of loop antenna with twisted pair leads, and I am not sure what might its impedance be. The antenna-side LNA and receiver adapter block (which, crucially, includes DC power injection) would have to deal with balun and impedance matching - from that loop antenna to 50 ohms coax than back to the antenna.

author: cdev
author_rank: Super Contributor
timestamp: November 28, 2018, 08:47:31 pm
content: A single decent LNA at the antenna is likely all you need. You can use a cheap power injector to supply it with power. Consumer equipment should work fine because often consumer LNAs now use pretty decent chips. You don't need a lot of gain. A low noise figure is nice. Check your coax, because it may have been damaged by water ingress or something.Emphatically, you should not need two LNAs.If you can get an appropriate MMIC you can make a device very rapidly. Its around as simple an electronic device as you could possibly imagine. Mini Circuits is a good manufacturer of nice RF LNA MMICs. Quite a few LNA MMICs share similar footprints. Lets say you are using one of these devices, which all have four pins. Two of which are grounds, the other two are 1. the input and 2. a combined DC input and RF output. Thats the pin that goes to the coax.(Once you build this, check that its not oscillating - this isnt so easy but use common sense. Is it working well? If it isnt its likely oscillating. Keep every lead super short.)If you just want to make something quick, you could likely mock something up in a few minutes using the 'dead bug' style You should cut a tiny square of PCB just large enough to fit into the space between your two SMA connectors (which go on top of the PCB, not with the PCB in the C as is done normally - Solder the SMAs last.) First you tack the capacitor to one SMA. Then tack the other side of the cap device to the input pin of the device - the antenna > the lone capacitor > MMIC. At the antenna end you just need the antenna input, a dc block cap and the mmic and then another coax connector.Put the chip upside down and tack the grounds to the PCB beneath with a little pieces of copper that goes over it like a bridge. Just dab the solder very lightly. The c shaped piece of copper just is there to provide a low inductance ground for both pinsconnect the input pin directly to a decent ceramic cap, value isnt so critical, but small. Thats to block dc (static electricity) and connect the output to your transmission line directly (the power will come up from below) The most important thing is that the grounds are short and direct. At the end just use a few more short dabs to tack it all together. then you are done.Then you have to make a bias-tee the same way. For the inductor you should use a decent quality inductor with a high self resonance. Or wind one yourself using very very thin wire. There you use a similar cap to block the DC from getting into your radio. Use a fairly high value cap to bypass the RF and keep it off of the DC connection. The inductance comes first as you leave the signal path. Have the inductor start right at the trace, no extra wire between it and the trace. This is all easier to do than to type. You likely already have an appropriate device somewhere on some junk board you can use. Try it.I find that its best to put the chip at a 45 degree angle with the grounds at either side. The top is the input, that goes straight to the capacitor and from it straight to the antenna connector. The output goes straight to the coax connector. This will all be very very small. Tack the two SMA connectors down to your piece of PCB.Quote from: technix on November 28, 2018, 09:44:12 amIf you want a broadband, (working at much higher frequencies too) low loss LNA, thinner PCB would be preferable but for 88-108 MHz it really doesn't matter much at all. If you are using 75 ohm coax make your traces appropriate to a 75 ohm environment. If you want best performance, you could perhaps should design your LNA so that it has a pronounced rolloff in response at higher and lower frequencies to prevent overload from strong cellular, etc. signals. Even to the point of adding filters to its input. That will ensure that your gain is used for FM and not other signals. Again you shouldn't need a lot of gain. 15db is likely about right. Some of that will be used up by the filter loss.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 29, 2018, 01:28:23 am
content: Quote from: technix on November 28, 2018, 06:42:59 pmThat is where your problem is---- a poor antenna.Any amount of amplification will not make up for a poor signal/noise ratio.If you are getting poor results without an amplifier, all the amp will do is amplify the signal & noise equally, resulting in no net improvement .QuoteYes, you could, maybe, get better results if you use the amplifier itself to do the impedance conversion, as is done in so-called "active antennas", but I still think you will be severely limited by the poor antenna.A wide band amplifier is not a good idea, as it will amplify out of band stuff to the point it may be able to overload the receiver front end, causing interference.Foradjacentstrong signals, even if you have a LNA with excellent overload performance, you are only transferring the problem to the receiver front end.

author: OwO
author_rank: Super Contributor
timestamp: November 29, 2018, 06:08:20 am
content: Quote from: technix on November 28, 2018, 12:53:10 pmThere are many widely available RF bipolar LNA transistors like the BFG591, BFG541, 2SC3357, BFG135, etc all available from taobao; beware though that I have seen some substandard 2SC3357s from aliexpress (hfe degradation after a while operating at high bias points, everything operating within datasheet limits), so make sure to pick a seller specializing in RF parts with a good rating, and keep an eye for circuit characteristic changes.Generally the IP3 of a simple transistor amplifier is linearly proportional to the P1dB, so a 10dB higher P1dB amplifier will usually have a 10dB higher IP3. I would look for beefy LNA transistors like those in SOT223 packages with a high current and dissipation limit. As for noise figure 2dB or less is usually considered very good for 100MHz band FM reception (environment noise is usually way higher). Most of these transistors also have available spice models and a simple test circuit will reveal the IIP3.

author: aandrew
author_rank: Frequent Contributor
timestamp: November 30, 2018, 02:09:30 pm
content: I bought the (very expensive!) eval board for the NXP BFU580 (the entire BFU5xx series in fact). It's a very very simple circuit but I wasn't sure what the layout requirements were.The transistor works pretty decently. I would suggest looking at NXP's app notes and datasheets for these transistors. UM10772, AN11499 and AN11500 were all helpful.

author: cdev
author_rank: Super Contributor
timestamp: November 30, 2018, 02:42:17 pm
content: I have had very good luck with the versatile Mini-Circuits PSA4-5043+ - It works extremely well at 88-108 Mhz.It has built-in ESD protection in addition to having very high IP3 and very low noise figure. Its rated as under 0.75 db but is often lower, around 0.65 db. Gain at 100 Mhz is probably a bit under 20 db. It runs off of any supply from 3.3 to 6 volts and takes around 56 ma. So should work well mounted at your antenna. Just keep it dry and make sure toclean the board completely of flux residue. (Which with this chip and many other similar chip can cause hard to figure out stability problems)."Mini-Circuits PSA4-5043+ is a E-PHEMT based Ultra-Low Noise MMIC Amplifier operating from 50 MHz to 4 GHz with a unique combination of low noise and high IP3 making this amplifier ideal for sensitive high dynamic range receiver applications. This design operates on +3 to +5V supply at only 33 mA at 3V and 56mA at +5V, is internally matched to 50 ohms and is supplied in a super small SC-70 (SOT-343) MSL 1 package."Ground the chip well, use lots of vias, and keep the PCB dry and clean and it will behave well. You will need to make a bias-tee to feed power to it but you likely can use a cheap power injector used in China for MMDS and satellite receivers. Make sure to not send more than 6v to the MMIC if you use the PSA4-5043+, which would be a very good choice.It also likes to have at least 100 cm - preferably 200 cm or even half a meter or more of feed line between it and your antenna, put it in a case a little bit away, don't connect it directly to the antenna. Tape the feed line to your grounded mast and waterproof both it and it's case.If you want to make a traditional one piece LNA with a built in voltage regulator you can use pre-made LNA PCBs you can buy on ebay with this chip. It shares a compatible footprint with a number of other similar LNA chips. Or you can use those boards for the bias tee and just make the very simple LNA I described before for the antenna end using the PSA4-5043+ and a piece of impedance appropriate junk PCB with vias that you have chopped out from a junk board or made - with the trace cut at the appropriate places with a knife. For use at 100 Mhz this is very easy. Just make sure it doesn't oscillate. It shouldn't if you keep the ground leads short - make sure to ground the chip well.Quote from: technix on November 28, 2018, 12:53:10 pm