RF preamplifier design: Getting both a low noise figure and high dynamic range? - Page 1
Forum section: electronics / projects

author: cvanc
author_rank: Frequent Contributor
timestamp: September 08, 2015, 12:01:43 am
content: Hi all-Just found this place and having fun trying to absorb everything.If you don't mind, let's talk about VHF RF preamplifiers, specifically ones for the FM broadcast band (88-108MHz). What are the obvious design decisions if the goal is to have a very low noise figure (less than 1dB) and also a high tolerance for strong local signals (wide dynamic range)?Some low NF commercial units exist but the ones I find all seem to have designs many years old. They often use a dual gate MOSFET (ex: BF998) in a simple one transistor circuit. Specs claim NFs around 0.6dB and I have no reason to doubt, or way to verify, that. But they overload with stronger signals where I live. Perhaps something better could be made.So what would the general design decisions be for a preamp that meets both of these needs? Is the old BF998 still a good choice for the active device, or is there something more appealing these days? And what about that one transistor circuit topology - wouldn't, say, a cascode setup show better strong-signal performance? (or am I showing my age and nobody does cascodes anymore?)Thanks for any input you might provide, and thanks to all who are regular contributors here - this place is pretty amazing.Best,-CarlChicago

author: rs20
author_rank: Super Contributor
timestamp: September 08, 2015, 12:04:37 am
content: If you have overwhelmingly strong signals, why do you need a low noise figure? Are you trying to receive both strong and weak signals?

author: moffy
author_rank: Super Contributor
timestamp: September 08, 2015, 12:22:09 am
content: To discriminate against strong signals you can have your narrow band/tuneable filter before the preamp/mixer. An old fashioned variable capacitor/inductor and frequency dial.Probably the best solution.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 08, 2015, 01:06:10 am
content: In Amateur Radio circles,there has been a move away from dual-gate Mosfets,for the same reason.It is a vexed question,though,as do you want to lose low noise and/or sensitivity to get better overload performance?I suggest you have a look at a fairly recent ARRL Handbook,as they do have a quite "in depth" treatment of this issue.At the moment,my only other suggestion is to use an attenuator at the input of the Rx when listening to localStations,or to try some kind of notch filter if they are interfering with your reception of much weaker "DX" Stations.A coaxial "stub" may work,but might be getting a bit big at 100MHz.

author: Howardlong
author_rank: Super Contributor
timestamp: September 08, 2015, 01:33:50 am
content: PGA103+ seems to be the flavour of the month with the EME guys, 0.5dB noise figure and reasonable dynamic range.Quick and dirty, Avago MGA 62563, reasonably high P1dB and OIP3 ~1dB NF if you bias it and match it right.

author: retrolefty
author_rank: Super Contributor
timestamp: September 08, 2015, 01:42:37 am
content: I wonder if a =< 1 db NF is justified at 88-108MHz? An old ham trick I read and used says to remove the antenna from the FM radio, turn off squelch, and turn up volume to get a decent noise background. Then re-hook up the antenna and if the noise level increases by any amount then there is no need for a lower NF in your front end.High dynamic range is a different animal and is environmentally dependent on how strong and how near various FM stations are from your location.

author: Howardlong
author_rank: Super Contributor
timestamp: September 08, 2015, 01:49:47 am
content: Perhaps we need to know what the OP is trying to achieve?Attempting to engineer high Q notch filters in Band III to notch out one or two signals isn't a trivial task, even harder would be a variable one. If this had to go in front of the LNA, you could probably kiss goodbye to a 1dB noise figure too.

author: nfmax
author_rank: Super Contributor
timestamp: September 08, 2015, 07:19:30 am
content: Many, many years ago I attended the inaugural lecture of our professor of communications. He set up a demonstration of tuned cavity filters at VHF, using an aluminium beer keg as the cavity, modified with a 'piston' in the top to control the resonant frequency. That was the coarse control: the fine control was at the bottom, with a tuning characteristic of 2 pints per MHz.

author: Richard Head
author_rank: Frequent Contributor
timestamp: September 08, 2015, 02:26:46 pm
content: A dual gate MOSFET is a cascode configuration. It's just all in one package. The BF998 is quite a decent device but the BF991 is a bit better at the lower frequencies as far as I remember.Unfortunately low NF and good front end filtering are at loggerheads. Every dB of insertion loss adds directly to the noise figure so for lowest noise figure you need the lowest filter insertion loss. The lowest loss filter (that will match from 50R to +/-2k) is an L-network and not a high Q tank. An L-network will provide virtually no front end pre-selection at all.Having said that I doubt that you would benefit from such a low noise figure at 88-108MHz anyway as the background terrestrial noise level will almost certainly dominate. Do the noise test as Retrofly suggests. Very low noise applications forfeit all front end filtering and have to accept the associated IMD problems that come from having the window wide open.A single resonator high-Q front end is often a good compromise if the local interference isn't too bad.Also, for lowest noise figure you must operate the dual gate MOSFET at maximum gain and add attenuation afterwards. Don't reduce the gain using gate 2 as the noise figure gets worse at lower gain settings.

author: mark03
author_rank: Frequent Contributor
timestamp: September 08, 2015, 02:42:23 pm
content: Quote from: retrolefty on September 08, 2015, 01:42:37 amFM demod output noise is not proportional to input noise, is it? I think you'd need to put your receiver in SSB or CW mode for this to work.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2015, 03:08:06 pm
content: Narrowband white noise is equivalent to a combination of phase noise and amplitude noise. The FM receiver rejects the amplitude part and detects the phase (well, frequency, but that's just off by a filter).One would expect, if the receiver is sensitive enough to produce full limiting even at zero input, then the noise output should be independent of input, because all noise has the same width, which is the IF bandwidth of the receiver.If the total gain is somewhat less, then amplitude noise will remain, which makes the detector output relatively noisy, but for less total output, therefore still being quieter.So, it depends.Tim

author: retrolefty
author_rank: Super Contributor
timestamp: September 08, 2015, 04:33:16 pm
content: Quote from: mark03 on September 08, 2015, 02:42:23 pmWhen unsquelched to an unoccupied channel on the FM bands and turning up the volume you will hear the band noise. With no input carrier signal to the IF amp(s) they will not be in saturation and just pass the (amplified signal noise onto the detector and audio amp.This simple test does not measure the NF, but rather just tells you quickly if external noise from the antenna is dominate or not.

author: cvanc
author_rank: Frequent Contributor
timestamp: September 08, 2015, 06:39:47 pm
content: Quote from: rs20 on September 08, 2015, 12:04:37 amYes, exactly. Crowded dial, lots of signals, and Murphy's Law says the interesting ones are weak and hard to get.

author: cvanc
author_rank: Frequent Contributor
timestamp: September 08, 2015, 06:42:56 pm
content: Quote from: moffy on September 08, 2015, 12:22:09 amAs I understand it the ideal location for a low NF preamp is right at the antenna (before any long coax runs). Placing a tunable filter up on the roof that would have to be adjusted for every change of station might technically work but presents some practical problems. Ideally I can find a solution that requires no adjusting when changing channels.

author: cvanc
author_rank: Frequent Contributor
timestamp: September 08, 2015, 06:48:48 pm
content: Quote from: Howardlong on September 08, 2015, 01:49:47 amHi, I'm just trying to listen to a wide variety of radio stations without fussing with anything each time I change the frequency.I have a directional antenna on the roof with a rotator. I have a large city in one direction and when I rotate the antenna in that direction I get hit - hard. But when I rotate the city into the back null of the antenna I stand a chance of getting weaker, more distant signals. What I have now does work, but it overloads when I aim it back at the city (strong stations appear at more than one point on the dial).

author: cvanc
author_rank: Frequent Contributor
timestamp: September 08, 2015, 06:49:43 pm
content: Quote from: nfmax on September 08, 2015, 07:19:30 amBrilliant! Cheers to your professor.

author: cvanc
author_rank: Frequent Contributor
timestamp: September 08, 2015, 06:56:09 pm
content: Quote from: Richard Head on September 08, 2015, 02:26:46 pmThank you; I hadn't really thought of the 998 as a premade cascode before. And I was considering making the G2 bias adjustable - appreciate the information.Higher noise when biasing for lower gain might actually work all right in my situation, as long as it got quieter when I cranked the gain back up. But it still suffers form needing adjustment when changing channels - something I wold like to avoid.

author: Richard Head
author_rank: Frequent Contributor
timestamp: September 09, 2015, 06:53:37 am
content: I have a directional antenna on the roof with a rotator. I have a large city in one direction and when I rotate the antenna in that direction I get hit - hard. But when I rotate the city into the back null of the antenna I stand a chance of getting weaker, more distant signals. What I have now does work, but it overloads when I aim it back at the city (strong stations appear at more than one point on the dial).It sounds like you may be suffering from intermodulation distortion generated inside your receiver. This becomes FAR worse with strong signals (even if not tuned to them). The symptom of intermodulation distortion is multiple distorted signals that are low in strength but repeat all over the band. What you may need to do is to attenuate the signal from the antenna to avoid the intermods. A front-end pre amp will probably just make the situation worse. If intermods are your problem the only real solution is to get a better receiver.

author: mark03
author_rank: Frequent Contributor
timestamp: September 09, 2015, 05:38:53 pm
content: Quote from: Richard Head on September 09, 2015, 06:53:37 amOr a switchable attenuator for when your yagi is aimed at the city. That would be easy enough.

author: cvanc
author_rank: Frequent Contributor
timestamp: September 10, 2015, 01:45:10 pm
content: Quote from: Richard Head on September 09, 2015, 06:53:37 amNo, it happens in my preamplifier. I can see lots of spurious spikes on my spectrum analyzer when the roof antenna is aimed at the city. This is with the preamp feeding the spectrum analyzer directly - no receiver involved. When I rotate away from the city, the spurious spikes all disappear.Thanks all for the discussion; it's an interesting topic for me.

author: AF6LJ
author_rank: Unknown
timestamp: September 10, 2015, 02:00:40 pm
content: Quote from: nfmax on September 08, 2015, 07:19:30 amThis is a good idea and will do the job.In commercially manufactured cavities Bi-metal washers are used to compensate for temperature drift.There are plenty of plans for construction of cavities on the net and in old ARRL handbooks.

author: AF6LJ
author_rank: Unknown
timestamp: September 10, 2015, 02:02:03 pm
content: Quote from: cvanc on September 10, 2015, 01:45:10 pmSounds like you really could benifit from some selectivity ahead of the pre amp.

author: cvanc
author_rank: Frequent Contributor
timestamp: September 10, 2015, 04:11:42 pm
content: Quote from: AF6LJ on September 10, 2015, 02:02:03 pmWell, sure, but when discussing NF's in the <1dB range wouldn't any additional circuitry ahead of the preamp shoot that NF to hell?I'd much rather think of it as needing more headroom *in* the preamp so it could just tolerate the mixed signal environment I am exposing it to. If I could somehow achieve that while preserving my low NF... it would be Hog Heaven, right? Bulletproof.

author: AF6LJ
author_rank: Unknown
timestamp: September 10, 2015, 10:44:58 pm
content: Quote from: cvanc on September 10, 2015, 04:11:42 pmYou have to decide what is most important.Do you really need a preamp with less than 1 DB NF?I haven't the slightest idea what you are using it for so I cannot make that decision.Passive parts will be less of an issue than active ones.QuoteIf you could Yes it would be nearly ideal. I am not so sure you can do that.The cable TV companies have nice ultra linear distribution amplifiers but don't count on them having a low noise figure. The receiving loop I just bought for HF came with a high dynamic range pre amp but it doesn't need to have the kind of NF you want, due to being designed for .1-30MHZ.Just wondering; what are you using this preamp for??

author: cvanc
author_rank: Frequent Contributor
timestamp: September 10, 2015, 10:55:45 pm
content: Quote from: AF6LJ on September 10, 2015, 10:44:58 pmThe FM broadcast band (88-108MHz). I enjoy DX'ing but live in a complex signal environment.

author: AF6LJ
author_rank: Unknown
timestamp: September 10, 2015, 11:24:13 pm
content: Quote from: cvanc on September 10, 2015, 10:55:45 pmThat was my guess, and a high Q preselector would go quite a ways to knock down the intermod.There is another way you can deal with the strong signals, it's not ideal but but worth experimenting with. Ether find yourself adouble stub tunerfor this portion of the VHF band or make one.The double stub tuner will work as a notch filter and you can tune out the station that is the worst offender for the portion of the band you are listening in.

author: cvanc
author_rank: Frequent Contributor
timestamp: September 11, 2015, 01:02:59 am
content: Quote from: AF6LJ on September 10, 2015, 11:24:13 pmThank you, but I think filtering just won't meet my needs here. First, there are a large number of hot signals to be attenuated, and which frequencies are hot is highly dependent on antenna rotor aim (my antenna front-to-back ratio is about 20dB). Second, some of the stations I want most are very weak and surrounded 200KHz on either side by signals 25-30dB hotter. It's quite the thicket on my spectrum analyzer.My current tuner (actually,tuners... will get to that in a moment) has a NF of ~7dB but is extremely selective. So a roof-mounted preamp with a much lower NF makes a large improvement in listenable stations.After the preamp I have long cable runs and splitters, because I have three of these tuners in different rooms. I haven't observed anything that makes me think the tuners ever splatter. As I said earlier I know the preamp does sometimes, but just barely - if I rotate maybe 5dB out of the worst-case aim it clears up. So it almost meets the bulletproof criteria I mentioned above, but not quite.Thanks again for the reply.

author: AF6LJ
author_rank: Unknown
timestamp: September 11, 2015, 01:37:32 pm
content: Quote from: cvanc on September 11, 2015, 01:02:59 amYou may already have the best of both worlds, good luck...If you do find anything that works better for your situation, please share it. I'm interested in this stuff.

author: cdev
author_rank: Super Contributor
timestamp: September 18, 2015, 12:05:12 am
content: This MMIC would be another good choice, its easy to work with, and it offers basic ESD protection.http://www.minicircuits.com/pdfs/PSA4-5043+.pdfThe only drawback is it's a 50 ohm device. But it's more than just a solid performer at 88-108 MHz, the performance is quite good that low, and the gain is more than 20 db.Overload performance is very good and also as it has ESD protection, it can be incorporated into a device with a very very low parts count, you could basically use it as a mast mounted device with simply a blocking cap and use a bias-tee downstairs at your receiver to inject power. That will always be the optimal setup for any VHF/UHF setup that uses an LNA.I suppose you could also design a device to use a varactor diode to allow you to tune it remotely, you could likely figure out a way to make a mast mounted pre-selector/filter. Then you could tune its notch or frequency peak with one or several variable DC voltage(s) instead of variable capacitors or inductors.

author: C
author_rank: Super Contributor
timestamp: September 18, 2015, 02:20:58 pm
content: Are you thinking that a preamp is different from a radio?As some here have said,Put a BW filter on the input.Put a notch filter on large signals.Both prevent the amp from seeing large signals that would cause overload.If there is a possible overload case, you could do like a radio does and use AGC on the Preamp. Doing the best the preamp can do could be better then the overload result.Also stated is one antenna to three radios. Are you switching to one radio at a time or using a splitter to feed three radios at once. Are you using an amplified splitter or passive splitter.C

author: AF6LJ
author_rank: Unknown
timestamp: September 18, 2015, 02:30:40 pm
content: Quote from: C on September 18, 2015, 02:20:58 pmThe Op implied that his main challenge was adjacent channel interference. He does FM BCB DX'ing. I think he has the best compromise for his situation.

author: retrolefty
author_rank: Super Contributor
timestamp: September 18, 2015, 02:38:53 pm
content: Quote from: AF6LJ on September 18, 2015, 02:30:40 pmWell if adjacent channel interference is the problem then the solution can only be handled at the I.F. filtering level. Normally stereo FM receivers have a pretty 'wide' I.F. filter. Some higher end tuners offered a wide/narrow filter option for the great vintage FM tuners. For DX on FM broadcast band I would think you would want a more narrow IF filter option then 'normal' as SNR required for full stereo quiting takes a pretty strong signal.

author: AF6LJ
author_rank: Unknown
timestamp: September 18, 2015, 02:47:42 pm
content: Quote from: retrolefty on September 18, 2015, 02:38:53 pmThis is true, his issue is the strong station in combination with others are causing intermod in his preamp.

author: retrolefty
author_rank: Super Contributor
timestamp: September 18, 2015, 03:10:53 pm
content: QuoteThen he has two separate problems each with it's own solution. If intermod from the existing pre-amp exists then he needs a stronger preamp with more headroom. That mini-circuit MMIC looks like a winner to me.

author: cvanc
author_rank: Frequent Contributor
timestamp: September 18, 2015, 03:20:29 pm
content: Correct, the issue is intermod in the preamp (combined with a desire to increase the intermod tolerance of said preamp, as opposed to removing it, or lowering its' gain, or prefiltering).The tuners have digital IF and extremely high adjacent channel selectivity (over 80dB). The tuners are not having any problems that I can see. But remember their NF is in the ~7dB range. There's room for improvement in the NF.A low NF preamp up on the roof is the good topology here. And my system, as it is now, works much better than no preamp at all (far more listenable weak stations).Between the preamp and the tuners are long coax runs and lossy passive splitters, so the (very hot) preamp signal is more reasonable by the time it gets to the tuners. There's probably ten, maybe twelve dB loss to the closest tuner, and more to the more distant ones.That's why I want to preserve the NF right at the mast-mounted preamp. In my situation the preamp is acting a bit like both a preamp and a distribution amp. I *want* the signal coming out of it to be much hotter than typical because of my lossy cable runs. Make sense?Thanks.

author: C
author_rank: Super Contributor
timestamp: September 18, 2015, 03:33:08 pm
content: QuoteAGC on preamp can remove/reduce intermod, it's just not common on cheap preamps. Most people would switch in or out the preamp. More headroom just lowers the need of AGC on preamp to prevent overload.There is a choice of a distribution amp that is separate from the preamp. The cable loss lets you boost the signal again to a limit just before the split while adding some noise.QuoteFrom this, Guess notch filters do not work or a very narrow BW rf amp.Some radios let you shift the RF band pass off center from the IF. Great for one side adjacent channel interferenceSome radios have very poor AGC while others are great.If the AGC system only works on the desired signal, the radio can be setting the RF amp to max gain which can cause the IF to be overloaded.How many of you have seen a radio that has a separate AGC system for each IF in the radio. If you were lucky you may have seen a separate adjustment tor each.There are many ways to change a radio system. You can gain something and in process lose something.One option may work great for one signal while be bad for others.If you have a choice then you can select options to use.C

author: KJDS
author_rank: Super Contributor
timestamp: September 18, 2015, 03:38:11 pm
content: put that in a balanced configuration,http://www.mitsubishielectric.com/semiconductors/content/product/hf/gaastransistor/smlsgn_gaas/smlsgn_gaas_lv4/mgf1954a_1104.pdfbut you need to be feeling brave to make it work.

author: AF6LJ
author_rank: Unknown
timestamp: September 18, 2015, 07:57:22 pm
content: As you can see from the OP's requirements he most likely has the best compromise.Earlier in the thread the OP stated the desire to keep his current low NF.

author: retrolefty
author_rank: Super Contributor
timestamp: September 18, 2015, 08:13:23 pm
content: Quote from: KJDS on September 18, 2015, 03:38:11 pmTo say the least. But how would you propose a "S to Ku" band preamp help with receiving on the FM broadcast band?

author: KJDS
author_rank: Super Contributor
timestamp: September 18, 2015, 09:01:57 pm
content: Quote from: retrolefty on September 18, 2015, 08:13:23 pmIt's a transistor, they don't stop working at lower frequencies, they just tend to be harder to match, though that's far less critical in a balanced configuration.