UHF downcoverter design sanity check - Page 1
Forum section: electronics / rf-microwave

author: Yansi
author_rank: Super Contributor
timestamp: June 02, 2019, 12:01:33 am
content: Hi!it is me again tinkering with stuff I shouldn't, hi hi. This time attempting a UHF to VHF downconverter.Could you please comment on the solution I came up with?Input signal level should be in the range of -65 to -25dBm or thereabout. On the input I have placed a third order low pass filter mainly to match impedance from 75 to 50 ohm, with the good side effect of getting rid of a bit of unwanted spurs.I chose a MiniCircuits RMS-2 level 7 mixer (had those available). The IF port of the mixer has a broadband termination, using the diplexer shown (145 MHz CF, Q=2). I have tried simulating that one for component value spread and it seemed fine (I need just 250kHz BW at the IF).After that a 5th order low pass filter is placed. Band pass would of course be better, but that is a rather complicated to do with SMT.Last part of the chain is a MMIC amplifier, currently MSA0686 for the prototype. I want to use a different one (BGA2771) for the final, unfortunately I do not have those yet. MSA0686 has a NF of 3dB, which is "meh". The BGA2771 shall be better.What is not in the schematic: The output after the amplifier will be split (resistively) into two output ports. (so another 6dB loss there).LO is not solved yet, but I am not that concerned with it, it is just LO (any suitable PLL+VCO will suffice).What I am not sure is whether this can be designed like this, blockwise. Would there be any benefit for example swapping the amplifier with the low pass filter in front of it? Or should be any other changes made?If I calculated right, the overall conversion gain of this test thingy should be like... 8dB? and the noise figure more than 12dB likely. Which seems rather high :-/ (But I am not sure how to deal with it, as the mixer has NF of about 8dB).Thank you for any hints and suggestions.Y.

author: chrisl
author_rank: Regular Contributor
timestamp: June 02, 2019, 05:40:51 am
content: It seems the BGA2771 has a 4.5 dB of NF which is worse than the 3dB of NF of MSA0686.If you want to have a better cascaded NF then add an LNA at the front of the mixer which will bring down the over all NF.However, it is not just the NF and gain you want to care for a RF system... you need to worry about the cascaded IP3 and P1dB as well.Also may want to move the LPF to the output of the amp to suppress any Nth harmonics out of the amp..... I think there is enough the L-I mixer isolation and the diplexer rejection (~10dB).Chris

author: German_EE
author_rank: Super Contributor
timestamp: June 02, 2019, 08:10:22 am
content: With the Minicircuits mixers matching each port to fifty ohms is everything and performance will suffer if you fail to do this. So, I suggest a low value attenuator on the RF input and the LO port to ensure that the device sees 50 ohms +/- j0.

author: Yansi
author_rank: Super Contributor
timestamp: June 02, 2019, 08:43:45 am
content: Well, you know what the diplexer (bridged T) does? (exactly a broadband 50ohm match, at least according to my wish). Input port is sort of matched to 50ohms using the input low pass filter. According to available literature, IF port is the most critical one for matching.I do not know what else could be done to match better to 50ohms, other than making the matching resistive and lossy, which in this case is not really affordable. There is enough loss already.It indeed should be good to add an amp in front of the mixer, but I am not sure if that is a good idea, if I almost do not know what the signal input level will be. (signal comes from satellite LNB) - I could select a low noise preamp chip, however its high gain would likely saturate the mixer with strong input signal. The input compression point of the mixer is about 10dB below LO level, according to the literature. (i.e <0dBm). By plugging an amplifier in front, that may severly limit the IP3 of the overall design.The output amp. Yes, I could swap it with the low pass filter, but I forgot why I dit it this way! The output of the amp will feed a resistive splitter into two output ports. With this configuration, the filter loading is mostly independent on what is on the outputs - I wanted to isolate the filter from the load.(I did a mistake, the part no. for the amp was not BGA2771, but MGA82563 which has a NF of <3dB, sorry for that)Are my thought and decisions correct?Now I can do only two things: Prototype and measure it and test what signal levels I am getting from the LNB.

author: Radiosonde
author_rank: Contributor
timestamp: June 02, 2019, 09:04:17 am
content: What about Quadrature Hybrids?The most important thing is deecent match.It is also very important to keep your IP3 in mind, as well as the system gain, too much gain is a very bad thing because it affects your noise figure.Regards

author: Yansi
author_rank: Super Contributor
timestamp: June 02, 2019, 09:17:53 am
content: You wont find any quadrature hybrids either for my input or output frequency. Way too much arbitrary frequency values to fit within any kind of off-the-shelf parts. And making custom hybrids? I am not sure if that is even a good idea, with the wavelengths involved and materials used (bog standard crap "FR4" PCB).Now it is lunch time, after lunch I will try prototyping the circuit as is to see what it is up to. Then improvements will be applied.

author: Yansi
author_rank: Super Contributor
timestamp: June 02, 2019, 10:35:06 am
content: Just for the giggles, I have hooked up the LNB to SA and this it what it gets me with the cobbled-together setup.(There is a long 15m absolute crap chinese coax cable - unfortunately currently have none better and the dish with LNB is positioned rather loosely than precisely)So, currently we are talking about -90dBm peak signal power. That is way too low, should be more, like 20dB more I'd expect. I will confirm signal power when this horrid setup will get its balls polished.

author: Radiosonde
author_rank: Contributor
timestamp: June 02, 2019, 10:54:04 am
content: Is this a downconverter to convert EsHail Signals from LNB to a 2m rig?

author: Yansi
author_rank: Super Contributor
timestamp: June 02, 2019, 11:00:11 am
content: Eh.. well.. um... I haven't said that!

author: Radiosonde
author_rank: Contributor
timestamp: June 02, 2019, 11:05:38 am
content: Nice idea, even if you havent said thatA fried of mine uses a SDR/ComReceiver for RX and a 70cm to 13cm Transverter with 20W PA as TX70cm because most people dont use SSB on 70cm, so you dont "waste" a TRX with this application.I am struggling myself if I should build a Transverter or a special 13cm SSB TRX only for EsHail with build in Receiver for LNB.

author: Yansi
author_rank: Super Contributor
timestamp: June 02, 2019, 03:05:24 pm
content: I have even some interesting ideas for an upconverter, including couple watt PA. But receive path first!So, circuit prototyped and tested. It works, kind of. I am getting quite some LO leaking to the output (about -40dBm of LO gets out). Which, if I look at it, seems right. The poor little RMS-2 mixer has just 26dB LO-IF isolation at such frequncies. Rest of the dBs come from the lowpass filter. So, I will probably switch to a better mixer (EMRS-11X ~ RMS-11X) which has 35dB isolation, and will make the succeeding lowpass filter more steep (7th order instead of 5th).It seems, that input LNA would be handy. I am getting a lot of signal output now, but the SNR is poor, about 15dB with my setup.So, little research into LNAs: I think it should be handy to be able to select LNA or bypass it. In case of strong signal input, it would drive the mixer nuts. Insted of having LNA and external bypass switches, I think it would be desirable to have LNA with integrated bypass. I have found so far only very few suitable ICs:First I have found ultracheapBGU7033. It seemed extremely nice, until I noted the 4dB NF and 75ohm impedance (and SOT363 package I hate).Then I have found a lot of interesting products from Guerrilla RF (Mouser), sifted through them and came withGRF4042as the only suitable candidate: (almost) 50ohm, very decent NF of 1dB, decent bypass mode loss of just 1.5dB and affordable price of 3$ (a bit high side to my liking) and kind of okay package (2x2mm QFN12).I have also found a couple of others, but I do not like them for some reason:MGA72543.Do you have any tips for other LNA chips with bypass, suitable for 5V supply operation with low noise figure?What I think would be appropriate to do with the LNA:75ohm input -> small low pass match to 50 ohm -> bypassable LNA -> filter (SAW, 50ohm) -> mixer RF input(I have found a possible source of suitable SAW filters)What do you think about this addition of LNA?

author: Radiosonde
author_rank: Contributor
timestamp: June 02, 2019, 03:12:46 pm
content: I would go down with 75 Ohm koax, its cheap and working...then convert the 75 Ohm to 50 Ohm, now we are talking.This is a very low noise Amp:https://www.modelithics.com/models/Vendor/MiniCircuits/PSA4-5043plus.pdfIts noise figure is partially under 1dBSadly it is hard to get minicircuits Parts, especially if you want specific ones.Regards

author: Radiosonde
author_rank: Contributor
timestamp: June 02, 2019, 03:18:29 pm
content: Other thing:I dont think a LNA is really necesary, if it really is you can let it in without bypass.As mentioned a LNB+SDR-Play is sufficient, so I dont really see the need for a high gain amplifier.

author: Yansi
author_rank: Super Contributor
timestamp: June 02, 2019, 03:41:20 pm
content: I have never said different, that the input is 75ohm! If you look at the schematic, the 3rd order lowpass on the input is used to transform 75ohm into 50ohm.LNB + SDR won't cut the mustard, for some specific reason, I need to down-covert further to 145MHz, to interface with existing stuff.Why adding amplifiers? It may not be obvious, but the overall loss in the path (see the schematic on top) is: 1+7+1+1 = 10dB, plus the output split another 6dB. So you have at least 16dB loss in the path, not including for example SAW filter, that would add another couple dB of loss. This has to be compensated for somewhere. The overall estimated noise figure of the circuit is ~12dB. Also quite high.Hence why I think adding amplifier on the INPUT may be better then a high gain one on the output of the chain.The important question now is, what NF the input amp should have? Low NF may be even unnecessary. After all, in the satellite LNB, there is LNA with about 0.5dB NF and 10-12dB gain, succeeded by a mixer + IF amplifier NF 7dB gain 40dB (heck, haven't expected that at all!)See this:https://www.nxp.com/docs/en/data-sheet/TFF1017HN_N1.pdf

author: Radiosonde
author_rank: Contributor
timestamp: June 03, 2019, 07:38:46 pm
content: How much gain is allways diffcult, I noticed that when I begun to design my Shortwave Receiver.The only solution to the problem is an accurate level plan of the whole signal chain, find some typical input level and look what level is present on each stage, then you can check if it is enough.It depends on the receiver but in general too much amlification is normally a bad thing as I mentioned above.Also watch for unlinearities and missmatch.My aproach would be like that:Input-Protection-matching pad-wide bandpass filter-amplifier via Hybrids etc.The most important thing is the input stage, dont forget input protection.Another thing I see is the LO input, it may be unnecessary but I would not go that straight into the mixer, a Bandpass and maybe a low attenuation pad for a good match should be a good thing.It depends on your signal source, however every bit of unwanted signal will interfere with the mixing process.Regards

author: Radiosonde
author_rank: Contributor
timestamp: June 12, 2019, 07:36:14 pm
content: What do you think about this approach?It is a complete carefree package, you can connect a LNB directly as well as the uplink antenna.The only catch is the fact that you need a TRX with split mode because the receive frequency is always the same.RegardsGesendet von meinem SM-J730F mit Tapatalk

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 12, 2019, 08:17:04 pm
content: what VCOs are you running on the ADF4111/2?

author: Radiosonde
author_rank: Contributor
timestamp: July 15, 2019, 08:11:41 pm
content: Sory for the late reply, but here is what I got.Right now I am working on the component level schematic.It would be nice to get some suggestions and ideas.Regards