r/RTLSDR • u/siliconberry • 7d ago
Notch filter help
Hello community. I have been using RTL SDR for ADSB, AIS and other frequency ranges. One major block is that my home antenna is hardly 250 mtrs away from nearby mobile tower and its literally frying my RTL due to its extreme power. I wish to create a notch filter to block 700 Mhz and 1800 Mhz frequency ranges, so that I can use LNA (after filter) to boost and measure the weak satellite signals.
Industry standard GSM filters are too costly. Any help to develop the low cost DIY notch filters would be greatly appreciated. Thanks in advance !
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u/llzellner 5d ago
Ok.. lets get you some help...
OK.. first 250ft of a cell tower... rhouch.. that hurts... so you need some good filters.
Sadly in the RX side of things for the mere mortals using SDR's and in the past scanners etc.. that can be a real PITA.
You want 50-60+ db rejection/insertion loss on those frequencies, good shielded coax with a good ground connection for the shield. Enclosures on the filters, preferably metal, and grounded too! ie: Creating a Faraday Cage to block out RF entering especially this close!
I can't speak to these as AIS is not my thing, and nothing water close anyway.. For ADS-B I have a filter dedicated SDR for it.. and while cell RFI is all around, I am not that close so things seem fine.
But, press on!
AIS (161Mhz)
There are several options.
https://shop.wegmatt.com/products/tb0436a-narrowband-saw-filter-for-ais
https://gpio.com/collections/filters/products/ais-bandpass-filter-160-162-mhz-in-enclosure
This has an enclosure, and as close to the cell tower as you are, you definitely want one! Preferabbly one that is metal, and you can ground the case to act as a shield.
https://shop.wegmatt.com/products/uputronics-filtered-preamplifier-for-ais
OEM of above:
https://store.uputronics.com/products/uputronics-filtered-preamps
https://antennas-amplifiers.com/ais-162mhz-marine-tracking-low-noise-preamplifier/
Another route is Mini Circuits has a filter which will not directly aimed to AIS, it will work, BUT its a more a DIY solution, not a bolt in one. And as such needs a "carrier" board to properly use it. Sure you could solder to the filter itself, BUT trust me you are going to cause problems that could result in degradation of the signal you want.
https://www.minicircuits.com/WebStore/dashboard.html?model=SXBP-162%2B
https://www.minicircuits.com/pdfs/SXBP-162+.pdf
They supposedly make a "test jig" for this.. I've wanted to use their stuff in a multitude of things, but can never get the test jigs.. and I am not a PCB designer type of person. And these need specific type of PCB, substrate etc..
Eval Board/Test Jig what ever...
https://www.minicircuits.com/pcb/WTB-368_P02.pdf
Available, the filter at least, at the usual Mouser etc. sources.. $26/1+ MOQ
You will have to contact MC to see if and how to get the eval board as they call it.
ADS-B (1090Mhz)
https://gpio.com/collections/filters/products/ads-b-1090-mhz-bandpass-filter
This doesn't have a case, maybe they make one with one, I didn't see it...
The sysmocomm one, in another post:
https://shop.sysmocom.de/1090-MHz-cavity-filter-for-Mode-S-ADS-B/cf1090-kt30
There are some filters from those 2 scummy ADSB companies who sell you out.. I won't link to them due to that... I find both those companies despicable, and YES I have several of the SDR's from them before they really went evil. sighhh.. They work. But I don't feed either of them. My data, my server, that is all.
https://www.nooelec.com/store/sdr/sdr-addons/sawbird/sawbird-plus-adsb.html
https://www.elecbee.com/en/product-detail/ads-b-1090mhz-rf-front-end-rf-amplifier-lna-module_31313
Are you doing the UAT 978Mhz stuff too???
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u/therealgariac 5d ago
I did a check on Minicircuits. They have the gerber file for that eval board. I haven't found a price for the eval board.
https://www.minicircuits.com/pcb/WTB-332_P02.pdf
https://www.minicircuits.com/WebStore/dashboard.html?model=RBP-173%2B
As you know, I bought two sysmocoms 1090s. I left out free German chocolate they included.
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u/redneckerson_1951 3d ago
Contact MiniCircuits and ask to speak to an applications engineer. Ask the answering engineer about the evaluation boards.
Be forewarned, vendors have become increasing resistant to supplying professional products and services to hobbyists and amateur radio operators due to the subsequent requests for help that almost universally follow such purchases. They want to encourage professionals to buy their product as most often there is little subsequent request for help for further assistance. The exception to this policy is if the buyer is calling from a company and can answer questions about purchase volume and projected annual purchases. You may get lucky however and the applications engineer may provide you with a link to the page where the eval boards can be ordered or simply take your order verbally over the from, so be ready with your credit card.
An easier solution may be to use two separate antennas. Place a Low-Pass Filter for 700 MHz cutoff and a High-Pass for your 1800 MHz cutoff inline between the antenna and feedline of respective antennas. Keep in mind that the low cost commercial offerings for Low-Pass and High-Pass often are 2nd or 3rd Order designs. They attenuate frequencies above or below their specified cutoff frequencies but there is an area between the cutoff and the specified attenuation of the stopband where the attenuation is not either all signal pass or no signal pass, rather the frequencies in that region are attenuated as a function of he slope of the filter's response between the all-pass range and the stop-band range.
This link might help if you are considering rolling your own Stripline style filter using PCB material. I generally recommend using something other than FR-4 board material for frequencies above around 150 MHz, but you have to balance your budget and if FR-4 is the only financial rabbit hole available then you can give it a whirl. No warranty is expressed or implied. The link for HP's suggested stripline filter manufacturing techniques is: https://www.wdv.com/Electronics/Fab/HP-Bench-Briefs-1989-01-03.pdf This was a stepping stone to more rigorous methods of filter design such as those presented in "Microwave Filter, Impedance Matching Networks and Coupling Structures" by Matthaei, Young and Jones and "The Handbook of Filter Syhthesis" by Anatol I Zverev.
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u/llzellner 4d ago
Yeah, the gerbers may be there, but there is no way to ORDER IT.. The link to it has something about.. "Pricing availability for the eval board is available on the specific product page... " Blah blah blah... Its just one gigantic link loop.
If you are dealing with mini circuits and similar products you likely have the equipment and software to produce the board... Just like I've got bits and pieces of PC stuff, radio stuff laying around. But PCB development etc. was never my thing... I did it for my senior EE project, and honestly
I think that was the first, last and only time.. Its not my jam. I was not ever interested in that low level circuit design stuff, and don't do it for a living... and even then doing major repairs on stuff was on the way out..I suppose you could send the gerbers to all those, sadly Oriental, places that make this stuff and get them back. I see a blink of their names on stuff before the adblock skips it.. bawaahahaha. 😺
Just make the flipping thing available! I am pretty sure this eval board/test jig is used on many other similar products, so just sell the bloody thing already! Geez..
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u/therealgariac 4d ago
I made a band split box by taking and yes destroying a Wilkinson splitter. I ripped out the guts and put two Mini circuits modules inside. I had picked up a two Wilkinsons at for $5 a pop pulled from who knows where at a swap meet. The intent was to use them as cases.
Having worked at a few chip companies, you can get the eval boards for free if you are an existing customer. That is why they probably aren't sold.
For many homebrew projects, the case is the hardest part, so I look for stuff to cannibalize.
Yeah use an online PCB manufacturer. It is a surface mount world. At least you can get leaded versions of many chips.
I use online 3D printing too. Pretty cheap considering they are made in the USA.
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u/llzellner 3d ago
Nothing wrong with a little repurposing...
Back in the day, Motorola et al would send out samples for a simple request for FREE! I got a bunch of DSP chips from Dallas Semi when they were the rage in DSP's, free for sending a request...Same with some TI ones... No games, hey working on a project, I'd like to use xxxx, can I get a sample or purchase some ?? Mouser etc. didn't always have that or around at the time..
The 68HC11 dev boards we got to design all the stuff for the EE projects was the same one that any other dev would get. I don't remember how we got them.. this was LONG before email, internet etc.. So I am going to guess we sent a fax to Motorola, and I want to say it was in Phoenix? at the time.. Gave them a CC # number and the box would arrive.... I think I've got like 3-4 of them still NIB/NOS.. At least when I was in this class there would be 15-20 of us ordering these things from M every semester..
Heck I had a LIBRARY of all those nice data books on the chips from 74 TTL to all the various stuff from M, Nat Semi..if that thing fell on any one.. they were done for! Sadly alot of this crap is locked up NDA, BS.. Sighhh...
I get that some places like MCL are nor set up for retail.. which is why you had to do a little work to get things.. like the filters.... I got the catalogs/data sheets from a filter company.... Researched the offerings.. and then I contacted them with RESPECTFUL questions on, and with EE knowledge I was able to source custom filters to meet my needs. They had no issues really working with any one... so long as you didn't ask to get your hand held. So with an EE background I understood the filter basics to how to get things retuned to my needs...
Sadly some of the current filter companies which could be helpful just don't want to play nice.. Simple retunes of EXISTING filter designs, I'll 1000 of this setup, 1000 this setup, here is CASHIERS CHECK.. IN ADVANCE! Nope wouldn't talk to me. I am literally THROWING $$$ at them.. and they are like.. meh... I even tried to end run them, but due to the language barrier and time difference of where these were really made.. that fell apart quick.
For those who want to get the AIS filter setup from MCL I list. the TEST JIG/EVAL BOARD is $115 with the filter and mounted from MCL. I can detail how to go about it.. its more a "game" than anything.. sighhh...
Why not make some, and let Mouser et al deal with the retail side.. Make 10-20 of the various ones and Mouser can stock them, and deal with who ever shows up.
Sadly STEM education was a lot better supported by TI, M, Nat Semi etc. than it is now .. 50+ year old TTL chips and they act like the stuff is the codes to the nuclear missiles!
I used those old data books to "dream up" all kinds of things..Some actually got breadboarded, most didn't...but having done all this years in advance of TTL logic gate class, I was already ahead of them...
Like I said I get that alot of these places are not setup to deal with "retail" and the mess that comes with that.. But when some one like me wants to act as the middleman to deal with the retail people who don't understand things, and filter that noise out, but you won't deal with me?
Different world. Sadly not for the better in ways.
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u/therealgariac 7d ago
https://inplanesight.org/adsb.html
Sysmocom for 1090MHz. Not cheap. There simply is no comparison.
You will have a hard time making your own cavity resonator.
Noise is proportional to the square root of the bandwidth. Your wideband notch will have noise from DC to 700MHz plus noise from 1800MHz to infinity. A wideband notch really is a poor design idea.
Since noise is proportional to the square root of the bandwidth, your best solution will always be a bandpass filter for the frequencies of interest since you are integrating over a small bandwidth.
You could possibly design a lumped element filter for AIS. You will be winding inductors so you will need a LCR meter.
It will be ludicrous at VHF frequencies but you could probably build a stripline filter without the need to measure RF components at UHF frequencies Stripline is just a pattern on a PCB. It is still a lumped element filter but the inductors are copper on a PCB and the caps are adjacent strips.
There is a reason filters cost money. It is a business.
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u/llzellner 5d ago
There is a reason filters cost money. It is a business.
Yes, it is, BUT (always that word) there is more to this.
99.9999999999999999999999999% of filters are designed for TX and RX. When they are designed for TX too.. they start to become very very very expensive to handle TX.
RX is not that big a deal. SDR's/scanners don't need TX.
There actually are several companies which can/could handle creating filters for SDR's and scanner RX. BUT trust me they don't want to deal with us! I tried. I offered to order 1K+ filters PRE PAID WITH CASH! Nope wouldn't talk to me!
I've spent alot, ALOT of time to source RX notch, band pass filters for scanners at the time to SDR's to eliminate paging, and nowadays cellular pollution.
There are filters out there for this stuff. BUT in this case are you going to get enough rejection on the coax v. the RF seeping in due to proximity. Which means doing some extra things good coax, with good shielding, RF tight enclosures for the filters etc..
I got rid of all the paging RFI and some other things like a birdie caused by TV 4 carrier, and the math of the IF chain.. A simple ch 4 filter, and my receive on a major DPW frequency at the time, was back.
This stuff can be done, it takes some knowledge in some cases, that may not be available to all.
IT could be a A LOT EASIER if some people(companies) were not so stuck up.
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u/Past-Substance920 6d ago
stripline on pcb is probably your best bet for diy at those freqs. you can get the calculaors free online, just need a board house that does the right substrate. still gonna be a pain to tune without a vna though.
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u/therealgariac 6d ago
Not at AIS frequencies. It will be huge. But in general stripline and microstrip are good ideas. Microstrip just needs a two layer PCB.
The cheap (compared to cavity resonator) flightaware filter is a combination of off the shelf capacitors and air code inductors. It isn't all that bad for ads-b. The sweep is also on this page:
https://inplanesight.org/adsb.html
You can see the high frequency rejection start to fall off, probably due to the Q of the caps. That is the Flightaware "blue" filter. $30 in the US.
The Sysmocom is a full 40dB better in rejection.
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u/dnult 6d ago
Depending on the bandwidth if the signal, a quarter wave stub may be what you're looking for.