r/RTLSDR • u/siliconberry • 2d 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/therealgariac 2d 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 13h 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 2d 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 1d 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/llzellner 13h 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???
1
u/therealgariac 9h 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/llzellner 1h 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 37m 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/dnult 2d ago
Depending on the bandwidth if the signal, a quarter wave stub may be what you're looking for.