SMD RF filter PCBs

[rhb]

Does anyone know of a cheap source of  bare PCBs laid out for building RF filters using SMD parts and end launch SMA connectors?  I'd like to build some hardware rather than spend my time learning yet another piece of software to layout a board myself.

I'd like something at 50 ohms  good to at least 3 GHz.


Thanks.

Have Fun!
Reg

[ejeffrey]

I have seen such things -- a microstrip with a gap for a series component and a ground plane to attach shunt components.  But I am not sure how much point there is these days.

They made some sense when PCBs were expensive, but you can now get affordable small PCBs, including 4 layer boards with Rogers high frequency laminates if you want.  And ordering a custom PCB means you can adapt it to your needs, rather than trying to use a generic layout.

Doing this in a free/cheap PCB layout package should be no problem even for a beginner, and is well worth it.

[DaJMasta]

You could go the old copper tape route on a substrate, though measuring and cutting the tape to get the right matching could be tricky by hand.


You could always start with FR4, though.  Yes, you get the loss from the material and the variability in the source (though there are some PCB manufacturers that will specify and guarantee this to some degree), but for one off prototypes and not super sensitive designs, it will probably suffice in this range.

An ENIG surface treatment will give you less variability in trace thickness and no solder mask will probably be a must, but even if the performance isn't fantastic, it would probably suffice for prototyping out circuits and it is so, so cheap.  I wouldn't say it's suitable for production, but it should be alright for experimentation and one-offs that you can manually tune component choice.

[hendorog]

https://www.ebay.com/itm/150203651783

Hand cutting 'slits' in FR4 has also worked well for me.

[rhb]

@hendorog  Thanks for the link.  A bit more expensive than I would like and not quite as many poles.  But it does give me an idea of how to search for them.  I'd stumbled across these or something similar, but could not remember where or how to find them.  eBay searches tend to make me want to beat my head against the wall.

@ejeffrey  I want to spend most of my time developing FOSS FW for  Zynq based DSOs rather than learning  a PCB design package. As you point out, custom boards are cheap.  I want something sitting around so that if I want a custom filter I can make it and stick it in a box in an hour or so.

I'm not really  interested in building "all in one" RF circuits.  I want a collection of modules that I can hook together with SMA jumpers.

I've got a lot of PCB design packages installed on my system.  But only so much time. I'd like to buy a solution.

[ejeffrey]

Having boards laying around for fast prototyping of filters is reasonable, although someone limiting -- at some level the board dictates the possible filter topologies you can use.  It also doesn't work for active devices, so you are going to be stuck if you need amplifiers, VGAs, mixers, of DC offset adjustment for a DSO.

I don't know exactly what you are thinking about in terms of your needs, but for a DSO maybe all you need are lowpass filters.  In that case, you could get a few pre-packaged mini-circuits filters.  You won't have the flexibility of building your own, but they will be better performing, and if your interest is primarily in the DSO firmware rather than the analog front end, it seems like you don't need a lot.

On the other hand, I think "I don't want to waste time learning to use a CAD tool" is short-sighted at best.  Compared to developing DSO firmware, learning to do simple CAD layout takes a trivial amount of time.  And if are serious about making a usable product you are going to need to do that, since you aren't going to build a high performance AFE and sampler with only prototyping boards.

[rhb]

The filters have nothing to do with the DSO project.  The filters are part of a project to have an assortment of connectorized RF modules which can be used to prototype an RF system.  The Mini-Circuits devices would be great if they were cheaper.  Except that even if they were cheap,  I'd be buying them rather than building them.
 
I  want to be able to get an occasional  quick solder fume fix to relieve the pain of Vivado and FPGAs without a lot of fuss.

It seems to me that someone must have designed an OSHW filter board, but web searches are a very hit or miss activity.  I find things one day, but when I try to find them a week later I can't.  So I thought I'd ask.

I certainly intend to learn a PCB tool, but I tried several of them a few years ago and found it very frustrating. If I learn a PCB package now, I'm not learning Vivado and FPGAs.

I have absolutely no interest in nor intention of building a DSO.  There is simply no way one can build a low volume DSO for less than 3-5x the cost of a commercial unit.  And that ignores the engineering cost of designing the board.  I'm pursuing the DSO analog to DD-WRT.  I'm an old hand at DSP and programming in general.  I can do much better than what the Chinese provide.  If an OEM would make there code available I'd just work on that.  But as they won't ,I decided I'd just start from scratch.  I'm also quite interested in seeing how compressive sensing performs on a Zynq.  I don't know of any OEM offering one, though I feel absolutely certain Keysight is experimenting with them.

[hendorog]

@hendorog  Thanks for the link.  A bit more expensive than I would like and not quite as many poles.  But it does give me an idea of how to search for them.  I'd stumbled across these or something similar, but could not remember where or how to find them.  eBay searches tend to make me want to beat my head against the wall.

Just in case you didn't notice, that price gets you 4 boards.

A couple of other ideas which may allow more poles - you could 'zig/zag' the layout using the small islands - or even cut the big islands into two parts using a knife or a dremel type tool.