Breadboarding at RF

[jgalak]

I am working on an RF project and have a 40MHz signal (it'll eventually be in the 140MHz range, I'm keeping it low for now since my scope is only 100MHz).  The signal currently goes from PCB, via coax, to the scope, and all is relatively well.

But now I want to insert a filter into the signal.  I don't have a coax inline filter, so want to breadboard something.  I have a couple of BNC-to-test lead adaptors, and tried to put the filter together on a breadboard and thus insert into the signal path.  The result is a disaster - both the input to the filter, and the output of the filter are horribly distorted.  A square-ish wave now looks like a jagged triangular wave.

My question is general - is there a way to use a breadboard at these frequencies?  Or should I just give it up as a bad idea and make my prototypes more with more permanent techniques (dead bug, varo board, etc.)? 

[TheUnnamedNewbie]

One of the biggest causes of problems on breadboards are the inductance of long leads. However, if you keep your leads short that can be, at least in part, reduced. In addition, remember that loop area is a key factor for inductance - keep the return path as close to the signal path as possible!

I think you should be able to do something on a breadboard to supprisingly high frequencies if you keep that in mind.

However, the stray capacitance will mess with your filter, so it will shift in performance (insertion loss, passband, etc).

Another thing that helps is using SMA connectors to right-angle board plugs such as these: https://www.amphenolrf.com/132136rp.html

I've found on some breadboards, with some tweaking of the connector (filing down the edges of the pins so the are a bit thinner) and some strategic use of sticky tape I can use these to connect to breadboards. That way you don't need long test lead adapters and you stay coaxial right up to the breadboard. Of course, this does need SMA Cables as well, but in my lab I've pretty much switched to SMA completely.

[tggzzz]

I am working on an RF project and have a 40MHz signal
...
My question is general - is there a way to use a breadboard at these frequencies?  Or should I just give it up as a bad idea and make my prototypes more with more permanent techniques (dead bug, varo board, etc.)?

You can use breadboards - as a doorstop.

You should do some Spice simulations including plausible parasitic capacitance and inductance. For the latter, assume 1nH/mm of wire. That will show you why breadboards don't work.

To find out how you should do it, look at ham (amateur) radio techniques; they are well documented. Fundamental principles: keep all leads small, have a groundplane, match impedances.

[Kalvin]

You may find this w2aew's video quite useful on how to breadboard things in RF frequencies:

https://youtu.be/kH110yjYZ2g?t=385

[hagster]

Just google 'die cast enclosure' there are hundreds of varients.

Depending on what you are doing you may not need a metal enclosure anyway.

If its for shielding you can get injection modled boxes with sheilding sprayed on the inside or you can buy rf shielding paint such as mg chemicals super shield if you want to do ot yourself.

[jgalak]

Most of what I'm doing on a bread boarding is experimenting with what will eventually go on a PCB - that's the nice thing about solderless breadboards.  Oh, hey, this cap didn't work?  Let me swap in another.  Fast and cheap (no wasted parts) compared to soldering things.  But, yeah, may just not work for these frequencies.

[cdev]

"Manhattan" style prototyping.

Vero board- you may be able to use that more effectively than manhattan for filters. Also ebay user "rfextra" sells a small filter PCB that you can use to make small filters very easily. It depends what frequency you need this for and how cheap you are..

Ugly style can work too the key to success being make everything incredibly short- especially the path the ground in critical places. This is why home made (but vias) or sent out PCBs using SMT is the way to go, ultimately, it allows you to have ground planes and predictability that is difficult to impossible to achieve in any other manner.

But FR4 is also lossy at higher freqs.

I am working on an RF project and have a 40MHz signal (it'll eventually be in the 140MHz range, I'm keeping it low for now since my scope is only 100MHz).  The signal currently goes from PCB, via coax, to the scope, and all is relatively well.

But now I want to insert a filter into the signal.  I don't have a coax inline filter, so want to breadboard something.  I have a couple of BNC-to-test lead adaptors, and tried to put the filter together on a breadboard and thus insert into the signal path.  The result is a disaster - both the input to the filter, and the output of the filter are horribly distorted.  A square-ish wave now looks like a jagged triangular wave.

My question is general - is there a way to use a breadboard at these frequencies?  Or should I just give it up as a bad idea and make my prototypes more with more permanent techniques (dead bug, varo board, etc.)?

Don't use a breadboard above the very bottom of HF, except at the very beginning. too much stray capacitance and inductance.

You could use the island technique for HF. Put SMA edge mount connectors for your connections but instead of clipping it to the edge as is done normally, have it higher, sitting on its edge mount connector like a little stand.  That will work fine for VHF too.

Doing this you can make some small filter prototyping boards.

Its easy, it takes longer for me to write it than do it, almost. The kind of SMA connector I get the most of is the edge connecting kind. Just soldertwo down facing each other a few cm apart from one another.. not grabbing the end of the PCB, sitting on it. Then build your filter in that space. If you anticipate maybe needing more room just use a larger piece of PCB so you have some room to expand. If its not big enough desolder it and move them a bit farther apart.

You can use the nibs chopped out by your nibbling tool as anchors for both through hole or SMT parts by using super glue, but ventilate well - with a fan blowing the smoke out your window  when you solder to its top..

[Yansi]

Even though FR4 is lossy, it is good enough up to 10 GHz homebrewing.  The increased loss tanget at very high frequencies even may be welcome when working with some crazy fast stuff, such as 25GHz ft transistors.

[vk6zgo]

I am working on an RF project and have a 40MHz signal (it'll eventually be in the 140MHz range, I'm keeping it low for now since my scope is only 100MHz).  The signal currently goes from PCB, via coax, to the scope, and all is relatively well.

But now I want to insert a filter into the signal.  I don't have a coax inline filter, so want to breadboard something.  I have a couple of BNC-to-test lead adaptors, and tried to put the filter together on a breadboard and thus insert into the signal path.  The result is a disaster - both the input to the filter, and the output of the filter are horribly distorted.  A square-ish wave now looks like a jagged triangular wave.

My question is general - is there a way to use a breadboard at these frequencies?  Or should I just give it up as a bad idea and make my prototypes more with more permanent techniques (dead bug, varo board, etc.)?

Even if you build it properly, a filter will inevitably distort your "squarish" wave.
By their nature, they are narrowband devices, so will not pass all the harmonics making up your waveform.
The input Impedance of the filter will also change radically outside the designed bandwidth.

[jgalak]

I _want_ it to distort my squarish wave   It's meant to be a low pass filter to kill the odd harmonics and get something reasonably sine-like.

But that's not what it's doing.  Just the act of going from coax to breadboard is causing the square wave to be horribly distorted, and what comes out of the filter looks like a jagged triangle wave.  It's a mess.

[radar_macgyver]

Just the act of going from coax to breadboard is causing the square wave to be horribly distorted, and what comes out of the filter looks like a jagged triangle wave.

Might also be due to your probing methods. High impedance probes with ground leads can add enough inductance to cause distortions like what you're seeing. Jim Williams had an app-note or two about this.

Edit: It's AN-47. http://cds.linear.com/docs/en/application-note/an47fa.pdf

Found a good companion too: http://readingjimwilliams.blogspot.com/2011/10/app-note-47-part-1.html

[jgalak]

Might also be due to your probing methods. High impedance probes with ground leads can add enough inductance to cause distortions like what you're seeing. Jim Williams had an app-note or two about this.

Edit: It's AN-47. http://cds.linear.com/docs/en/application-note/an47fa.pdf

Found a good companion too: http://readingjimwilliams.blogspot.com/2011/10/app-note-47-part-1.html

In this case, I wasn't using a probe, but going back into coax to the scope.  But I get your point.

[Kalvin]

Might also be due to your probing methods. High impedance probes with ground leads can add enough inductance to cause distortions like what you're seeing. Jim Williams had an app-note or two about this.

Edit: It's AN-47. http://cds.linear.com/docs/en/application-note/an47fa.pdf

Found a good companion too: http://readingjimwilliams.blogspot.com/2011/10/app-note-47-part-1.html

In this case, I wasn't using a probe, but going back into coax to the scope.  But I get your point.

My guess is that you are seeing reflections due to mismatch with the filter, coax and the oscilloscope's input impedance. The impedance of your filter should match with the coax, and the input impedance on the oscilloscope should match with the coax.

[cdev]

the free program RFSIM99 is useful

[jgalak]

My guess is that you are seeing reflections due to mismatch with the filter, coax and the oscilloscope's input impedance. The impedance of your filter should match with the coax, and the input impedance on the oscilloscope should match with the coax.

The signal source is 50 Ohm, as are  the cables.  There's a 50 Ohm through-terminator on the scope input.  Obviously when it went to the breadboard, that wasn't any particular impedance - it's just bits of random wire and the breadboard itself.

I'm not sure what you mean by impedance of the filter - it's just a simple RC lowpass filter - 22k series resistor and a .1uF parallel capacitor.  How would one match impedance on that?

[Kalvin]

My guess is that you are seeing reflections due to mismatch with the filter, coax and the oscilloscope's input impedance. The impedance of your filter should match with the coax, and the input impedance on the oscilloscope should match with the coax.

The signal source is 50 Ohm, as are  the cables.  There's a 50 Ohm through-terminator on the scope input.  Obviously when it went to the breadboard, that wasn't any particular impedance - it's just bits of random wire and the breadboard itself.

I'm not sure what you mean by impedance of the filter - it's just a simple RC lowpass filter - 22k series resistor and a .1uF parallel capacitor.  How would one match impedance on that?

Typically the RF filters are designed for specific impedances for the best performance. If the impedance of the source feeding the filter input and/or the impedance of the filter output do not match with the load, the filter response may not be as intended.

It looks that you have terminated the coax properly at the oscilloscope's end, so there should not be any reflections. So, my guess was wrong

[ogden]

I'm not sure what you mean by impedance of the filter - it's just a simple RC lowpass filter - 22k series resistor and a .1uF parallel capacitor.  How would one match impedance on that?

Your filter cutoff frequency is 72 Hz. No wonder it does not pass 40MHz signal 

For frequencies as high ar 40MHz or 140MHz, you would want to design LC filter:

http://www.wa4dsy.net/filter/filterdesign.html

Ugly layout and soldering, but still illustration pictures of island method:

http://www.rowetel.com/?p=4804

[fattowave]

There used to be a nice product for rapid RF prototyping and debugging called "Mini-Mount" in Europe and "Solder-Mount" in the US sold by Wainright Instruments. The idea was self-adhesive transmission lines (50 ohm, 75 ohm) and package footprints that could be cut to size and stuck down onto a large sheet of Cu-clad which acted as the groundplane. I believe it's discontinued now, but you could implement a DIY version by fabricating some cheap, single-layer PCBs with all the footprints, microstrip lines, bends, junctions etc. that you might need, and gluing them to a copper plate or Cu-clad board with double-sided tape. I built complete radio system prototypes using this stuff in the early 90s and it works reasonably well up to 2 GHz with care.

Some relevant info here:
http://www.lpkfusa.com/articles/prototyping/edn_2_96.pdf

[jgalak]

I'm not sure what you mean by impedance of the filter - it's just a simple RC lowpass filter - 22k series resistor and a .1uF parallel capacitor.  How would one match impedance on that?

Your filter cutoff frequency is 72 Hz. No wonder it does not pass 40MHz signal 

The possibility that I'm a bloody idiot can never be ruled out...

No idea how I came up with those values.  I know I was using an on-line filter calculator, but no clue what I did....  Changing that to 270 Ohm and 10 pF made thing work much better.  I am now, in fact, getting something resembling a sine wave.  It's not clean by any means (since this is still on the breadboard), but at least the filter is now making things better rather than worse.....

[ogden]

No idea how I came up with those values.  I know I was using an on-line filter calculator, but no clue what I did....  Changing that to 270 Ohm and 10 pF made thing work much better.  I am now, in fact, getting something resembling a sine wave.  It's not clean by any means (since this is still on the breadboard), but at least the filter is now making things better rather than worse.....

Now it is time for you to watch this video to understand why breadboard for RF is "no go":

[medical-nerd]

the free program RFSIM99 is useful

I have been unable to get it installed in Windows 10, any suggestions since this is the only windows version I have now.

Cheers

[Yansi]

Use a Virtual machine for it.

RFSIM99 runs fine on Win7.

[cdev]

RFSIM99 also works great on Wine

[james_s]

Dead bug it on blank copper clad PCB. It's not pretty but it's easy to do and can perform very well. It's actually challenging to make an etched PCB that works as well.

[Yansi]

I disagree on the etched PCB.