Breadboarding at RF

[whalphen]

I just use 0 ohm resistors.  At HF and VHF, I don't think it adds enough inductance to be significant.  You can add these to an Elsie model.  After you have the circuit design done using the topology in Elsie, just edit the circuit and add a stage using Insert Part Prior to This Stage.  Put an inductor there.  Anyway, the parts you buy and install will likely have tolerances greater than the reactance from a nH or two that may result from the 0 ohm resistors.  Elsie is a great tool for modeling this.  Add the inductor as I described.  Then set the stepsize values and run the Monte Carlo analysis to see just how much effect the variation has.  If you find it to be significant, just add a bit of capacitance in series to offset the reactance.  Just calculate the amount of capacitance needed to produce a similar magnitude of reactance as the inductance at the frequency of interest. 

I seldom stick to the design Elsie comes up with initially.  I always set limits and run the optimizations several times, improving the filter to meet my specific needs and to minimize sensitivity to the higher tolerance parts.  A couple of Elsie hints: check the transmission angle to make sure you don't get a 180 degree phase shift at the frequencies you are using.  For some modes, this will distort your signal.  You can put limits and run the optimization to correct this.  Check the return or VSWR and set limits if you want to control reflections during optimization.  I also sometimes use the 'swap end - end' feature and check return or impedance from the tail end of the filter, too, if I'm concerned about that -- for example, with filters used in diplexers.  And, as Yansi suggests, set reasonable Q factors in the Elsie analysis tab.

By the way, another very useful tool is the free Iowa Hills Smith Chart software.  I use it often.  It gives the benefits of a Smith Chart without having to spend too much time learning how to use it.  It's great for coming up with impedance matching circuits.

[jgalak]

Thanks. I'm only understanding maybe 1/4 of the things in Elsie, so lots of learning to do

I had to tweak the design extensively, since it generates oddball L and C values that don't exist in real components.  "Nearest 5%" helps, but not enough.  So took quite a few iterations.  The filter is for an APRS tracker, so my frequency of interest is pretty limited, which makes it easier.

Here's what I ended up with:

[jgalak]

Note that this is a "first approximation" filter.  It's not going to be used on the air.  For now, it's just there so I can kill extra harmonics when feeding the signal into a SpecAn/receiver.

For on-air use I'll either get a commercial one or, hopefully, will have a real SpecAn by then so I can actually test the filter "as built".

[whalphen]

Looks good.  And it meets the FCC requirements, so you could use it on the air.  Attached is the one I used on one of  my tracker designs.  It's very similar to your's.  I had Elsie design the filter.  Then I set the limits -- shown in red -- and had Elsie optimize it.  Then selected nearby inductor values from my inventory, set their step sizes to 0, and had Elsie optimize the capacitors to work with my inductors.
When I built that tracker I didn't have a spectrum analyzer.  I just fed the output of the tracker through an attenuator and then to an SDR dongle to look at the harmonics. It's not as accurate as a spectrum analyzer, but it was good enough to confirm that harmonics were well below the limits.

[jgalak]

Yeah, SDR dongle is the best I have right now. 

[jgalak]

So I finally got myself a Spec An, and was able to characterize the filter I made.  Looks pretty good to my eye:

[ogden]

So I finally got myself a Spec An, and was able to characterize the filter I made.  Looks pretty good to my eye:

Looks nice indeed. Always do "reality check" as well - full freq range of your SA. Depending on layout/enclosure sometimes RF can find leakage/reflection path around your filter at high frequencies.

[jgalak]

So I finally got myself a Spec An, and was able to characterize the filter I made.  Looks pretty good to my eye:

Looks nice indeed. Always do "reality check" as well - full freq range of your SA. Depending on layout/enclosure sometimes RF can find leakage/reflection path around your filter at high frequencies.

So that was an interesting point and I decided to do just that.  Below two images were taken with the exact same setup (slightly different cable and connectors than previous one).

First shows 0-500MHz.  The second shows full scale, 0-3.2 GHz.  Is it really not filtering at all at those frequencies?  At 2.9GHz, it appears to actually be 2dB _above_ reference!  I'm assuming its because at those frequencies you get weird coupling effects on the PCB, and the trances and components are no longer working as expected.

Now, in practice, this probably isn't a huge deal, as there shouldn't be any energy that high up going into the filter in the first place, but it's definitely good to know...

[Howardlong]

I disagree on the etched PCB.

Same here.

My rule of thumb is HF and low VHF is fine for Manhattan as long as you take care (e.g. short leads), mid-VHF and above you need to consider fabricating your own boards.

At the risk of flogging a dead horse, the Busboards with solid copper backplanes are my preferred alternative to Manhattan up to about 100MHz or so for RF and mixed signal prototyping.

[ogden]

Now, in practice, this probably isn't a huge deal, as there shouldn't be any energy that high up going into the filter in the first place, but it's definitely good to know...

Strange coupling at 2.9 GHz. Filter could have built-in "antennas" I wonder how your filter layout looks. Did you "fold" your filter so in/out is close to each other? Use non-SMD capacitors? I would be worried about poor filter performance around 2.4GHz. Nearby WiFi transmissions can saturate LNA or impact AGC operation, if any.

Note that by adding two shunt capacitors at input/output, thus converting filter from T to PI topology, you will get 9th order filter using same number of inductors in place of current 7th order filter.

[jgalak]

The layout is whalphen 's, above.  All components are smd.  Filter is inside a piece of 1/2" copper pipe with end caps (though caps are not soldered on), as per his design.

The goal of this is to be an APRS tracker output filter.  The particular "transmitter" I'm using is a bit weird, and outputs a square wave, not sine.  Running the unmodulated carrier through this filter gave me just one unwanted spike, at the third harmonic, but down about 50dB from the primary.  Unfiltered, it gave a beautiful rake at the odd harmonics.

[xaxaxa]

just a tip: SMA connectors from aliexpress/ebay are notorious bad because of their fake gold plating; the connector looks and functions fine for a month until they corrode.

It's easy to test though; just prepare a salt & vinegar solution (adding salt until saturated is fine), put one drop onto the connector and wait for it to dry. If there is visible discoloration then there is no gold plating; a good connector will go almost untouched by the solution.

You can also tell by price; if it's below $0.4 usd per connector it's guaranteed fake; good quality connectors with real plating average about $0.5 usd in china, and about $2 usd from mouser/digikey. I would look for a listing that specifically say gold plating and do the vinegar test; if it fails the test I'd have a message with the seller.

[daslolo]

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

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

What are the spiral wires for?

[Howardlong]

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

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

What are the spiral wires for?

They're hand wound inductors, there's a ferrite toroid that they're wound around.

[eb4fbz]

So I finally got myself a Spec An, and was able to characterize the filter I made.  Looks pretty good to my eye:

Looks nice indeed. Always do "reality check" as well - full freq range of your SA. Depending on layout/enclosure sometimes RF can find leakage/reflection path around your filter at high frequencies.

So that was an interesting point and I decided to do just that.  Below two images were taken with the exact same setup (slightly different cable and connectors than previous one).

First shows 0-500MHz.  The second shows full scale, 0-3.2 GHz.  Is it really not filtering at all at those frequencies?  At 2.9GHz, it appears to actually be 2dB _above_ reference!  I'm assuming its because at those frequencies you get weird coupling effects on the PCB, and the trances and components are no longer working as expected.

Now, in practice, this probably isn't a huge deal, as there shouldn't be any energy that high up going into the filter in the first place, but it's definitely good to know...

You get re-entry response at high frequency due to inductors parasitic capacitance (interwinding capacitance). When designing high frequency filters you should have all parasitic effects into account, and even use RLC models for every component. Check the self resonance frequency (SRF) of the inductors you have used, the inductors will act as a perfect short at that frequency.

[jgalak]

This filter is designed for APRS, so the frequency of interest is at 144MHz.  The source, feeding into the filter, puts out a square wave, so there's also the odd harmonics of 144MHz.  Shouldn't be anything anywhere near the re-entry response, I'd think.

Looking at the signal with the filter, I get a single extra spike at the 3rd harmonic, about 40dB below the fundamental, and nothing else is visible within the dynamic range of the SA. 

So I think I can safely call it "good"

[Dwaine]

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":

I learned something new today.   

[tggzzz]

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":

That's half the story; the other half is the inductance of the wires - assume 1nH/mm.

Then calculate the resonant frequency of a wire connected to a breadboard.

[Wolfgang]

IMHO, solderless breadboards and RF are not a good combination.
What does work, is "Bronx" style using components above a groundplane.

https://electronicprojectsforfun.wordpress.com/prototyping/

I used techniques like this (with 50Ohm striplines made from FR4 PCB strips) successfully up to 23cm.
RF feels OK in a defined impedance environment. A chaotic rats nest of wires is a no-no (except if you care for all kinds of surprise effects).
Deblock everything with caps, use a groundplane, keep wires extra short or put signals on 50Ohm lines, apply shielding, use SMA edge connectors,
avoid probes, ... and your life will be easy

[ogden]

IMHO, solderless breadboards and RF are not a good combination.

Yes, Mr.Obvious.

This is exactly what we are talking here. You are advised to read thread before posting.

[Wolfgang]

If you have a problem that I agree with something, I probably cannot help you.
BTW: I have read the other entries.

[ogden]

If you have a problem that I agree with something, I probably cannot help you.
BTW: I have read the other entries.

There's huge difference between statement and agreement.

Could you point out something like "I agree to what's said multiple times before" in following sentence?

IMHO, solderless breadboards and RF are not a good combination.

[Wolfgang]

The simple answer is : forget a breadboard at 40MHz.

If you need to try something out at 40MHz, it should look similar to this one:

https://electronicprojectsforfun.wordpress.com/an-noninteger-frequency-divider-with-sinewave-output/

If you scroll down, you can see a lot of prototypes built on copperclad PCBs. There is also a bandpass filter ("BPF"). All these work fine,
and the technique will be OK up to 150MHz if you keep all leads very short and all components close to the copper ground.

The picture also shows a finished amp module in a shielded box. This is a safe way to prevent unwanted coupling or radiation.

[james_s]

For a long time that deadbug construction technique looked so sloppy I just couldn't bring myself to do it. In more recent years I've played around with it some and found it performs extremely well, I've really come to appreciate it.

[Wolfgang]

I had the same learning curve. One has to differentiate between optical and electrical uglyness 

A lot of "neat" circuits suffer from extra lead lengths, unwanted couplings due to "all in a row" aligned components, ...

Ugly construction (I call it "Bronx" style to separate it from the very nice "Manhattan" style by Jim Kortge) can be used will into the GHz region if ou know hat you are doing.