Breadboarding at RF

[james_s]

You disagree that it's challenging to make a good RF PCB? I guess maybe if you're a skilled RF engineer, but I certainly find RF boards to be a challenge. Not impossible by any means but more difficult than dead bugging a prototype.

[ogden]

I guess maybe if you're a skilled RF engineer, but I certainly find RF boards to be a challenge. Not impossible by any means but more difficult than dead bugging a prototype.

BTW it depends on what you consider as RF frequencies. For 900MHz I would disagree that deadbug gives better results than PCB

When you dead-bug, then anyway you shall know RF properties of circuit & components, same knowledge apply to PCB as well. Don't be scared of PCB, especially at <= 100MHz frequencies. Make everything preferably in straight line (at least for starter prototypes), short signal traces as possible. To be on safe side - always use 2-sided PCB but etch only one side, leave bottom as solid ground plane. Obviously don't be shy of some extra vias for ground traces on top layer, put them close to "load" - filter capacitors or RF components. Also preferably you shall learn how to make 50Ohm microstrip trace. There's various online calculators on internet for that. Basically that's it. Up-to 100MHz - just try PCB to see it's not that scary and usually it happens to work.

[Neomys Sapiens]

RFSIM99 also works great on Wine

Could you be a bit more specific? 
I use the software myself but I have not noticed a correlation between results and my state of inebriation.
So: white or red? vintage? HOW MUCH OF IT?

[KJDS]

RFSIM99 also works great on Wine

Could you be a bit more specific? 
I use the software myself but I have not noticed a correlation between results and my state of inebriation.
So: white or red? vintage? HOW MUCH OF IT?

Rather like dancing, there's an optimum amount, sufficient to lose your inhibitions but not enough to leave your legs wobbly.

[3db]

RFSIM99 also works great on Wine

Could you be a bit more specific? 
I use the software myself but I have not noticed a correlation between results and my state of inebriation.
So: white or red? vintage? HOW MUCH OF IT?

Nice one 

[whalphen]

I found that I frequently have a need to prototype RF filters.  I keep a good assortment of SMD capacitors and inductors.  And I had a supply of generic filter PCBs made.  They have SMD pads to allow a variety of filter topologies.  I can just leave unneeded positions empty or put jumpers.  The PCBs have an SMA footprint at each end and are narrow enough to slide into a short piece of 1/2 inch diameter copper pipe.  For the ends, I add copper pipe caps drilled out for the SMA connectors.  Bulkhead nuts on the SMA connectors can hold the caps in place.  I usually don't use the copper pipe unless I need the shielding or if I want the filter for long term use.  I've used this technique for a long time and use it for other things, too.  I keep some PI pad and T pad PCBs, too, for attenuators, DC blocks, and impedance matches.  I've even used it for RF amps -- usually with larger copper pipe, and feedthru caps to feed the power thru the end cap of the pipe.  I recently had a board made for use as a directional coupler to fit in a 1 inch pipe.

By the way, Tonne Software's free Elsie program is the filter designer I typically use.  I pick a topology, specify the filter, add in some limits for the various parameters, and let it optimize a filter design for me.  I can then quickly prototype a filter and check the results on a spectrum analyzer.  It almost always matches the expected results.  If anyone is interested in the PCB layouts, let me know and I can make them available.

[jgalak]

Oooh, that's clever.  I was going to do something similair on my next board, leave in space for a few passives in a filter configuration, but I really like this idea...

[cdev]

I'd be interested in your filter design PCB templates.

Thank you.

Ive use copper pipe a bit but need to revisit it because I've only recently gotten a decent drill, so its time to buy some copper caps and go to town, so to speak.

I've found with small LNAs the case makes all the difference.

What (brands?) caps and inductors do you prefer? I would be interested in your observations on this.

 I have enjoyed homebrew of small RF passive filters and receiver gain element blocks a lot.

I found that I frequently have a need to prototype RF filters.  I keep a good assortment of SMD capacitors and inductors.  And I had a supply of generic filter PCBs made.  They have SMD pads to allow a variety of filter topologies.  I can just leave unneeded positions empty or put jumpers.  The PCBs have an SMA footprint at each end and are narrow enough to slide into a short piece of 1/2 inch diameter copper pipe.  For the ends, I add copper pipe caps drilled out for the SMA connectors.  Bulkhead nuts on the SMA connectors can hold the caps in place.  I usually don't use the copper pipe unless I need the shielding or if I want the filter for long term use.  I've used this technique for a long time and use it for other things, too.  I keep some PI pad and T pad PCBs, too, for attenuators, DC blocks, and impedance matches.  I've even used it for RF amps -- usually with larger copper pipe, and feedthru caps to feed the power thru the end cap of the pipe.  I recently had a board made for use as a directional coupler to fit in a 1 inch pipe.

By the way, Tonne Software's free Elsie program is the filter designer I typically use.  I pick a topology, specify the filter, add in some limits for the various parameters, and let it optimize a filter design for me.  I can then quickly prototype a filter and check the results on a spectrum analyzer.  It almost always matches the expected results.  If anyone is interested in the PCB layouts, let me know and I can make them available.

[whalphen]

Attached are some PCB layouts (Kicad files and Gerbers.) Schematic PDFs are there, too.  I've shared them all on OSH Park in case you just want to get the boards. (Links are in the attached file.)  Because they're so small, they are very cheap on OSH Park.  (The filters are $2.55 for three PCBs, delivered.)  The filter modules use 0805 parts, but 0806 and 0603 parts can fit, too.  You may notice a couple of the boards are laid out linearly.  One is laid out with adjacent inductors orthogonal to minimize interaction.

I typically purchase the capacitors and inductors from DigiKey.  I use low cost 50V ceramic caps C0G/NP0 or X7R.  For the HF and VHF stuff I do I haven't found any noticeable difference with the higher cost RF caps.  For inductors I prefer wirewound, low series resistance.  I also keep a couple of full sets of cheap capacitors from AliExpress to use in case I run out or need an uncommon value.  I've had good experience with the ones from China, except that I sometimes find them mis-lableled.  For example 15pF caps labeled as 13pF.  I always measure the values before using them.

[jgalak]

A bit off topic, but since we are talking about this:  is there a good source (book, website, ebook) folks can recommend on filter design that covers very VHF/UHF?  Experimental Methods in RF design only goes to HF, but something at about that level for 2m and 70cm would be welcome reading.

[jgalak]

I'm getting errors from Kicad for a "wbhLibrary".  Is that perhaps your personal library?

[whalphen]

Yes.  That's a library with my custom symbols.  I think what's happening is that my custom library is still in the list of Component Library Files (under Preferences) that you downloaded.  I think you have three options to resolve that: 1) download the files again because I've corrected and replaced them, 2) go into Preferences and delete the reference to my library, or 3) just ignore the message.
 
Also, attached is another picture I shared on another thread.  Maybe it will give you some ideas.  The larger diameter one is a preamp I use for satellite comms.  The power is supplied thru the two feedthru caps.  As you can see, I sometimes put a male SMA connector on the module.  This is actually soldered on the outside of the cap with holes drilled for the center pin and the four PCB mounting pins. 

In almost all cases, I solder the SMA connector on one end to the cap and then solder it to the PCB.  Thus the PCB has an SMA connector and a cap attached to one end and only an SMA connector on the other end.  The cap on the opposite side is soldered to the pipe, but not the SMA connector. The pipe and its attached cap are slid over the PCB and held in place by the other cap on the far end and by a bulkhead nut on the SMA connector at the near end.  This way it can still be disassembled if needed.

Sometimes I coat the outside of the copper shell with clear spray paint to protect the copper.  And, as you can see in the photos, I sometimes use heatshrink tubing over the entire shell.  The pipe caps are called 'test caps' and are available from plumbing suppliers.  I usually get them from Zoro.com.  It's a bit tricky to hold the small cap while drilling.  I keep a 1" long piece of copper pipe for that.  I put the cap on the pipe and clamp both the cap and the pipe in a drill vise or a lathe chuck.  It helps a lot to use lubricant while drilling.

[Neomys Sapiens]

What (brands?) caps and inductors do you prefer? I would be interested in your observations on this.

I suppose that this question was not exclusively directed at whalphen, so:

C= Eurofarad(Excelia), CalRamic, American Technical Ceramics, Presidio Components, Novacap, Syfer, SRT Microceramique, DLI, Johansson, Cornell Dubilier
L= Microspire, Fastron.de, Renco, Piconics, AEM, Coilcraft, API Delevan, Venkel, Coiltronics, Johansson, NIC Components,
Other: Yantel (Temperature Compensation Attenuator, tunable SMD inductor), Anaren, State of the Art Inc

(also some Ind/Cap lines by AVX, Kemet, some branches of Vishay, but thats probably common knowledge)

Those elements shown by whalphen look like fine work. I wish I had a service for that!

[jgalak]

Hey whalphen, do you know the specific brand/model of the SMA edge connectors you used on the boards?  I can't find one that matches your pad dimensions, and I don't really know what's "close enough"...

[whalphen]

I usually buy them by the hundred from AliExpress. Here's one vendor I've used recently:
https://www.aliexpress.com/item/100pcs-lot-Gold-SMA-female-jack-Plug-Straight-Receptacle-solder-PCB-clip-edge-mount-RF-connector/32285453963.html?spm=a2g0s.9042311.0.0.mxrBlK

Also recently, this vendor sent me a hundred that got destroyed by the US post office equipment.  All I received was a mangled package with an apology form letter from the USPS.  (Must have been interesting to see 100 SMA connectors pinging around in a high speed mail sorting machine!)  The vendor sent me replacements at no additional cost.  https://www.aliexpress.com/item/Hot-Sale-100-Pcs-Gold-SMA-Female-Nut-Bulkhead-Solder-Deck-PCB-Clip-Edge-Mount-RF/32512250123.html?spm=a2g0s.issue_5ptha.0.0.p8rRER

For special sizes or lengths, I usually go to http://www.rfsupplier.com/, another Chinese company.

The pads on the PCBs are sized for the standard 0.25" wide connector.  Some of my PCB designs can also accommodate a wider one that I no longer use -- but even these work with the 0.25" connector.  You have to be careful to order the ones for 1.6 mm thick PCBs if that's what you want.  The ones for 1.2 mm PCBs are also common on AliExpress.  If you're not careful, you can end up with those.  I keep both.  When I order PCBs from China (usually PCBWay) I often order 1.2 mm boards to cut down the shipping weight.  (All my 1.2mm PCBs are masked in Blue so I don't get them mixed up with my 1.6mm green PCBs.)

The SMA connectors from China are a bit lower quality than US sourced connectors.  Dimensional tolerance is not quite as good.  But, for non-commercial use at HF and VHF, I find them to be completely adequate.  Of course you can get all these from major manufacturers via major distributors at higher quality, but the cost is several times higher.

I know some readers will be critical of Chinese sources.  But, over the past 3 years, I've placed nearly 500 orders for tens of thousands of parts from AliExpress vendors and have no complaints.  On rare occasions I've had a problem with a purchase, but the vendors nearly always were eager to resolve it.  In the few cases where I haven't gotten an adequate response from the vendor, AliExpress has always quickly refunded my money.  My experience with US vendors on eBay has been about the same rate of success as with Chinese vendors on AliExpress.  When I want high quality parts right away, I go to DigiKey (but then I pay a lot more.)

[jgalak]

Do you buy the bulkhead nuts separately? 

[Yansi]

Rfsupplier.com ... never came across that one. Very nice find. Already seen "Superbat" appearing a lot on Aliexpress.

Y.

[whalphen]

Do you buy the bulkhead nuts separately?

They usually come with the bulkhead connectors.  And since I don't always use the bulkhead connectors in bulkheads, I end up with extra nuts.  For the copper pipe caps, you can use the bulkhead connector with its supplied nut.  Usually, I install a regular connector on the end where the cap is soldered to the connector and I install a bulkhead connector on the end where the cap is held on by the nut.

[whalphen]

Rfsupplier.com ...

They are also a good source for enclosures.  They have a huge variety.

[jgalak]

Ok, so stupid question for whalphen.  I had some of your filter 1 boards made up and used elsie to design a chebyshev low pass filter.  It doesn't use every pad on the board, but fits nicely into the topology.  Problem is, it requres three of the pads to be shorted - L3, L5, L7. 

No matter how much I struggle, no matter how much flux and solder I use, I can't get a solder bridge to form across the two sides of these pads.  The solder mask is doing its job way to well...

So what do you use to short these out (if anything)?  I've thought of using a bit of wire, or a 0 Ohm resistor, but I'm not sure if they are going to introduce stray inductances or capacitances that'll mess up the circuit...

(for everyone else, these are 0805 SMD footprints that I'm trying to short out)

[KJDS]

Just use a short length of wire. That wire will have about 1nH of inductance. Add it to the circuit analysis and see what happens

[Yansi]

Iti is pointless to model a 1nH inductor as a wire jumper accross SMD 0603/0805 pads, unless you model the whole PCB with its all parasitics.

[KJDS]

One step at a time

[jgalak]

The software I'm using, Elsie, doesn't really let me model the extra inductance (afaik) - this jumper would not be in a place where the designed circuit has an inductor, so I can't just add an extra nH.  Elsie is more of a design tool than a modeling tool.

I suppose I could model the whole thing in LT Spice and see what happens.

What about using a 0 Ohm resistor, instead?  I have some of the right footprint as part of a resistor kit.  Would that introduce less stray inductance than a wire?

[Yansi]

O ohm resistor might have a bit less inductance than a plain piece of wire, due tonhe fact it is a flat thin film wire. But as i say, the difference ma be in %.

To simulate any kind of paasive rlc structure, get a copy of RFSIM99. It is a great good simple software for this task. Otherwise get any kind of suitable simulator, preferably one that does S-parameters as the rfsim. I have just stumbled upon the QUCS STUDIO. Also feee and very easy to learn and work with. Has plenty of extensive step by step tutorials.

Dont forget to add loss components when simulating the filter structure.  The real inductors definitely do not have infinite Q factor.

//sorr mistyped from my mobile phone.