Author Topic: Making low value air core inductors  (Read 8852 times)

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Offline Wolfgang

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Re: Making low value air core inductors
« Reply #50 on: September 24, 2018, 09:35:11 pm »
Sorry, I wanted to be helpful in avoiding errors or a nonfunctional design.
I would propose to:
- use a quality substrate (Rogers)
- transmitter grade caps (ATCs are fine, but I would choose larger sizes so ratings are not a problem. 1206 should be fine for 10W)
- air core coils instead of SMD parts where power is at the limit. This also gives some tuning possibilities.
- Use a small box with a top lid ferrite absorber.
- simulate everything with all frequencies, power leves and loads involved.
That should do  :)
 

Offline G0HZU

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Re: Making low value air core inductors
« Reply #51 on: September 24, 2018, 10:02:09 pm »
OK but ATC600S caps with a small value of a few pF have a typical ESR of 0.05-0.08R up at 0.5-1GHz with low package inductance. At 1.7GHz it is 0.1R ESR.  Put three in parallel and the equivalent goes even lower. The ATC 600S (0603) package rating for voltage at DC is 250V but ATC briefly test them at 2.5x this voltage to prove them.

https://www.atceramics.com/UserFiles/600s.pdf

To imply these 0603 caps are only suitable for receive filtering or low power Tx is not right. They can be used in typical LPFs at 10W even with a relatively poor load mismatch especially if the PA has a sensible source impedance. The old dev test amp I used was a balanced GaN PA between two IPP hybrids.

« Last Edit: September 24, 2018, 10:14:49 pm by G0HZU »
 
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Offline G0HZU

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Re: Making low value air core inductors
« Reply #52 on: September 24, 2018, 11:27:05 pm »
Minicircuits do a HPF in 1206 that is similar to the original 1600MHz filter requirement.

https://ww2.minicircuits.com/pdfs/HFCN-1320+.pdf

If you click on the data here

https://ww2.minicircuits.com/pages/s-params/HFCN-1320+_VIEW.pdf

You can see that their filter gets quite lossy by 8GHz. It even has 2dB loss at about 5.5GHz! It has 5dB loss by 8GHz. But it is OK at 1700MHz. They rate it at 7W max despite the tiny 1206 package but if it was used at 1700MHz at 5-7W I'd expect it to get quite hot. There are several caps and inductors crammed into that package and I think the inductors will generate most of the heat when used at 1700MHz especially into a poor load. If you look back at my homebrew HPF using decent low loss caps and hand wound inductors the loss across 5-8GHz was less than 1dB.

The Minicircuits 1206 HPFs are quite cheap at a couple of dollars and no inductors to find or wind! These are similar to the LPF and HPF filters seen in various RF analysers in youtube teardowns.
« Last Edit: September 24, 2018, 11:30:16 pm by G0HZU »
 
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Offline G0HZU

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Re: Making low value air core inductors
« Reply #53 on: September 25, 2018, 12:01:59 am »
As promised, here's the response of the 1700MHz lumped BPF. This was built in a hurry using hand wound inductors and it is built the same as the earlier simulation in terms of the component values.

However, I used the Kemet CBR caps and not ATC caps and I don't know how accurate they are. The response looks to be very close but if you look closely the return loss is not as good on the real filter and the passband is slightly wider and less rounded. This is because the shunt cap values aren't quite right and probably need to be tweaked a tiny fraction of a pF. But this is good enough I think and the insertion loss is about 1.0dB at 1700MHz. It would improve slightly with tweaked shunt caps but I'm too lazy to bother. I've made loads of filters like this over the years so I'm not so keen to spend too much time and use up any more of my Kemet caps on it. I think it's good enough already :)

See below for a copy of the Genesys simulation and a slightly fuzzy image of the filter being tested on the VNA. In the lower left corner you can see part of the big old wideband 50W GaN test amp I've borrowed.
« Last Edit: September 25, 2018, 12:14:57 am by G0HZU »
 
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Online coppercone2

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Re: Making low value air core inductors
« Reply #54 on: September 25, 2018, 12:14:35 am »
does anyone think the idea of a rice sized heatsink for a SMD filter is adorable?

Or would it detune and require a ceramic heat sink?
 

Offline G0HZU

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Re: Making low value air core inductors
« Reply #55 on: September 25, 2018, 01:37:31 am »
Having a heatspreader under the PCB is a good way to get heat away. On the exotic Rogers + FR4 multilayer boards the best way to do this would be to have the PCB manufactured with a pocket or cutout area within the 12(?) layer board that is only 2 layer. i.e. the 0.02" width of the top Rogers layer would be under the parts that get hot. This pocket could be under a filter or a TR switch PIN diode for example. Then make sure this sits against an aluminium heatsink and this can be part of a milled chassis. This would get the heat away really well and would prevent the local PCB area from gradually creeping up in temperature over time like a pedestal. But this PCB will be very expensive!





« Last Edit: September 25, 2018, 01:49:16 am by G0HZU »
 

Offline G0HZU

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Re: Making low value air core inductors
« Reply #56 on: September 25, 2018, 10:57:15 pm »
If you want to see a typical heatspreader then look again at my middle image in post reply#53 that shows the VNA plot of the lumped filter. In the lower left corner is the big old prototype GaN amplifier. This is a gold coloured PCB about 22cm x 18cm and I think the PCB is either Rogers 4003C or 4350 and it is only 0.02" or 0.5mm thick. This is very fragile and would snap in half like a cracker if it was flexed because the material is brittle and because of all the via holes in the PCB. If you look you can see it is screwed to a 10mm thick slab of aluminium and this provides support and it also provides a great way to suck heat away from the PCB. The aluminium then sits on another heatsink below this and this can be force air cooled. This really does keep the PCB components much cooler.

I also had a quick go at designing an interdigital BPF at 1700MHz using microstrip. I used a slightly thicker PCB material but it was Rogers 4003C again and you can see the result after a Sonnet simulation. I think the freebie Sonnet Lite program could cope with this filter as it only needed 29Mb of memory. I think the free version of Sonnet allows up to 32Mb.

You can see the insertion loss of the interdigital filter is about 1.6dB at 1700MHz even though the PCB material is Rogers 4003C. I've designed loads of filters like this but this was many years ago now. However, the simulation plot below should be representative of what you might get with such a filter. The simulation includes an enclosure with a metal lid over the PCB and this does affect the response slightly.

To give an idea of size, the vertical microstrip fingers in the filter are each about 28-29mm long.
« Last Edit: September 25, 2018, 11:45:37 pm by G0HZU »
 
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Offline yl3akb

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Re: Making low value air core inductors
« Reply #57 on: September 26, 2018, 06:28:56 am »
G0HZU, great stuff!

What is the procedure You use to design such microstrip filters? Do You start with some initial model based design (do You use any available calculators for that?) and then move to EM simulation? Do You use any built-in optimization tools or just tune it manually?

Also, what is there at 1.7 GHz band for which this filter is needed? As far as I know, closest stuff is GSM at ~1.8 GHz and GPS/GLONASS at ~1.6 GHz
 

Offline yl3akb

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Re: Making low value air core inductors
« Reply #58 on: September 26, 2018, 06:35:49 am »
Sorry, I found the answer to last part of the equation at start of this thread ;)
 

Offline G0HZU

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Re: Making low value air core inductors
« Reply #59 on: September 26, 2018, 10:36:49 pm »
To design that interdigital filter in Genesys is a fairly clunky process but here is how it goes...

The design equations used by Genesys for the interdigital filter are fairly basic and are based on a few classic equations and these are usually given in the literature that gets bundled with each version of the software. I don't think this part of the software has changed in 25 years. These equations give the microstrip lengths, widths and spacing and the tap point based on user inputs for start and stop frequencies for the passband and the design ripple and impedance of the resonant sections and the specs of the PCB material. The software automatically generates the complete circuit as a linear model based on the equation results.

However, the equations are crude and the next step in the process is to simulate the design using Genesys' own enhanced linear models for coupled microstrip lines and for the via holes and end effects etc. This takes the accuracy to a much higher level.

This quickly shows that the basic design equations are inadequate on their own because the simulated response will usually be off frequency and will also be ripply with a poor match. So the next thing Genesys does is to automatically assign some optimisation goals to try and drag the response back to where it should be. The user can edit the optimisation goals if required (really easy to do) but usually they are OK. The optimisation is automatic and takes maybe 10 seconds to fiddle about and optimise the response.

This gives a tweaked version of the original design and it is often quite close if then built as a real PCB. But usually there will still be subtle issues with bandwidth, match and stopband performance in the real PCB version when built. So the next step is to set up the PCB layout such that it can be exported to the Sonnet EM simulator. This is mainly a manual (fairly tedious) process for the user and takes a few minutes to set up the filter and the ports in a 3D enclosure suitable for Sonnet.

Then the design is exported to Sonnet and it gets analysed. This is a very simple circuit for Sonnet and it takes a minute or so to simulate depending on the accuracy required. Usually Sonnet will reveal an issue with slight mismatch and it will give more realistic stopband performance due to the PCB layout and the enclosure. The results are automatically sent back to Genesys and presented to the user in Genesys and the end result you see on the screen is the result of me tweaking the layout (takes a bit of experience to do this quickly) to optimise the match and response such that it looks good after the final simulation in Sonnet :)



« Last Edit: September 26, 2018, 10:43:30 pm by G0HZU »
 
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Offline G0HZU

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Re: Making low value air core inductors
« Reply #60 on: September 26, 2018, 11:18:07 pm »
The final stage is to then to use the inbuilt PCB layout editor such that it can have extra pads and vias and connectors (and the artwork for a screen/lid)  as you would get in a real test PCB. This can be re simulated by Sonnet if required and the final PCB layout is then exported as a Gerber or DXF file. This can be imported into the SW that controls a PCB mill and the PCB can be milled and drilled and routed in about half an hour. Here at home I have an old T-Tech 7000S PCB milling machine that can do this for me. It isn't as fast as a modern LPKF machine but it can mill the PCB really accurately and it's possible to be testing a real PCB on a VNA within half an hour of finishing the design on the computer :)
« Last Edit: September 26, 2018, 11:22:27 pm by G0HZU »
 
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Offline yl3akb

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Re: Making low value air core inductors
« Reply #61 on: September 27, 2018, 06:39:14 am »
Thanks for the insight of Your procedure.
I myself have little experience with AWR microwave office and process is quite similar. First, built in tool (called iFilter) generates perfect model based circuit, then it can be exported to AWR and simulated using AWR models (it takes in to account cross-coupling, vias, etc). I usually optimize the filter at this stage with built in optimizer (usually for particular return loss). Then filter can be simulated and optimized with one of AWRs EM simulators, like Axiem. I never tried more than ~2 GHz, but usually I need to trim resonators of actual filter, because pas band usually turns out little low, even with all the EM sim. which includes vias and enclosure.

You mentioned milling - from experience with such process, milling takes off  a little bit of  material also, not just copper. Does it affects the results of filter, knowing that coupled edge region is very critical for these kind of filters ?

 

Offline G0HZU

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Re: Making low value air core inductors
« Reply #62 on: September 27, 2018, 05:55:03 pm »
Quote
You mentioned milling - from experience with such process, milling takes off  a little bit of  material also, not just copper. Does it affects the results of filter, knowing that coupled edge region is very critical for these kind of filters ?

There isn't a definite answer to this but a lot depends on the skill of the operator and the quality of the tools and also on the filter design itself. At work, our LPKF machine is only used by one experienced operator and he is really careful and he has access to new tools all the time for critical work. So he can usually mill a filter like this using a fresh/accurate end mill tool and he can set the depth so it only just skims the copper away with a straight vertical edge. So at work we usually get very good results when we transfer the same gerber to an etched PCB from Labtech or Graphic or Exception PCB.

Here at home I'm a bit less critical and I would probably use a narrow V cutter and a worn end mill to make a filter like this. Otherwise I end up using and wearing too many expensive end mill tools. Once the end mill tools start to get some advanced wear they tend to give a ragged edge and this is worse than using the V tool. Most of my tools are part worn freebies from work. So in my case my results don't look as neat but I'm usually pleased with the RF response. I have sometimes really messed up the depth and it doesn't seem to make a significant difference but maybe it depends on the filter type and specs. I tend to design fairly narrow BW filters so the spacing is wider and I think the milling isn't as critical. At work we sometimes use end mills as narrow as 0.005" for filters with very narrow gaps between fingers. But usually it's 0.015" for stuff like this.
« Last Edit: September 27, 2018, 06:09:20 pm by G0HZU »
 
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Offline G0HZU

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Re: Making low value air core inductors
« Reply #63 on: September 27, 2018, 06:08:28 pm »
To show a real world example where some people really do use 0603 caps at fairly high RF power levels, I had a rummage through some old GaN amplifier app notes at work and found this 30W PA from Cree. This uses a single GaN device and you can see in the output network below that they used 0603 caps in the output matching/filtering. Each capacitance is effectively two 0603 caps in parallel. You can see they are 0603 when compared to the size of the SMA connector. These will be ATC 600S caps and this amplifier is a fairly narrow design up at 1.4GHz. You can also see that the skinny PCB is mounted on an aluminium spreader/heatsink and this helps suck heat away from this filter network.  Obviously, this is an evaluation circuit aimed at demonstrating the capabilities of the GaN device rather than a demonstration of the robustness of ATC 600S 0603 caps in a 30W amp. But it does show that it isn't 'unthinkable' to use these tiny capacitors at 10W in a transmit filter network :)
« Last Edit: September 27, 2018, 06:21:15 pm by G0HZU »
 

Offline G0HZU

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Re: Making low value air core inductors
« Reply #64 on: September 27, 2018, 06:33:09 pm »
Another supplier of HiQ ceramic caps is PPI

http://passiveplus.com/

You can get some idea of sizes and prices here:

http://passiveplus.com/kits.php#0603N - (.060 x .030) Ultra-Low ESR

We have various demo kits of these PPI caps at work in large and small packages but I don't think anyone has used them (except me).

Note that the Kemet HiQ-CBR 0603 kit I showed earlier has confusing data for price/qty at Farnell. My kit cost about £50 and it has qty 50 caps on tape for each of 36 values across 0.3pF to 47pF. So that is a total qty of 1800 HiQ 0603 caps for £50. They won't be as good as ATC 600S caps but they are pretty good value.

See below for what the Kemet kit looks like inside. You get two (and a bit) pages of 0603 caps all on tape and clearly marked. It's easy to use but not so easy to return caps to the kit that have been used or have fell out of the tape (when you got two but only wanted one!)

https://uk.farnell.com/kemet/cer-eng-kit-34/rf-microwave-capacitor-kit-0603/dp/2456895?krypto=nRv4lv5p7oYuI8syLvS3aGyGpmHYylT15G8zId6kXkBgSMcxMJojjX7pVrkdcJoNsUtY14BJf%2BbOEW2fEKs7sQ%3D%3D&ddkey=https%3Aen-GB%2FElement14_United_Kingdom%2Fsearch
« Last Edit: September 27, 2018, 06:44:59 pm by G0HZU »
 

Offline yl3akb

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Re: Making low value air core inductors
« Reply #65 on: September 28, 2018, 06:27:59 am »
I would also recommend Johanson S-series kits:
https://uk.farnell.com/w/c/passive-components/capacitors/capacitor-kits-assortments/prl/results?brand=johanson-dielectrics|johanson-technology&sort=P_PRICE

They also offer capacitor modeling/database software, which allows to export S-parameters of each capacitor:
https://www.johansontechnology.com/software
 

Offline jujunTopic starter

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Re: Making low value air core inductors
« Reply #66 on: September 28, 2018, 10:20:55 am »
I think it's important to have the simulation model of all the capacitors of the kit.
For the Kemet ones, there is a paying option, and also this : http://ksim.kemet.com/

Also about SMD Coils, I noticed that coilcraft do a very cool feature in the design kit : there is a free refill !

Now I need to learn a design flow to be able to simulate layout with real models, and then I will buy a kit.

Thank you all for your precious help :)
 


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