Author Topic: Making low value air core inductors  (Read 2606 times)

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Offline jujun

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Making low value air core inductors
« on: September 17, 2018, 06:44:33 am »
Hello,

For the filter I am building, I need quite low inductor value (3nH) and high Q, for now with the help of this : https://hamwaves.com/antennas/inductance.html I found that I need only 1 turn ! Is it correct to do a low value air core inductor this way ?
Is there any practical tricks to build this?

The filter I am making is a high pass for 1,6 Ghz.

Thank you

J
« Last Edit: September 17, 2018, 07:51:54 am by jujun »
 

Offline mmagin

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Re: Making low value air core inductors
« Reply #1 on: September 17, 2018, 08:35:15 am »
I think that it is more popular to use microstrip techniques today, but I think that wouldn't be a strange sight in UHF stuff from 50 years ago.

Considering you probably have to, at a minimum, design a PC board to build stuff at >1.6 GHz (though I think you'd still do okay with ordinary FR4 materials unless your needs are exotic / high power), I wonder if it would be preferable to realize the whole filter with microstrip techniques?
 

Offline coppercone2

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Re: Making low value air core inductors
« Reply #2 on: September 17, 2018, 09:56:35 am »
did you try varying wire diameter ?

if you don't like 1 turn, I managed to make a 6 turn using 36awg wire and a 0.25mm core :-DD

I can see it being made under microscope very carefully if you have a ceramic core to wind it carefully.. just paint the core with a very thin layer of wax before you wrap and heat it a bit to melt the thin layer of wax and it might work. Q is like, 14,

but there is kind of a trick, if you want broadband performance you wind it in a cone/bugle shape.
http://www.piconics.com/conical-inductors/

3nH is really really low though. I wonder if you can get a really good signal inductor by winding a tiny tiny cone out of tiny tiny wire. If I had my HFSS with me right now I would try to simulate it maybe, its super interesting.
40ghz performance.

They also make them really small too:
http://year2000.manufacturer.globalsources.com/si/6008848736032/pdtl/Choke-coil/1156304522/Choke-Coils.htm

Maybe you can outdo PCB material this way.

there is also this
https://www.digikey.com/products/en/inductors-coils-chokes/fixed-inductors/71?k=inductor&k=&pkeyword=inductor&pv2087=u0.1nH&pv2087=u0.2nH&pv2087=u0.3nH&pv2087=u0.33nH&pv2087=u0.39nH&pv2087=u0.4nH&pv2087=u0.47nH&pv2087=u0.5nH&pv2087=u0.56nH&pv2087=u0.6nH&pv2087=u0.68nH&pv2087=u0.7nH&pv2087=u0.8nH&pv2087=u0.82nH&pv2087=u0.9nH&pv2087=u1nH&pv2087=u1.1nH&pv2087=u1.2nH&pv2087=u1.3nH&pv2087=u1.4nH&pv2087=u1.5nH&pv2087=u1.6nH&pv2087=u1.65nH&pv2087=u1.7nH&pv2087=u1.8nH&pv2087=u1.9nH&pv2087=u2nH&pv2087=u2.1nH&pv2087=u2.2nH&pv2087=u2.3nH&pv2087=u2.4nH&pv2087=u2.5nH&pv2087=u2.55nH&pv2087=u2.6nH&pv2087=u2.7nH&pv2087=u2.8nH&pv2087=u2.9nH&pv2087=u3nH&pv2087=u3.1nH&pv2087=u3.2nH&pv2087=u3.3nH&pv2087=u3.32nH&pv2087=u3.4nH&pv2087=u3.5nH&pv2087=u3.6nH&pv2087=u3.7nH&pv2087=u3.8nH&pv2087=u3.85nH&pv2087=u4nH&pv2087=u4.1nH&pv2087=u4.2nH&pv2087=u4.3nH&pv2087=u4.4nH&pv2087=u4.5nH&pv2087=u4.55nH&pv2087=u4.6nH&pv2087=u4.7nH&pv2087=u4.8nH&pv2087=u4.9nH&pv2087=u5nH&sf=0&FV=ffe00047&quantity=&ColumnSort=0&page=1&pageSize=25

They looks like their made with a few turns in an acrylic resin, their like 3x3x3mm
https://www.digikey.com/product-detail/en/wurth-electronics-inc/744913050/732-7280-6-ND/5353422

#1

Inductance    5nH    
Tolerance    ±5%    
Current Rating    4A    
Current - Saturation    -    
Shielding    Unshielded    
DC Resistance (DCR)    1.8 mOhm Max    
Q @ Freq    140 @ 150MHz    
Frequency - Self Resonant    6.5GHz



#2
Inductance    3.85nH    
Tolerance    ±5%    
Current Rating    1.6A    
Current - Saturation    -    
Shielding    Unshielded    
DC Resistance (DCR)    6 mOhm Max    
Q @ Freq    100 @ 800MHz    
Frequency - Self Resonant    7.5GHz
« Last Edit: September 17, 2018, 10:27:10 am by coppercone2 »
 

Offline T3sl4co1l

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Re: Making low value air core inductors
« Reply #3 on: September 17, 2018, 04:23:27 pm »
3nH is the stray inductance of a single 1206 chip component.

I think you've asked about this before, and it was noted that absurd values like this indicate a change in strategy is required.

Tim
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Need engineering assistance? Drop me a message!
 

Offline coppercone2

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Re: Making low value air core inductors
« Reply #4 on: September 18, 2018, 12:00:42 am »
what is your opinion of the inductors of 3nH that they have for sale?

I am actually curious in comparison to stripline and such

my simulations showed that most of the stripline filters have modeing that occurs at 2x the design frequency so you for sure need to follow them up with subsequent filters if you want a response similar to that of low frequency filter circuits (no one expects their filter to have a fit at 2x its design frequency, which is what I see with microwave stuff, same with waveguide/cavity filters, you need to find the bode plot to be sure of anything, the ones sold by pasternack seem to account for it).

IIRC the hairpin was kinda OK but compared to the bode plot i got from simulations to something like one of those modules from pasternack the response looked like dog shit. I mean the s21
« Last Edit: September 18, 2018, 12:22:07 am by coppercone2 »
 

Online Wolfgang

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Re: Making low value air core inductors
« Reply #5 on: September 18, 2018, 01:54:04 am »
I may be old fashioned, but I am very sceptic about discrete components with values in the same order of magnitude as their parasitic inductances and capacitances.
If you can, try to make a stripline design with tunable elements.
 

Offline coppercone2

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Re: Making low value air core inductors
« Reply #6 on: September 18, 2018, 03:41:48 am »
any bored people  wanna look at those parts and measure some filters?
 

Offline jujun

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Re: Making low value air core inductors
« Reply #7 on: September 18, 2018, 07:10:10 am »
So I explain a little bit why this question, and what I did.

I really wanted to be able to record the data of a NOAA weather satellite on 1.7Ghz, the pass was in a few hours, so I needed a working filter in a few hours. So I choose the lumped components way instead of the microstrip or cavity to be able to have something very quick.

Because now I understand what cause losses in filters (thank you all :) ), I choose a 7th order high pass. I found that I needed 1pF and 1,5pF luckily I had them in my SMD C0G capacitor bag. Then I used the kicad calculation tool to have the size of the track at 50ohm. Then with my swiss army knife I made the pcb on scrap dual side FR4. Then for the coil I first tried with a longer coil, thinking that I could space the winding for tunning the filter. I ended to build several smaller coils. Each time I was able to see the filter band pass going higher in frequency. At the end I only had a little pice of wire of maybe 4mm. And the match was good, maybe -25dB RL, and in the band pass (of interest) the losses are only 0.8dB to 1,5dB.
For a quick little filter it's good :)
But I managed to miss the satellite pass because of the power supply of the LNA ...

« Last Edit: September 18, 2018, 08:31:45 am by jujun »
 

Offline coppercone2

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Re: Making low value air core inductors
« Reply #8 on: September 18, 2018, 09:11:45 am »
 :'(

good ' battle engineering' story though :popcorn:
 

Online Gribo

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Re: Making low value air core inductors
« Reply #9 on: September 19, 2018, 05:44:20 am »
There are wire wound air core inductors at these values, but expect them to be in 0402 or smaller packages. The pads parasitic capacitance will ruin your day at these frequencies. 0402 pads have about 1pF of parasitic capacitance each.
 

Offline coppercone2

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Re: Making low value air core inductors
« Reply #10 on: September 19, 2018, 06:32:14 am »
with the inductors I linked above, are you supposed to budget with them or just use air construction?
 

Offline G0HZU

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Re: Making low value air core inductors
« Reply #11 on: September 19, 2018, 07:32:49 am »
It's only 1.7GHz! There's nothing 'absurd' about using (0603 or 0402 SMD) inductors of just a few nH up at 1.7GHz. For many years now the manufacturers have been releasing wideband s2p models of their SMD inductors and it's possible to simulate a PCB based design like this using an EM simulator such as Sonnet.

On a typical design you would probably have to select a 2.7nH SMD shunt inductor (in the place of a theoretical 3.3nH) to allow for the ground via hole inductances but a typical design would use multiple ground vias per shunt inductor and a very thin PCB substrate. To save frustration and time it really is best to simulate the PCB layout and use accurate models for the inductors. The best SMD lumped component models come from companies like Modelithics but you have to pay for these. For DIY, even a simple tiny loop of skinny wire would be OK for a 3nH inductor in a HPF design at 1.7GHz.

A lot depends on how high something like a 1.6GHz HPF needs to work in terms of passband response. It should be possible to get good passband performance to about 10GHz or so but at some point the response will degrade due to transmission line effects in the components etc. But Jujun only needed operation to 1.7GHz :)

However, I think it would have been worth trying for a 1.7GHz bandpass filter rather than a 1.6GHz highpass. The BPF could have been designed as a lumped design using SMD LC parts or it could have used microstrip for some of the design.  A reasonable target for a 3rd order lumped BPF would be 1dB insertion loss at 1.7GHz and maybe 200MHz bandwidth. It wouldn't give the steep LF rolloff of the 7th order HPF but it would give some useful rejection of 2.4GHz wifi etc.
« Last Edit: September 19, 2018, 08:10:15 am by G0HZU »
 

Offline jujun

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Re: Making low value air core inductors
« Reply #12 on: September 19, 2018, 06:43:45 pm »
However, I think it would have been worth trying for a 1.7GHz bandpass filter rather than a 1.6GHz highpass. The BPF could have been designed as a lumped design using SMD LC parts or it could have used microstrip for some of the design.  A reasonable target for a 3rd order lumped BPF would be 1dB insertion loss at 1.7GHz and maybe 200MHz bandwidth. It wouldn't give the steep LF rolloff of the 7th order HPF but it would give some useful rejection of 2.4GHz wifi etc.

Yes, I must definitely try to build a band pass. It would be a lot cleaner.
I choose an HPF also because there is a lot of strong signals that I need to filter that are under 1.7Ghz, typically FM broadcast and GSM, TV. Above 1.7Ghz there is less strong signals. (but still 3G)
Maybe the best would be to build a BPF with a stub to block the only one very strong FM broadcast signal I have.
 

Offline G0HZU

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Re: Making low value air core inductors
« Reply #13 on: September 20, 2018, 05:02:31 am »
There's lots of ways to design a fairly narrow BPF up at 1.7GHz but you need to be careful how you build and test it because it's easy to get confusing or false results due to a poor choice of filter topology, or poor component choice or poor layout or poor test gear. If you want to make a lumped BPF I'd recommend you do it on a conventional (thin) PCB with microstrip lines leading into the filter and use good quality SMD caps (suitable for use at 1.7GHz) and you can either wind the inductors yourself or use SMD. But SMD inductors won't be adjustable. The nice thing about doing it on a PCB is that the SMD caps will be easier to fit and so will the inductors. But the effect of the PCB tracks and pads will need to be modelled on a decent simulator like Sonnet. Otherwise, you would have to be prepared to do a fair bit of 'suck it and see' in terms of choosing the correct component values that offset the strays in the PCB layout. Any sloppy practices or poor component choices will result in poor results and frustration and anything you build will need to have a tight layout if it is to work well.

If the circuit and PCB layout is simulated properly beforehand it should work first time or at least be very close. Ignore all the comments above that say it isn't possible to use SMD inductors (of just a few nH) to make lumped filters up at 1.7GHz. These comments are from 'experts' who either have no practical filter design experience at 1.7GHz or they tried and failed and assumed their failure meant that everyone else must therefore fail too  ;D

« Last Edit: September 20, 2018, 05:09:40 am by G0HZU »
 

Offline KJDS

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Re: Making low value air core inductors
« Reply #14 on: September 21, 2018, 05:22:08 am »

If the circuit and PCB layout is simulated properly beforehand it should work first time or at least be very close. Ignore all the comments above that say it isn't possible to use SMD inductors (of just a few nH) to make lumped filters up at 1.7GHz. These comments are from 'experts' who either have no practical filter design experience at 1.7GHz or they tried and failed and assumed their failure meant that everyone else must therefore fail too  ;D

I would prefer to do it as an interdigital, possibly on suspended substrate and would be confident that it would work first time with excellent performance, but given the timescales I know I could get a lumped bandpass filter to work first time.

Offline G0HZU

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Re: Making low value air core inductors
« Reply #15 on: September 22, 2018, 04:20:20 am »
Suspended stripline is a bit exotic  :)

But yes, it could also be done as an interdigital filter in microstrip although this would be a fair bit bigger than a lumped BPF. At work, I would traditionally design a 1700MHz BPF as a combline filter to get the size down.

Getting back to the original 1600MHz HPF I did a quick simulation in Genesys and Sonnet this afternoon using basic models for the caps and the inductors. I also managed to find an old dev board that I hacksawed to just contain the artwork for this HPF. This is a Rogers 4003C PCB 0.02" thick. See the simulation below and then I built it on the dev board. I used Kemet 0603 Hi Q capacitors rather than waste ATC caps but it still gave good agreement with the simulation for S21 and S11 up to about 5GHz or so. I made the inductors by hand as a tiny loop of wire.

The simulation is run in Genesys but it exports the layout to the Sonnet EM simulator to simulate the effect of the PCB layout. Hopefully you can see the VNA display of the real thing is quite similar to the simulation. The real filter has some cheapo SMA PCB end launchers and the loss and mismatch of these will degrade the result a bit up by 8GHz. But the loss is still only about 0.6dB by 8GHz. The VNA was calibrated with a 2 port ecal module so this should be a decent measurement in terms of overall uncertainty.
« Last Edit: September 22, 2018, 04:30:57 am by G0HZU »
 
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Offline jujun

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Re: Making low value air core inductors
« Reply #16 on: September 22, 2018, 06:20:13 am »
It's a very clean and professional work !
Can we see a picture of the filter ?

I really need to find some SMD hi Q capacitors and some roger substrate.

Because the filter I build was on FR4 from aliexpress and SMD capacitors from aliexpress too ... so not very good quality.

Any clues on how and where to find high Q capacitors and roger substrate ? My guess is that there is no really cheap way to get them.

I need to improve my knowledge in Genesis, because I never generated the layout with it.
« Last Edit: September 22, 2018, 06:45:20 am by jujun »
 

Offline G0HZU

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Re: Making low value air core inductors
« Reply #17 on: September 22, 2018, 06:39:11 am »
See below for a picture. It's a bit scruffy because it is a crudely hacksawed section from a larger PCB. I had to bodge the connections for one of the SMA endlauncher connectors as you can see in the picture. Also, this PCB wasn't really designed for UHF so the 0603 parts aren't fitted as snugly together as I would like. But it was good enough. I didn't try and make it look neat so the soldering could be better.

But if something bodged together like this can work OK then a tighter PCB layout with better quality caps should be slightly better.
 

Offline G0HZU

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Re: Making low value air core inductors
« Reply #18 on: September 22, 2018, 06:46:41 am »
Quote
Any clues on how and where to find high Q capacitors?

I used caps from this Kemet dev kit from Farnell.

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

Farnell stock number 2456895. I think it was about £50 for the kit. Not cheap but not exactly expensive either.

These caps aren't as good as ATC but they are going to be better than jellybean ceramic 0603 caps and you get lots of small values, eg 0.3pF to 1pF in 0.1pF steps and then the steps get a bit larger. But you still get lots of values up to 47pF.

Correction: Actually, I think the two outer 2.2pF caps are ATC 600S caps ( I forgot that I had loads of these to hand) but the centre one is definitely a 1pF Kemet cap from the kit.
« Last Edit: September 22, 2018, 06:51:00 am by G0HZU »
 

Offline jujun

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Re: Making low value air core inductors
« Reply #19 on: September 22, 2018, 06:47:24 am »
I dont understand why your layout (in simulation) dont have a bigger ground plane on the top.

Yours coils look like mine !
 

Offline G0HZU

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Re: Making low value air core inductors
« Reply #20 on: September 22, 2018, 06:54:23 am »
Quote
I dont understand why your layout (in simulation) dont have a bigger ground plane on the top.
Because I'm lazy and I did it in a hurry :) I didn't think the extra ground artwork on the top was relevant so I didn't bother with this. I missed out a lot of via stitching as well but I don't think it matters much in this case. I just did a quick and dirty Sonnet simulation of what I thought was relevant. It simulates much faster like this.
« Last Edit: September 22, 2018, 06:56:00 am by G0HZU »
 

Offline jujun

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Re: Making low value air core inductors
« Reply #21 on: September 22, 2018, 07:00:25 am »
Is it a good idea to buy for for example this https://www.ebay.fr/itm/100-pcs-CERA-TRIM-Johanson-M-C-0-6-2-5pF-250V-2320-RF-Trimmer-Capacitor/253027684130 instead of a rf smd capacitors kit  ?

The main problem is the footprint that is different, so I need to take it in account in the design and simulation, and I cannot use standard SMD capacitor on the board instead of a trimmer.
 

Offline jujun

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Re: Making low value air core inductors
« Reply #22 on: September 22, 2018, 07:09:05 am »
Also, I see that you don't add the SMA edge connector in the layout, and you don't simulate it too.
So I wonder if it's because it's rated to be 50ohm so you count it as not really part of the circuit,
or because you didn't took the time to add it ?

I always wonder how the layout under an SMA edge connector should be to have a really good match,
and I feel that the way the track enter the connector is very important to have a good match.
 

Offline G0HZU

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Re: Making low value air core inductors
« Reply #23 on: September 22, 2018, 07:15:09 am »
Quote
Yours coils look like mine !
Yes, just a bit of wire. I did use 0603 SMD inductors at first but I only had 2.2nH parts here and I had to fudge the cap values to keep the cutoff near 1600MHz. But it still worked OK. But I thought it was more relevant to the thread to show it working with handwound coils.

Quote
Is it a good idea to buy for for example this https://www.ebay.fr/itm/100-pcs-CERA-TRIM-Johanson-M-C-0-6-2-5pF-250V-2320-RF-Trimmer-Capacitor/253027684130 instead of a rf smd capacitors kit  ?
At work, trimmer caps aren't allowed for various reasons so I don't use them much here at home either. But I suppose you could try them as the price looks cheap, but I'd expect to see issues at these frequencies with strays etc so I can't really recommend them as an alternative. I'd really rather have the Kemet kit of fixed parts.... or buy both?

Quote
Also, I see that you don't add the SMA edge connector in the layout, and you don't simulate it too.
Yes, I just simulated the filter section and I did expect to see some extra loss up at 8GHz because of my fudgy SMA to PCB interface. But I think it is OK for a demo :)


 

Offline jujun

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Re: Making low value air core inductors
« Reply #24 on: September 23, 2018, 12:25:23 am »
Is it true to say that the lumped componement aproch for this frequency is only valid for RX and TX at low power ?
For example If I wanted to be able to use this filter on a 10W TX, capacitor that could handle this tension would be very big, and because of theire size it would act as an inductor.
So is it true that for TX, the microstrip or the interdigital would be a lot better ?

What is the maximum allowed power for a filter with small SMD capacitor like this ?
 


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