Making low value air core inductors

[Wolfgang]

Only sick people even think of making a transmit filter from 0603 components.
Reasons are:
- in a filter the voltages across components can be several times larger than the applied voltage due to resonances
  This is especially true in case of mismatch conditions. Ultra small SMD cases just can stand a few ten volts maximum, often a lot less.
- The loss tangent of small lumped SMD parts is poor. This means that a significant part of the input power is converted to heat,
  and the dissipation properties and heat conductivity of small parts is also very small.
- At 1.6GHZ, also substrate losses can become significant. This lowers Q and also generates heat.

In order to play safe:
- Simulate your filter with the maximum power level you have and all output terminations including, opens, shorts and other mismtaches.
- Find the maximum voltage levels for all caps. If your SMD stuff is overloaded, you need RF-grade caps with voltage margins. They will
  probably be larger than 0603, howver.
- Use a quality dielectric instead of FR4

[coppercone2]

when it comes to power you would need to have a reason for the electrical properties to change, such as thermal drift or some kind of saturation parameter. It won't have more inductance for no reason. They are C0G and stable.

[Wolfgang]

one of the obvious reasons is maximum voltage. Stability does not help when a cap just burns out due to arcing.
Have a look at antenna tuners or filter banks if you want to see burnt caps, and those are a lot larger than SMD C0Gs.
Simulation is key !!

[ogden]

Only sick people even think of making a transmit filter from 0603 components.

What you say about TI engineers? Sick? PCB picture, 2.4GHz TRX matching network:

http://www.ti.com/lit/ml/swru528/swru528.pdf

[T3sl4co1l]

That's a considerable amount less than the 10W mentioned above.

Tim

[ogden]

That's a considerable amount less than the 10W mentioned above.

I was kind of looking for power mentioned but managed to miss it. Thank you for pointing out.

10W is way too much for 0603 size components. Agreed.

[edit] High altitude, thus low pressure can make things even worse.

[Wolfgang]

At 10W your 0603 SMD stuff would blow up even at the bottom of the Marianas trench. 
For transmit filters, good Q and robustness is of prime importance, size is secondary.

[G0HZU]

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 ?

A lot depends on the filter type. If you want a narrow bandpass response from a lumped filter then it's generally best to design for power levels of <<0.25W. This is because of the high impedances within the filter. So you have to be wary of the RF voltages that might get generated within the filter at higher power levels as the voltage can easily zap the lumped components.

So if you definitely wanted a narrow bandpass filter at 1.7GHz at a 10W power level you would typically be looking at an expensive inline cavity filter from the likes of K&L or BSC or Spectrum/FSY.

But you can run fairly high power into a basic cheby1 low pass or high pass filter and still use lumped parts. It would generally be OK to use 0603 caps at 10W in a typical LPF or HPF as long as you used decent quality caps. For example, from ATC. The Kemet HiQ-CBR 0603 caps I linked to earlier might be OK in a 1.6GHz HPF at these power levels but that is just a guess.

A lot will depend on the frequencies you expect to use and the range of load (antenna) impedances it might have to work into. A HPF ought to be OK up to frequencies of several times the cutoff frequency but the inductors will see some thermal stress when transmitting very close to the cutoff frequency as in your case of Fc =1600MHz and an operating frequency of 1700MHz. I really don't know how good the Kemet parts are but I'd be disappointed if they failed when used in a basic LPF or HPF at these power levels into a 50 ohm load for example. ATC 0603 caps would be a better option.

[jujun]

Also to return to the initial topic, it would be very interesting to have a comparison of handmade coils Vs air core SMD coils.
What is the maximum Q possible by hand ? Is it better than air core SMD coils ?

On a test filter what will be the differences between a filter that use hand made coils instead of RF SMD air core ?

[G0HZU]

I'd guess that ultimately there would be no difference because you could copy the SMD coil dimensions with a hand wound coil. However, you would need a microscope, plenty of dedication and and steady hands! For a 3.3nH coil at 1.7GHz I'd think that a Q of 200 would be a practical maximum either for hand wound or SMD if you optimise the dimensions.

The main advantage of handwound coils is that they are adjustable. 

[T3sl4co1l]

SMD coils are only limited by what's on the market; whatever's out there, is what's out there.

You can make arbitrarily good inductors, give or take how much space you can spare and relative to the wavelength of interest.  At most frequencies I think a Q over 1000 or so is rather difficult, using regular metals and insulators.

Also as you go up in Q, sizes get larger and larger, until eventually the component is more like a piece of transmission line or resonator, and the geometry of your circuit overall matters very much (like "plumber's special" designs), so that it's not very meaningful anymore to speak in terms of single components soldered onto pads.  Which really shouldn't be a surprise anyway, as maintaining a Q so high is certainly a whole-circuit consideration!

At microwaves, dielectric resonators become interesting, too.  Think total internal reflection but at fractional-wavelength scales.  They can be very good, but obviously break the "component with pins" concept even further.

Beyond that, you need physics.  A transducer to a mechanical resonance, or an electron or nuclear resonance*, or superconductors.

*Although NMR has rather low density to bother with.  You can make an oscillator locked to proton resonance, but I don't think you'd make a real signal filter with much over -60dB insertion loss that way.  EPR is usable, for example YIG spheres.  Note that resonant frequency is proportional to magnetic field, making these useful for variable filters/oscillators but not so much for stable fixed references.

Tim

[G0HZU]

To show the advantage of hand wound coils, here's a simulation of a lumped (SC Coupled) bandpass filter design at 1700MHz. In order to tune this on frequency the inductors need to be tuneable. So it would be hard to make this with fixed SMD coils. The PCB layout will affect the centre frequency as you can see in the graphs. This simulation uses a PCB made from Rogers 4003C 0.02" thick. The graph in the top right corner doesn't take into account the PCB layout but the Sonnet S21 graph does and it shows the filter response is bang on 1700MHz. The insertion loss will be dominated by the Q of the inductors although even the cap Q will have some effect on loss for this type of filter. I chose a Q of 150 for the coils based on experience. You can see the filter has a bandwidth of about 200MHz but because it is only a 3 section filter the stopband performance isn't that great. But add a couple more sections and it would be much better for only a bit more insertion loss.

This bandpass filter would only be suitable for low power, definitely not suitable for 10W signals!

[coppercone2]

what if you just use copper plate and use shim-stock to elevate the parts over a copper plane and solder them in mid air to get rid of all the balony relating to PCB material?

You can get bits of metal to lay the parts down on, solder them together in an air bridge, then remove the shim-stock.

Personally the idea of using a structural reinforcement as a electrical property bothers me.

https://www.ebay.com/bhp/shim-stock

If you really solder it all down flat, maybe you can use your own controlled dielectric material as a sheet to lay the parts down on, if its necessary for some reason?

Or what if you glued the parts to a dielectric, soldered it together, then elevated the dielectric over the ground plane on supports?

[G0HZU]

Well you could just about make it ugly style over a bare copper PCB but it would be a fragile labour of love with 0603 sized parts. Even when fitted to the skinny Rogers 4003C PCB the 0603 caps can crack in an instant if the PCB is only flexed a tiny amount. The Rogers 4003C material in 0.02" thickness is easy to bend or flex and it is also quite brittle. So I had to solder a metal sheet under the earlier high pass filter PCB to keep it rigid enough to allow gentle handling. Otherwise, even the act of fitting and tightening SMA cables to it would probably crack the caps.

The SC coupled BPF is very prone to detuning if there is stray shunt capacitance to ground at the series LC node so this is a major reason why the PCB affects the frequency response. But it's no big deal to model it and correct the design to suit the PCB strays and the component strays.

[coppercone2]

isent the PCB horribly drifty as a dielectric because of internal stress?

[G0HZU]

No, the Rogers 4003C material is very stable. It's just a bit flexible and fragile if it is only 0.02" thick. For real production boards the 0.02" thick 4003C material is usually fitted as the top and bottom layer of a multilayer PCB. The central core of the PCB might be anything from 4 layers through 16 (very skinny) layers of FR4 material. This usually results in a PCB of a couple of mm thickness and it is very stiff.

I borrowed a wideband test RF amplifier from work today that can produce up to about 50W up to nearly 3GHz so I set it up to produce 10W at 1.7GHz and fed this through the little 1.6GHz HPF board I showed earlier. This was fed through the board into a decent (8GHz) Weinschel 30dB 50W attenuator and into a lab thermocouple power meter. With +10dBm on the power meter, there will be at least 10W at the filter. I looked at it on a Flir thermal camera and after a few minutes the inductors were at +56degC and the centre 0603 cap was a few degrees cooler. The main PCB was at about 36degC. I had to put a tiny smear of white Tippex on the inductor to get a reliable reading on the camera. But the caps didn't fail or explode

If I had used a PA with a poor source match and a load with a poor VSWR I'd expect the inductors to get very hot very quickly if the load was at a part of the smith chart that caused a higher voltage at the filter. But I didn't try this. I'd expect it to get very hot but I'd expect it to survive. Normally, there would be a metal heat spreader directly under the PCB and this would help in removing the heat from the board. So with a proper heat spreader, the temperatures on the thermal camera would have been lower.

[G0HZU]

I've also found another Rogers 4003C 0.02" PCB I can cut up with a hacksaw and I'll have a go at making the 1.7GHz lumped BPF. I think I'll have to cut it about a bit to get all the 0603 components to fit and I'll post up the frequency response but it might not be this evening. However, it's going to look just like the simulation as I've made quite a few filters like this over the years.

A microstrip interdigital or combline filter would probably be of more interest to most people. I can simulate designs of both of these and post them up as well? The simulation is usually very close to reality so I don't think I'd want to make them for real.
 

[Wolfgang]

Hi,

in order to make this test meaningful you have to tune the input power source thru all the band of interest and see where hot spots occur. Especially in a higher-order filter the hot spots move thru the structure as frequency is changed. When Qs are high the "hot spots" can be only small fractions of the center or corner frequency.
Another reason for filter destruction is load mismatch.

IMHO, a proper monte carlo simulation of input frequencies and load conditions could help a lot. Then you know what the worst case really is.

[G0HZU]

Only sick people even think of making a transmit filter from 0603 components.

At 10W your 0603 SMD stuff would blow up even at the bottom of the Marianas trench.

I guess I'm responding to your hyperbole above. Maybe your argument is that if you can cause one 0603 cap to fail in a filter at 10W with a certain PA type and a sweetspot load impedance then the above argument stands?

But I think it depends on how you define the requirements. This especially applies to the range of source and load impedances it is required to work over. It also depends on the filter type. LPF and
HPF are generally very low Q networks and so can operate at higher power levels than a typical narrow BPF.

https://www.atceramics.com/UserFiles/1206_LPF_HP.pdf

ATC make little 1206 packaged lowpass filters using thin film technology and this package contains the equivalent of a complete high order LPF. But they rate them at 12W. So that's possibly the
equivalent of 4 caps and three inductors crammed into this tiny low profile space yet they claim a 12W rating. The 12W rating will presumably apply only if you use the filter within sensible limits for source and load impedance and at sensible test frequencies.

[Wolfgang]

I am maybe a bit conservative, but in my logic a transmit filter must never blow up over the whole frequency range  and all possible load impedances.

Here is why:
- in case of antenna or cabling problems, almost *any* impedance can appear at the filter output, depending where the fault is and how long the cable to the filter output is
- during PA tuning, the feed impedance can also take a vast range of values, so does the power level
- filter components can fail by 1) overvoltage (w.g. du due resonances in mismatch) 2) overcurrent (same reason) and simply too high power loss (bad design or cheap cap)

I simply do not understand why the most important issue is to use minimum size components, not robustness, longevity and fault tolerance. Why create work for yourself by
making a marginal design only working on sundays with good weather.

Are you trying a planned obsolescene approach ?

[G0HZU]

Are you trying a planned obsolescene approach ?

I'm not sure what you mean but I've never used the 1206 ATC 12W LPFs. I've only shown them as an example of a 12W rated filter that uses the equivalent of tiny parts.

[Wolfgang]

... the best of the filters is the LPF1206HP6000, this one even has a negative insertion loss !  at least according to their datasheet.
But seriously: Their power handling data is given with a 50Ohm input and output termination and a CW signal.
Mismatch tolerance is not even mentioned. I guess this stuff is used in mobile phones where power is just a small fraction of the 12W specified.

[G0HZU]

I simply do not understand why the most important issue is to use minimum size components

Here's one example for you. If you use several ATC 600S (0603) caps in parallel in a LPF you get lower overall ESR and lower overall series inductance for the same overall volume as a single big cap. So this means lower losses (better efficiency) and better filter stopband performance.

[Wolfgang]

All agreed for a receive filter. In a transmit filter its a different story.

[KJDS]

Mostly, Tx filters have an isolator on the output, or some other form of antenna fault circuitry, or both.

Having said that, common sense says use a reasonable level of component derating, but for a discussion of what's possible on a forum then whilst it can be worthwhile mentioning something, it isn't beneficial to keep banging the same drum without coming up with something else constructive.