Need advice on bandpass filter design (uni project). - Page 1
Forum section: electronics / rf-microwave

author: Refrigerator
author_rank: Super Contributor
timestamp: May 13, 2020, 09:23:25 pm
content: I've been assigned to make a bandpass filter as one of my semester projects for university.I think designing it is no big deal but some questions have popped up along the way. I'm usinghttps://rf-tools.com/lc-filter/to help me design this filter and ADS 2020 to test the design.Basically i've noticed that stopband attenuation plays a big role in the filter design but in my case the stopband attenuation wasn't given, meaning that i have to choose it myself.I don't have much experience in RF design so i don't know what level of attenuation is a good ballpark figure to base my filter off.For example, the calculator linked above defaults to -50 dB, but i don't know whether it's normal or some pie in the sky figure. What's a common real-world number i should be aiming for?Parameters are: center frequency 145 MHz, -3 dB at 144-146 MHz (passband), 5-th order, discrete components, FR-4 substrate, SMA plugs.Impedances are not given but they'll be the standard 50.Also i'm not sure which type of filter to choose. At first i was leaning more towards elliptic or Chebyshev, but now i'm thinking more along the lines of Butterworth.Problem with Butterworth is that the calculator gives me capacitor values in the femto Farrads and i don't think i can get my hands on those.Some freedom is given in the filter design as long as the initial criteria is met so i think some microstrip action wouldn't hurt.How hard would it be to design a discrete 490 fF or 151 fF capacitor on a PCB? I don't think stray inductance would be much of a problem given the µH inductors next to the fF caps.

author: FenTiger
author_rank: Regular Contributor
timestamp: May 13, 2020, 10:55:32 pm
content: I think those picohenry valued inductors are going to give you bigger problems.I've only done a little bit of VHF filter design, but I've never had much luck at getting realistic component values out of online calculators. I've had far better results from Elsie:http://tonnesoftware.com/elsie.html

author: TheMG
author_rank: Frequent Contributor
timestamp: May 14, 2020, 12:23:39 am
content: The required attenuation outside of the passband (how far out from the passband?) really does depend on the intended purpose of the filter, and is not entirely easy to predict as real-world components, the PCB layout and the physical construction all have parasitic inductance and capacitance that is going to affect the stopband characteristic at certain frequencies, especially if the aim is to have a large amount of attenuation.Since no specific requirement was given, just design the filter for the given requirements, following best practices for component choice and layout, and what you end up with for attenuation outside of the passband, is whatever you end up with. Find a calculator that doesn't require entering an attenuation requirement, or do the calculations manually to determine the required component values.A practical VHF filter is normally going to have inductor values in the tens or hundreds of nH. Discrete inductors in the pH and capacitors in fF values do not exist, as parasitic inductance and capacitance of leads and traces would exceed that. PCB inductors and capacitors of such values would not typically be used for anything lower than microwave frequencies, at which point you'd be using stripline filters which is a completely different animal.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 14, 2020, 03:17:49 am
content: Not to mention the other capacitors that are fractional-pF, in series with 2.5uH inductors that need a hell of a lot less than 0.5pF stray between their ends and to ground.Yeah, this ladder topology needs to be transformed to a coupled-resonators topology. This is better at high Q like this. Roughly speaking, tune up a bunch of resonators (helical resonators are okay at this frequency) to 145MHz, then couple them together just enough to achieve the desired bandwidth. Couple say a magnetic loop to the input and output and you're done.Probably better off buying one, the 2m HAM band is a standard thing after all. Not to say it's not a good learning experience to design and construct a filter, but it will take a while and if you have other priorities in this project, don't let it hold that up.Tim

author: Refrigerator
author_rank: Super Contributor
timestamp: May 14, 2020, 07:58:35 am
content: Quote from: T3sl4co1l on May 14, 2020, 03:17:49 amBuying is not an option because the point of this project is to apply the knowledge we gained in this course by designing, making and testing an RF filter.

author: M0HZH
author_rank: Regular Contributor
timestamp: May 14, 2020, 08:18:38 am
content: I don't think it's possible to meet the requirements (2MHz -3dB bandwidth at 145MHz from a 5-pole filter) with discrete components. My best attempt would be coupled resonators, designing for a high working impedance (1-2kohm) and using impedance transformers at the input & output.For these requirements, the most common practical solution would be some sort of helical resonator.

author: cdev
author_rank: Super Contributor
timestamp: May 14, 2020, 12:38:49 pm
content: Two helical resonators only lightly coupled (with a short piece of mini coax) or allowed to influence one another some ideally varying amount with a hole in the shielding with short taps near the shorted ends at the appropriate point for your low impedance ports. (but they are interchangeable) So each one is an LC resonant circuit at the desired frequency. There are lots of designs online. You dont really need test equipment if you have a receiver and signals you want to receive are audible and constantly available. But it would be very helpful if you do have it. Thicker wire works best by far but you can use thinner stiff wire make sure you use a metal box and that it is shut because any movement or deformation of the coil will change the frequency it resonantes at and you will need to retune it. Solder on thin bare copper wire makes it stiffer. You can use this for higher frequencies too. (but it will be smaller) Capacitor quality matters more as you go higher in frequency, Ive read. Some trimmer capacitors are too lossy to work well.

author: cdev
author_rank: Super Contributor
timestamp: May 14, 2020, 12:59:46 pm
content: Did they give you any other requirements like size?Quote from: Refrigerator on May 13, 2020, 09:23:25 pm

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2020, 02:28:21 pm
content: The first thing I thought of when reading the requirements is helical resonator. I will not say that a discrete implementation is impossible but tuning will be a problem. Below is a photograph of a 2 meter helical resonator which I designed and made by hand several years ago.In the design process, I started with the wire diameter which I had available to adjust the other dimensions. This was actually the second helical resonator I designed and built; the first one did not have overlapping seams for greatly increased strength and used plastic film trimmers which severely limited performance compared to the piston trimmers.If I built another one, I would implement variable coupling between sections using sliding windows. This one is over-coupled although that ended up working out well in practice.

author: WPXS472
author_rank: Regular Contributor
timestamp: May 14, 2020, 03:33:52 pm
content: As someone with some experience in building filters on a commercial scale, let me just say that good filter people are born, not made. I agree with the above statements that a helical resonator is probably your best bet. Filter design is all mathmatics, but filter building is a whole different matter. I would try to get an old, VHF high band radio, something like a GE MASTR2 and rip the preselector out. Then study that and build your filter in a similar manner. Don't copy, but use what you learn from studying that old preselector. The company I worked for in a former life made a series of helical filters that worked from about 125 MHz to 500 MHz all in the same housing, and meeting the same specs. They were pilot selection filters for an older CATV manufacturer, There were challenges in doing this, but we managed to overcome them and made filters for several years. They finally settled on a standard pilot frequency and had ceramic resonator filters produced. Something they should have done from the beginning, in my opinion.

author: cdev
author_rank: Super Contributor
timestamp: May 14, 2020, 03:37:25 pm
content: David, That looks very nice!I notice you have what appear to be threaded caps over the adjusthent trimmers. Thats to protect the values from being accidentally detuned, if the filter gets knocked around, right?

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2020, 04:00:52 pm
content: Quote from: cdev on May 14, 2020, 03:37:25 pmThe caps came with the piston trimmers and include a plastic gasket on the inside. The piston trimmer adjustments are pretty stiff so I doubt routine shock would change them so the caps are more to prevent contamination and idle maladjustment. If you look carefully, you can see the hex nuts which secure the adjustment side of the piston trimmers through the copper clad board but I think I soldered them on the inside once mounted.The piston trimmers are 1 to 10 picofarads and provide a very smooth adjustment over a couple of MHz.I opted for direct coupling as shown on the open end and you can see where I adjusted the coupling for best matching and never cleaned it up. Because the open end is the low impedance side, attaching the cover has little effect. A better implementation would silver plate the inside and seal the cover.

author: cdev
author_rank: Super Contributor
timestamp: May 14, 2020, 04:25:23 pm
content: What is a good way to silver plate copper items without breaking the bank? I suppose, electolysis might work, if you have some silver. I remember seeing this discussed here, I should just search for it.Thats pretty cool if you can actually brush on or paint on or rub on silver.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 14, 2020, 04:34:47 pm
content: Don't worry about plating. A few more percent in component Q isn't important at a system Q of only ~100. Plain copper will give a Q around 500 to 1000 at this frequency, I think??Tim

author: grouchobyte
author_rank: Regular Contributor
timestamp: May 14, 2020, 06:11:25 pm
content: Quote from: Refrigerator on May 13, 2020, 09:23:25 pmThis may help.....PM me if you need some additional details. I haven't been on the forum lately so it might be hit or miss reaching me. Hope I got the details of your requirement more or less correct-Bob/ Grouchobyte

author: Refrigerator
author_rank: Super Contributor
timestamp: May 14, 2020, 07:13:13 pm
content: Thanks for the input but i don't think i could get away with all microstrip design when my requirements call for discrete components.I think i might have painted myself into a corner with this one.Too bad the whole corona thing messed up my plans and i couldn't submit my own parameters for a custom filter in time.I'll try playing around with various filters and parameters until i get something usable.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 14, 2020, 07:24:35 pm
content: Quote from: FenTiger on May 13, 2020, 10:55:32 pmI tried running the program but it didn't run. Says it's 32-bit is there a way to run it on win10 64 bit?Edit: got it running, turns out the installer just takes ages to start running, in my case at least.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 14, 2020, 08:09:45 pm
content: Here's an idea i had, can the circled components be made into microstrips?

author: grouchobyte
author_rank: Regular Contributor
timestamp: May 14, 2020, 08:14:12 pm
content: Quote from: Refrigerator on May 14, 2020, 07:13:13 pmYou can try and submit the microstrip design. It does have some discrete capacitors and there is really nothing wrong with the critical structures being microstrip....so its kind of a hybrid design. Your prof or TA might like the simplicity or elegance of an less standard approach. During my teaching years, I would have allowed it.Anyways, Good luck finding suitable discretesBob/grouchobyte

author: grouchobyte
author_rank: Regular Contributor
timestamp: May 14, 2020, 08:18:15 pm
content: Quote from: Refrigerator on May 14, 2020, 08:09:45 pmWhat you circled are the microstrip structures in my proposed filter designAre you finally coming around to my way of thinking?Bob

author: Refrigerator
author_rank: Super Contributor
timestamp: May 14, 2020, 08:29:26 pm
content: Quote from: grouchobyte on May 14, 2020, 08:18:15 pmOh i think i didn't quite understand your first post then, i thought it was purely a microstrip design.But i agree on the hybrid idea, it says discrete components but not how many (or to what %) so i think it would be ok.

author: radioactive
author_rank: Regular Contributor
timestamp: May 14, 2020, 09:35:12 pm
content: Quote from: David Hess on May 14, 2020, 02:28:21 pmThat looks pretty cool! Here is some more VHF bandpass filter porn. This one is tunable. I didn't make it though. Something I bought on ebay in order to measure really low level 2nd order harmonics from a high power amplifier. (block the fundamental while passing the harmonics).

author: radioactive
author_rank: Regular Contributor
timestamp: May 14, 2020, 11:08:44 pm
content: Here is a Qucs simulation that might be a start. I modified a 150-174 MHz BPF I designed in the past (made it a bit narrower and centered on 145 MHz). I've included the simple inductor and capacitor models along with the simulation in the zip file. I think if you construct on FR4 and use air-core wire-wound inductors / ceramic capacitors, it should match pretty close to the analytical simulation. It will not meet your requirements for passband BW. This is more like 10 MHz vs 2 MHz as is. Still might be useful in some way.

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2020, 10:54:55 am
content: Quote from: T3sl4co1l on May 14, 2020, 04:34:47 pmI seem to recall that I got a Q of about 900 but measurement was difficult. The references I worked from suggested building it 10% oversized to make up for the loss of Q if silver plating was not used.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 29, 2020, 05:24:15 pm
content: I received the go-ahead to make the helical resonator so that's what's i'm going to doI've found an online calculator for helical filters athttps://www.changpuak.ch/electronics/Helical_Bandpass_Filter_Designer.phpAre there any good ways to simulate such filter?

author: Refrigerator
author_rank: Super Contributor
timestamp: May 29, 2020, 10:17:18 pm
content: Also how close to the ideal calculated filter should i expect the real life filter to be?I've had positive experience with simulated 2.45GHz antennas turning out very well in real life, should i expect something similar with this as well?

author: radiolistener
author_rank: Super Contributor
timestamp: May 29, 2020, 11:17:12 pm
content: In real life you will get more wide bandwidth, about 10-15 MHz instead of 2 MHz and more loss in pass bandIf you will get 5 MHz bandwidth it will be good result.I don't know how good result can be done with helical filter, but the best result that I got with usual SMD capacitors and FR4 pcb is bandwidth = 18 MHz. May be a little better result is possible, but I could not stand tuning it. So, I think 2 MHz is unreal...

author: cdev
author_rank: Super Contributor
timestamp: May 30, 2020, 08:46:56 pm
content: Try to keep it possible to move things around. Placement makes all the difference, also obviously the Q of the capacitor is very important. (this kind of circuit is useful in measuring Q. Maybe you can get construction tips from reading artcles on Q.I agree with radiolistener that 5 MHz is a good starting result. The capacitor probably makes all the difference. I could show you what kinds of capacitors I would pick. Do you have any deoxit? You should clean the crap out of any capacitors you use, esp the bearing and put some deoxit on it. Clean out any gunk, if you have an ultrasonic cleaner, use that. then use a tiny bit of deoxit.Use a trimmer capacitor a good one.Quote from: Refrigerator on May 29, 2020, 10:17:18 pm

author: cdev
author_rank: Super Contributor
timestamp: May 30, 2020, 09:16:09 pm
content: Interesting and useful variation, a helical resonator notch filter. I have wanted to build this one for the same reason David, its author did, extremely annoying and strong pagers.https://satsignal.eu/wxsat/filters/HelicalNotch.htm

author: Refrigerator
author_rank: Super Contributor
timestamp: June 14, 2020, 10:54:12 am
content: I've been looking for ways to simulate the helical filter but couldn't find any good way to do so, any advice?I'd like to at least calculate and simulate the equivalent LC circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 14, 2020, 11:27:13 am
content: I don't think there's really any good way to do that...You could calculate the LF equivalent inductance of the helical waveguide, but you need formulas including the shield around it. Most assume a helix in free space.You could perhaps adjust it by making an assumption about how much mutual inductance there would be to a coil the same dimension as the shield, and assume it's shorted.The capacitance, and the inductance for that matter, are distributed, so that neither acts on exactly the full length of the coil.Coupling is the easiest, as you need only know the coupling coefficient, and use that to couple between adjacent inductances, or tapping or capacitive coupling, whatever the case may be.If no connection is made to the helix itself, purely magnetic coupling, I don't think it actually matters what its impedance is -- only the coupling factors, resonant frequency and Q. That is, you might use normalized L and C, and adjust them proportionally to match the resonant frequency, and not worry about their ratio (Zo = sqrt(L/C)).Tim

author: Refrigerator
author_rank: Super Contributor
timestamp: June 14, 2020, 07:37:41 pm
content: I'm playing around with ANSYS Discovery AIM right now to see if it's possible to simulate the resonator with it.But seems i'm stuck at the helix part, man i must be dumb or something it can't be this hard to create a simple helix.