Help with high insertion loss in my 112Mhz bandpass filter - Page 1
Forum section: electronics / rf-microwave

author: sab
author_rank: Contributor
timestamp: November 07, 2018, 08:41:24 pm
content: Admittedly I'm being picky but I was wondering if anyone had suggestions on how to improve my 112 Mhz band pass filter. I used a circuit attributed to Ashhar Farhan, VU2ESE and Wes Hayward W7ZOI for a Spectrum Analyzer project (Specan). My simulations in LTSpice show a 6db insertion loss at 112Mhz as do the experimental results reported by Ashar. However, despite many hours of twiddling I'm unable to do better than a 10db insertion loss as measured by my R&S CMU200. I know I can easily compensate for the additional 4db but I was wondering if I had overlooked something obvious. Any ideas will be much appreciated. I've attached a picture of the filter and the circuit.Thanks,Sanjay

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2018, 10:18:40 pm
content: Did you tune for minimum insertion loss or also maximum return loss?If not the latter, then mismatch is probably swallowing a lot?Did you correctly model the real Q's of the L and C used in circuit?Tim

author: sab
author_rank: Contributor
timestamp: November 08, 2018, 04:47:57 pm
content: Thanks for your suggestions Tim.I never thought of measuring the return loss. I'll try that. Time to find myself a directional coupler.I had hacked together a model of stray inductances and capacitances in ltspice but I didn't notice much difference in insertion loss. I could refine it though.Thanks again,Sanjay

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 08, 2018, 06:51:39 pm
content: Yeah, correctly modeled parasitics will mainly show realistic notches and peaks in the stopband.You could also modeling with exact value parts, but measuring what values you've actually bent up isn't exactly trivial, so that won't be of much value either.Tim

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 08, 2018, 07:22:36 pm
content: It looks like you used double sided pcb material? In that case all the islands you did cut out will be little capacitors too, and the same for the shields.Also it's a bit hard to see but is there a direct connection between the bottom of the coils and the cap parallel to it?

author: sab
author_rank: Contributor
timestamp: November 09, 2018, 12:44:09 am
content: Thanks for your suggestions PA0BZ.Can you explain what you meant exactly by 'direct' connection? The live end of the cap is on the same pad as the live end of the inductor, but the ground end of the capacitor and the ground end of the inductor are both connected to the ground plane on the board. They are connected at different points on the board though. Could this be what you meant by a not direct connection?I thought it was considered good practice to use the unused side of the double sided pcb as a ground plane. You are right, of course, that this will create many little capacitors everywhere though.Thanks,Sanjay

author: cdev
author_rank: Super Contributor
timestamp: November 09, 2018, 02:43:08 am
content: You could probably redo it pretty quickly, why not try that making all the leads and connections short and directly to the components, and see if the loss improves?Don't tack them down until you have the layout the way you want it! Just set them in place. Keep leads super short.What I do is I use edge mount coax connectors (typically SMAs) and I just set them on top and dab the sides with solder to tack it down. I use a single piece of PCB for the ground, the bottom is completely useless and not used, for a passive filter, and my 'islands' always sit on top of that. You can use super glue to glue your islands down at the end if you must but if the ends (the smas) are anchored down and some of the components go to ground you dont even have to glue your islands since they are just for support.(if you use superglue, the smell is toxic if it gets hot not pleasant- You really don't even need to affix them if you have other parts that are grounded and of course you likely do.)I use little square pieces of PCB nibbled out with a nibbling tool as platforms for the components. They are so small the capacitance is likely very low. They are not of uniform size and shape, the way I do it is semi ugly.But I keep the leads to everything short. For transmission line I either use a piece of PCB or mini coax. Inductors I put at right angles to one another to minimize coupling between them. The SMAs would go directly to the first L or C component(s). The copper between the inductors is good. You can also use variable capacitors. It looks like you deformed one of the inductors laterally - I don't know how that would effect Q. I would keep it cylindrical and just pull the turns apart or compress them.10 db is totally unacceptable insertion loss.Try to make it neat so you can look at it and see exactly what it does in one glance.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 09, 2018, 08:52:46 am
content: Quote from: sab on November 09, 2018, 12:44:09 amPlease look at the original, there are no islands for the ground connection:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2018, 01:16:01 pm
content: Meh. The coupling between pads is minuscule, and the self capacitance of pads only detunes things slightly (you can't buy capacitors that precise to begin with, you'll have to tweak it to tune/match correctly). The resulting tempco (FR-4's tempco is pretty horrid) is the only dominant error leftover, and that's not a big deal for low power filters inside the shack.I have no problems with this style of construction, at this frequency. By 500MHz, say, you might want to cram the caps a bit tighter, use smaller pads, use 0603s, that sort of thing.Tim

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 09, 2018, 01:19:33 pm
content: Quote from: T3sl4co1l on November 09, 2018, 01:16:01 pmIt's not the coupling between the pads that worry me, but if you look closely at op's picture you can see that the cold connection between the coil(s) and the tuning cap across it takes a detour via the underside of the pcb, and that can't be good…

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2018, 01:29:44 pm
content: I don't see any obviously cold joints. Sloppy yes, but that happens.What about the ground? Oh, there's what looks like a shorting wire across a grid of pads, making a ground for the capacitors... but this doesn't apparently bond with the shields or the inductor ends? Is that true, OP?Tim

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 09, 2018, 01:35:51 pm
content: Quote from: T3sl4co1l on November 09, 2018, 01:29:44 pmCold connection, not cold joint, eg. not the hot side of the coils.QuoteNow you see it too

author: cdev
author_rank: Super Contributor
timestamp: November 09, 2018, 01:42:19 pm
content: Depending on the thickness of the PCB you use and its dielectric properties there is an optimal trace width for the impedance you are targeting. The width of a standalone trace with air on both sides (as opposed to one surrounded sitting on top of a PCB where the back is entirely the ground plane) is likely to be slightly different but for this purpose, you can assume its not, if you cut a piece of PCB that width you can use that as your transmission line, and then slice it off with a knife where you need to interrupt it for the capacitors. That makes it easy to have the grounds short and the signal path as short as is practicable.You could also make a PCB design to put your filters on and make up a bunch of them at some PCB house. Maybe it might be good also while you are getting the design right to use some small trimmer caps instead of discrete values.As it seems to me as if the individual parts of the filter are supposed to resonate individually at the pass frequency you can perhaps use a siggen to make a modern equivalent of a grid dip meter (?) and tune them first separately by themselves as tank circuits, (the simplest LC circuit by itself) and then add the (small) capacitances between them maybe with trimmers so you can adjust those values too, and re-tweak the coupling values.Think of the whole thing as a sort of row of tuning forks that you want to all ring together like a bell with sympathetic vibration when any other one nearby starts ringing.That said, if the best theoretical performance is 6 db loss your original 10 db could likely be improved on but optimizing it to be only 6 db you may need to use variable caps and a sytem of tuning the individual parts to resonate first, then together, to get it right.

author: sab
author_rank: Contributor
timestamp: November 09, 2018, 06:40:16 pm
content: I've attached my image with some annotations.To answer the question posed by Tim and PA0BZ, the shorting wire across the grid of pads connects to the ground plane on the underside of the board. I also drilled a couple of holes on the board and inserted wires to connect the ground plane on top of the board and the bottom thinking it might help.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 09, 2018, 06:50:14 pm
content: Most of the cuts make no sense at all, look at the red lines in the attached picture, this should be one big ground plane. What you did is introducing a lot of parasitic inductance and what not. Also have a look at the picture I posted from the original designer and you will see the difference.Start again with a blank pcb and only isolate the junctions that arenotconnected to ground, try to get the coils looking a bit more neat (higher Q!) and I guarantee you that the result will be much better.

author: sab
author_rank: Contributor
timestamp: November 09, 2018, 06:57:16 pm
content: I'll try that. I made the cuts incorrectly at first and then just thought I'd get away with shorting them. I guess I didn't

author: cdev
author_rank: Super Contributor
timestamp: November 09, 2018, 07:30:24 pm
content: For 112 MHz it should fairly easy to use a long skinny piece of PCB for the main signal trace. Use a single flat piece of PCB material for the body of the filter which is also the ground plane.In the future when you are making PCBs make sure to put lots of plated vias connecting top and botton ground planes. This is such a pain in the ass but its also important to not get unpredictable peaks and valleys in the filter responsetons of parasitics. If its not practical,best to just forego the copper on the sides and put the entire filter 'on top'with no grounding on the sides, instead the whole base is the ground.This is good because all leads to ground will be short.The places where multiple components meet that are not grounded can be self-supporting or perched on pieces of PCB which could be glued or possibly even soldered down (this probably not even necessary because the other connections likely offer enough rigidity). You likely could build prototype filters on a generic filter board you could reuse over and over for similar prototypes.Try to stay reasonably close to the right impedance.Here is a nifty program you can use to calculate what the impedance of arbitrary transmission lines might be.http://atlc.sourceforge.net/Its very easy to use!-----------In order to perform 1, 2, 3 or 4 above, the programme atlc needs to know shape of the transmission line's cross section. This cross section is stored in a data file, which happens to be a Windows bitmap file. The bitmap file is read by atlc, following which the programme performs the analysis.There are two ways to generate the bitmap for atlc.As you might expect, you can draw the transmission line's cross-section in a graphics programme such as Gimp, Photoshop, Windows Paint, Corel Photopaint etc. The programme must be able to save images as bitmap (.BMP or .bmp) files, in 24-bit (true colour) mode. The graphics programme can run under whatever operating system you like. If for example you have some U-section brass and some hollow rectangular tube, and wanted to know the properties of a transmission line formed from these, you might draw something like this, and save it as a file, which we might call ushape.bmpU-sectionIn the case of simple cross-sections, the bitmaps can be generated automatically. Currently 7 such programmes have been written:create_bmp_for_circ_in_circcreate_bmp_for_circ_in_rectcreate_bmp_for_rect_cen_in_rectcreate_bmp_for_rect_in_rectcreate_bmp_for_symmetrical_striplinecreate_bmp_for_stripline_couplercreate_bmp_for_microstrip_couplerto quickly generate bitmaps for transmission lines. The first five are for two-conductor transmission lines, whereas the last two are for directional couplers. See the section Automatic Bitmap Generators for information about what cross sections these programs generate. Using these programs to create bitmaps is very fast - much faster than creating them with a graphics programme.There is another similar related program, I think it may be a fork of atlc, maybe its named atlc2?Also, you can use other materials, or of course, also coax for transmission lines.

author: sab
author_rank: Contributor
timestamp: November 09, 2018, 09:00:51 pm
content: Thanks Cdev. Those are great suggestions. I'm going to remake this filter following all the suggestions I've received. This has become a personal challenge for me now!

author: deBug
author_rank: Contributor
timestamp: November 09, 2018, 10:29:48 pm
content: it's a 13 pole bandpass at VHF! 10dB loss is quite OK.VHF filters are hard, you will have loss in every component and with so many stages it is going to be lossy.You cite two gigants and what they have achieved and that your experience is different, well that is hardly surprising.Looking at the schematics and just counting the serial capacitors in the signal path, 10dB loss seems OK. To lower that make sure all the caps are high Q high frequency NPO super good quality well known brand name capacitor or you will have loss, that's the just the way it is.Sorry to sounding negative but I think your expectations are wrong. You have to realize that getting up to the level of Wes Hayward is not easily done.//Harry

author: G0HZU
author_rank: Super Contributor
timestamp: November 09, 2018, 11:52:45 pm
content: Quote from: deBug on November 09, 2018, 10:29:48 pmIt's just a three resonator BPF and the original design is an attempt to make Hayward's filter easier to make. But the response of that filter isn't as selective as Hayward's filter. So is it worth making anyway? If it was made properly I'd expect the loss to be much lower than 6dB but I don't think it's as selective as Hayward's filter.I think Hayward's original BPF designs are here:http://w7zoi.net/mixed-bag/mixed_bag.htmlIn my opinion, the first two filters on Hayward's page look to be very difficult to make and his insertion loss of 5dB isn't that special even taking into account the selectivity his filter achieves.To put this in perspective, if a 5-6dB insertion loss budget is permitted then I think you could get close to the same performance as Hayward's first two filters just using point to point (ugly) construction but with a different filter topology. I think the insertion loss would be about the same at 5 or 6dB even if (far from ideal) toroid formers were used like the little T37 toroid example below. Such a filter could be tacked together on bare copper PCB in a few minutes with no need for a scalpel or screens or exotic gimmick components. It would be just three little toroids and a few chunky sized SMD caps soldered together in ugly style.

author: cdev
author_rank: Super Contributor
timestamp: November 10, 2018, 12:40:08 am
content: It would be hard to impossible to tune toroids and SMD caps as optimally as one can tune a loosely coupled filter made with separate chambers and high quality piston capacitors and well-made inductors.A resonator filter may be a PITA but its the go-to design for getting weak signals that are next to strong ones for a reason. For example, APT weather satellites on the 137 MHz band next to very strong, dirty, pager transmitters.Two resonators rather than three is another option.

author: G0HZU
author_rank: Super Contributor
timestamp: November 10, 2018, 01:20:22 am
content: Quote from: cdev on November 10, 2018, 12:40:08 amI'm just trying to explain to deBug that the performance of those filters isn't exotic or unreachable. However it is possible to make it look difficult and to make it difficult and time consuming to build and I think Hayward's filters are in this category.It is possible to get similar performance as the first two Hayward filters just tacking together a few basic SMD parts on a sheet of copper PCB. It would take a few minutes with just a soldering iron. No scalpel, no drill, no exotic piston trimmers, no gimmick caps floating in the air, no PCB walled box with various peep holes in it. I'm not saying this alternative approach is better, I'm just saying that there isn't any unreachable guru magic going on with the Hayward filters but deBug seems to think there is

author: cdev
author_rank: Super Contributor
timestamp: November 10, 2018, 02:29:49 am
content: I agree, if you just put the inductors a bit away from and at right angles to one another, like in a zig zag, and also make them out of rigid enough wire that they maintain their shape very well. This really should be in a box to protect them from cats, etc. Once tuned they cant be disturbed. If you put them in a closed metal box you do need to separate them with shielding less so if its left open. But then they will also act like antennae somewhat. (Tuned loop antennas, which they are).Illogical as that might sound.You'll have to spread or compress the coil to tune it and that will need to be done with something like a wooden ice cream stick or wooden darkroom tongs that don't change the inductance. It would he helpful to have something like a tracking sweep generator or something like that to see what the response is as you tune it.An SDR that let you visualize a wide bandwidth at least 10 MHz, would be my second choice. (An RTLSDR isnt wide enough)(assuming there are some known signals in there to 'peak' it on.)That could work well and later on if you managed to rustle up a network analyzer you might be surprised at how well you managed.Quote from: G0HZU on November 10, 2018, 01:20:22 amOh, and the capacitorshaveto be decent quality ones.

author: G0HZU
author_rank: Super Contributor
timestamp: November 10, 2018, 02:52:36 am
content: QuoteUsing an insulated tool is actually more frustrating in my opinion because it can't make significant changes in inductance easily. I find it is quicker and easier to just use my fingers (and then let go and look for a change) if I already know which way the inductance needs to change. It is VERY fiddly to do even when experienced but there are ways to speed up the process. It isn't a nice or rewarding way to tune a filter but it does work and is fine for doing a one off BPF like this. The trick is knowing which toroid needs adjusting and which way it needs to be adjusted. Once you know this it makes it much easier. Otherwise, it would be like jelly wrestling with the response, often making things worse rather than better each time a toroid is tweaked. Very easy to get frustrated and admit defeat if done blind like this

author: cdev
author_rank: Super Contributor
timestamp: November 10, 2018, 02:59:15 am
content: Toroids are 'self shielding' which is a plus but getting the inductance exactly right with a toroid is not easy because you basically have no in between turn values available.Ive fooled around with these kind of filters a bit but have yet to make a permanent one.You do kind of need a nice box if you want to keep out stuff like pagers. The pager transmitters are strong and they splatter and detune receivers by overloading their front ends. If you have a semi-crappy receiver like an RTLSDR, you will be hearing the same pagers all up and down the bands if you cant apply a filter to get rid of them.Getting rid of that junk is difficult.A notch filter resonator is appropriate if there are only one or two really strong local stations. Unfortunately I live hear a major transmitter site so there are several really strong broadcast signals, plus pagers.

author: G0HZU
author_rank: Super Contributor
timestamp: November 10, 2018, 03:31:40 am
content: QuoteWhatever...My main point is that you all seem to be helping the OP polish a turd because the filter schematic in the OP shows a filter with inferior performance to Hayward's filter(s) in terms of stopband performance. It should have lower insertion loss than the Hayward filter but that comes at the price of inferior selectivity compared to the original Hayward design.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 10, 2018, 09:55:52 am
content: Quote from: cdev on November 10, 2018, 02:59:15 amThey aren't, and you do! We're talking VHF here, where you'll be using a few turns each -- squish the turns closer together for more L, or stretch apart for less. The near-field around the individual turns is quite "interesting", not well shielded. It takes a large, evenly distributed winding to get the "self shielding" effect that a smooth, symmetrical toroid has.(A pot core on the other hand is quite well shielded, but those are hard to come by for VHF purposes!)Tim

author: G0HZU
author_rank: Super Contributor
timestamp: November 10, 2018, 11:29:40 am
content: QuoteHow about 30 turns of skinny wire on a T37 toroid like the image in my earlier post?That's how 'unspecial' the 5dB insertion loss performance is of the first two Hayward filters despite the sexy looking gimmick caps and impressive looking solenoids. I think I could get similar performance with a filter made using those T37 toroids (shown below) with about 28-30 turns and it could be built in minutes ugly style (point to point)using SMD caps and toroids with no special layout requirements. Yes, it looks wrong to use toroids like this at 112MHz but I'm trying to show that there isn't any voodoo magic required to get that shape factor and insertion loss at 112MHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 10, 2018, 12:45:39 pm
content: Yeah, 30 turns is enough for reasonable shielding.Of note, #0 (air core), #1 and #2 are about the only powder materials suitable for VHF use. Permeability isn't much above air, so you still need a lot of turns and inductance is that much more sensitive to bunching of turns. But it's worthwhile if you need the boost in size or packing density without shields.Tim

author: G0HZU
author_rank: Super Contributor
timestamp: November 10, 2018, 02:34:56 pm
content: Yes, I would use phenolic #0 toroids. If I'm allowed to 'cheat' and use a more sensible size toroid such as T50 I'd expect to be able to beat Hayward's filter for insertion loss whilst matching it for bandwidth and selectivity. Such a filter could be tacked together in maybe two minutes with a soldering iron. but this would be a different filter topology that didn't need the exotic gimmick caps. Just tack together the coils and caps with no layout issues and it will work once the toroids are adjusted for lowest insertion loss. No need to adjust any caps, just use fixed values. Only three inductors to adjust...However, if you gave the toroid filter to 100 novices in an untuned state I doubt many would be able to tune it correctly even if the correct number of turns is on each toroid. My guess is nearly all of them would declare it impossibly frustrating to tune up and they would give up after maybe 15 minutes of squishing around the toroid windings. Toroids are not nice to tune, especially on a narrow filter like this. But with 30 seconds of training and the right tool(s) they could all do it in about a minute or two. Suddenly it becomes a piece of cake

author: cdev
author_rank: Super Contributor
timestamp: November 10, 2018, 03:11:15 pm
content: since we're talking about resonators what about having a tap instead of a capacitor.

author: G0HZU
author_rank: Super Contributor
timestamp: November 10, 2018, 06:29:53 pm
content: Here's a simulation of the original Hayward filter using his inductor Q and cap models. It shows a 5dB loss and pretty good selectivity across 60-160MHz. Much better than the filter design in the OP of this thread.However, there are alternative means to design Hayward's filter and one way is to transform away the problem components like the gimmick caps and I tried to design an equivalent filter with a topology that gave the same bandwidth and the same shape factor. But a key design aim was to make it easier to make with tame toroids and no daft component values like 0.092pF. I built it today and with little T37 toroids and some decent SMD caps I got about the same loss as Hayward did at just over 5dB. But with T50 toroids I get 3.8dB loss and a plot that looks very close to the simulation of Haward's filter below.It took about 2 minutes to solder together and about the same to tune it up. Note that the circuit of my filter is different to the parallel resonator circuit below and this is how I managed to get away from the gimmick caps. I also chose to use toroids because they are much less touchy and sensitive to strays from nearby metalwork etc.

author: G0HZU
author_rank: Super Contributor
timestamp: November 10, 2018, 06:53:13 pm
content: If built correctly with decent components, the filter in the OP of this thread would look something like the simulation below. You can see that it isn't an equivalent to the original Hayward design because it doesn't have the same selectivity. So is it worth making if you want/expect the same performance as the Hayward filter(s)?

author: cdev
author_rank: Super Contributor
timestamp: November 10, 2018, 08:34:44 pm
content: It depends on where the interference you want to eliminate is.

author: G0HZU
author_rank: Super Contributor
timestamp: November 10, 2018, 09:30:29 pm
content: If the filter is going to be used in a spectrum analyser then I would have thought that rapid rejection of the swept first LO is one reason to try for a very narrow IF1 filter here. The VU2ESE filter has about twice the bandwidth and this also lets in more signals to the second mixer.I think the VU2ESE filter is here:http://hfsignals.blogspot.com/p/specan-reboot-of-w7zoi.htmlUsing a symmetric design with solenoids for the input 70MHz filter isn't ideal either. It could be made much better with a few subtle changes to that filter to improve the stopband performance up into UHF.

author: cdev
author_rank: Super Contributor
timestamp: November 10, 2018, 09:44:26 pm
content: That looks like a great project.

author: sab
author_rank: Contributor
timestamp: November 11, 2018, 05:40:31 am
content: So, I followed the advice (single sided board, short leads, no unnecessary pads) from many generous people and, lo and behold, things certainly improved. 6.5db insertion loss and a pretty sharp frequency response (using my BG7TBL noise source which is at -10dbm). I've included some images.Thank you very much for all your help.Sanjay

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 11, 2018, 11:20:48 am
content: A bit lopsided, looks like one of the resonators is high or low?Tim

author: cdev
author_rank: Super Contributor
timestamp: November 11, 2018, 02:10:10 pm
content: You shouldn't assume the top and bottom of the PCB are both grounded unless you have lots and lots of vias between them. So many it looks like a checkerboard of vias. More around the traces.Otherwise they are a capacitor/inductor combination, not a ground.

author: David Hess
author_rank: Super Contributor
timestamp: November 12, 2018, 03:32:55 am
content: Quote from: sab on November 07, 2018, 08:41:24 pmBuild a helical resonator instead. 112 MHz is right in the range where this is very practical. The one shown below is for 2 meters and has a tuning range exceeding 144 to 148 MHz. The coupling window shown is way too large but suitable for higher power (coupling loss in this case is less than 0.3dB which is not ideal for a narrow filter) and covering a good part of the entire 2 meter band. When I designed and build it, I did not have good information on how large to make the coupling window.Alternatively, use better (higher Q) capacitors and piston trimmers or air capacitors for adjusting the sections.

author: sab
author_rank: Contributor
timestamp: November 14, 2018, 07:23:52 pm
content: Yes, a helical resonator looks quite promising from everything I've read.Thanks,