Designing a practical RF filter (1.8 MHz high pass) using online calculator? - Page 1
Forum section: electronics / rf-microwave

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 16, 2021, 08:42:25 pm
content: I'm trying to build a high pass filter or bandpass filter to reduce strength of AM broadcast signals entering a homebrew 160 meter/1.8 MHz ham band receiver. The filter would be prior to the first gain stage, so passband insertion loss is a consideration.I've tried two different designs (the first was a bandpass filter, the second a high pass filter, both seeking to differentiate between AM broadcast band and 160 meter/1.8 MHz ham band signals) using the LC filter calculator at rf-tools.com, and my (as-built) measured filters have far more gradual transitions from stop to pass band than the transmission response suggested by the web page's calculator. The 5-pole bandpass filter had minimum (in-band) insertion loss of over 20 dB as if it's high pass and low pass properties badly overlap; the high pass filter has insertion loss of over 20 dB at 1.8 MHz, about 25 dB at 1.7 MHz; an undesirably gentle transition for a 9-pole filter. The filters have 50 ohm in/out impedance and I'm using a nanovna to evaluate the realized filters.My question is this - is it useful/practical to specify and realize a RF filter from a web page calculator?Thanks for any suggestions

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 16, 2021, 08:59:00 pm
content: What impedance is your antenna and amplifier?At all frequencies I mean, not just whatever they're tuned to. That being the key fact: if they're tuned, they'renot flat 50 ohms!If they aren't the flat 50 ohms assumed by the filter prototype, you're going to have poor results.Solutions:- Add attenuators to ballast the impedance(s) closer to 50 ohms. (Obviously, bad here.)- Equalize the impedances, cancelling or dampening out-of-band reactances or resonances. Doable, may be tedious. May require a network analyzer.- Construct a constant-impedance or diplexing filter. This is effectively the above option, with the stop band explicitly linked to a port, rather than to resistors spread within the network.- Tune the filter to the antenna and receiver, straight up. May not be possible (you can't exactly filter that which is already reactive), in which case you'll effectively end up with some mix of the above. Probably needs a NA. Best case, measure s11 and s22, dump 'em into a simulator and solve for the inbetweeeny-bits. Kind of the enterprise-grade solution here, but not actually as intimidating as it sounds.Tim

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 16, 2021, 09:08:06 pm
content: Thanks for the quick reply!I haven't yet tried the filters on-air; at this point the terminating ports of the filter(s) are what as the nanovna ports present (which is to say they're intended for use in 50 ohm environment, and certainly are not the real-world behavior of an antenna as you point out.) Even in that setting the results are very far from expectation to the point where I wonder whether an online filter calculator is even a valid starting point.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 16, 2021, 09:29:47 pm
content: Oh okay, testing the filter as is.What type did you build, and with what components?If you chose a narrow (high Q) bandpass, you may find accidental impedance transformations or other effects, due to stray capacitance or inductance.In which, the series resonant tanks can be especially tricky. In which case, a transformation to all parallel resonators can be profitable. Which should be higher Z for reasonable (component) Q, so you end up needing impedance transformers coupling into them in the first place. Usually C dividers, taps on the coils (esp. if hand wound), or magnetic coupling. This sort of topology:http://www.jrmagnetics.com/rf/doubtune/doubccl_c.php(Note that the C coupling (Cc) between resonators, skews the response, giving a shallower HF asymptote. Magnetic coupling (mutual inductance) gives a symmetrical response, and inductive coupling (a very large inductor instead of a very small capacitor) gives a shallower LF asymptote. Same as a typical crystal filter, where the crystals are series resonators this time, alternating with shunt C's, giving a shallower HF asymptote for the same reason.)Tim

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 16, 2021, 10:05:51 pm
content: The bandpass filter is Chebyshev "direct coupled, series capacitor", order 3, lower cutoff 1.8 MHz, upper cutoff 2.0 MHz, passband ripple 3dB, direct coupled inductor value 4.7 uH. The inductors are Coilcraft 1008CS (SMD, wirewound); the capacitors are garden-variety NPO ceramic 0805 package.The highpass filter is Chebyshev, "shunt first", order 9, cutoff frequency 1.8 MHz, passband ripple 3 dB. The inductors are Coilcraft 1008CS (SMD, wirewound); the capacitors are garden-variety NPO ceramic 0805 package.The inductors (type 1008CS) have a series DC resistance of 2-4 ohms (depending on value used) per the datasheet and measured here.In both cases, the tool suggests in-band insertion loss of 0-3 dB (per the ripple spec); this is somewhat suspect to me as I can't imagine the filter can do it's job at any frequency with 0 dB insertion loss. The actual insertion loss in both cases is 20 dB or more. Component values were 5-10% tolerance (measured with a LC meter before assembly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 16, 2021, 11:48:37 pm
content: Oh OK, so that is a coupled-resonators style. Now, the passband itself is 10% (0.2MHz out of 2MHz) or a system Q of 10, and I suspect the resonator Qs are much higher than that to get the tight Chebyshev response. C4 for example is around 61 ohms reactance, which means your 2-4 ohm inductors give a maximum Q of only 30.If nothing else I think you'll find the response is a lot softer than expected, due to losses. A less aggressive filter may behave better, or you can use larger and/or cored inductors for higher Q.Component Q is the Q factor of a component, by itself, at some reference frequency (here, the cutoff or band-center frequency). System Q is the Q factor of that component, embedded in the system (between source, network and load), and is lower than component Q. System Q can never be higher than component Q; you could think of component Q as the limiting case, for a system of 0 or \$\infty \Omega\$.What happens to a filter, due to component Q?- As long as it's many times higher than system Q, the components behave near ideal, insertion loss is low, and frequency response is close to predicted.- As component Q falls, system Q starts to fall a little bit. "Real Q" so to speak, goes something like the parallel combination of intended system Q and component Q, i.e., Qr = 1 / (1/Qs + 1/Qc). Insertion loss is degraded, passband slopes off a bit (for a one-pass, it sags around Fc; for band-pass, it sags either side of Fc), and peaks soften. Some tuning can still be done to address the sag -- imagine an ideal filter that's over-peaked at the edges than in the middle of the pass band; after adding in representative losses, the edges sag down to meet the middle. Insertion loss suffers a bit, but you get the response you wanted at least.- For Qc ~ Qs, the response sags badly, to the point that no amount of tuning can recover the sag at the edges, or the sharpness. Either settle for less (softer transition band (lower system Q required), or high insertion loss), or get better components!Note the 1008CS-472XJEC only boasts a Q of 20 at 25MHz. Q generally goes as ~F to ~sqrt(F), so we should expect it to be pretty useless at 2MHz.Coilcraft provides models for all their RF parts; you can run them in a simulator to see something closer to expected. Huh, y'know there's several online (web based) simulators out there, but I don't know offhand which if any support embedding SPICE models. Maybe Multisim?As for the high pass: 9th order, holey moley that's aggressive! ~1uH at 2MHz is 12.5 ohms reactance, and you need huge Q for something this sharp (maybe 100 or more?). Like, take the width of the last passband peak, about 1.8 to 1.9MHz, that's at least a system Q of 21. So a component Q of several times that, is needed to get acceptable insertion loss.It can be done -- but you need very nice inductors to build these, and they need to be precise to get the passband exactly so. Hand wound cored or coreless inductors are the way to go here. Use litz if possible.You may consider a lower order, softer (lower dB or Butterworth) cutoff type to relax the required Q, and add a zero or two to the stopband, particularly if you know of some strategic frequencies to knock out. (The calculated version is the Cauer/Elliptic, which it looks like a -25 to -30dB stopband puts a zero on the edge of the AM BCB. I'm a bit skeptical of the values calculated for the highpass version: alas, it may still be just as demanding on quality and precision.)Note the tool specifically does not model losses, all it's doing is converting standard filter tables (ideal L and C calculated at normalized i.e. omega = 1, Z = 1 conditions) proportional to the parameters entered; and using a few standard transforms to implement the other types (direct-coupled and tubular).And, consider relaxing your attenuation requirements -- completely blotting out the interference (>80dB att.?), is less practical than just getting them within your receiver's dynamic range. I assume that's what the problem is, it's not that they're in their usual places and just annoying to tune past, it's that they're blowing out its range and splatting images everywhere?Further out there, is improving the front end altogether. Not exactly something for a novice ham -- I mean more along the lines of, if this is a home-built receiver, it would be a good improvement project. I don't know which case applies here, just putting that out there.Tim

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 17, 2021, 12:08:43 am
content: Tim this is an excellent (!) crash course/overview!I have some studying to do for sure, but this provides a starting direction.Thank you so much,Dave

author: iMo
author_rank: Super Contributor
timestamp: February 17, 2021, 10:00:31 am
content: You may compare your design with an another RF designer (free) here, for example:http://iowahills.com/9RFFiltersPage.htmlThere is an option to define the tolerances of your L and C and provide an Worst case analysis..

author: Co6aka
author_rank: Unknown
timestamp: February 17, 2021, 12:22:02 pm
content: Change your cutoff frequency (the -3dB point, remember) to at least 1.7MHz, or better 1.6MHz. Don't use SMD inductors in filters in this frequency range; wind your own on ferrite toroids, and 73-mix would probably be optimum. Use high-Q capacitors; SMD are OK here but higher-Q leaded parts (with preferable higher voltage ratings**) are abundant. Due to the wavelengths here it's not necessary to get small and fancy; a housing with some solderable standoffs will be fine. Reactances and impedances of what you connect to your filter will affect its "tuning" slightly, and the RX input of your radio is probably everything but 50 Ohms resistive (which you can see with the VNA.)**Remember that anything connected to a large HF antenna WILL BE subjected to a significant amount of energy, so design with robust components to reduce the possibility of poppencorken mit spitzensparken und blownenfusen.

author: A.Z.
author_rank: Super Contributor
timestamp: February 17, 2021, 01:09:01 pm
content: hello, you already got some good replies, so I'm going to just add my two cents herefirst of all, most/all calculators are useful to get you in the ballpark and have a start model, then as usual, the real thing may (and will) differ from the ideal assumed by the calculators (the same goes for antenna modeling using NEC stuff), that said, I don't know which tool you used, but I found this onehttps://rf-tools.com/lc-filter/to be pretty good, so if you used a different one, you may try it to see if it concurs with the values you have; then, the coilcraft inductors, assuming RX applications, are quite good, so if you already have them, I believe you may keep using it, just check the datasheet to ensure it will work at the desired frequencyAnd finally, as already pointed out, you should ensure to keep the input/output impedance if the filter at or near the design values, that may be obtained in a number of ways, but it's important to ensure that the filter will work as designed

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 17, 2021, 01:22:59 pm
content: Very good and thank you for the new replies (!).I had planned today to wind toroids for the inductors; I don't have any 73 mix but do have 43 mix and powered iron -2, might have one other lower frequency range powered iron but haven't checked the junkbox.I am indeed using the rf-tools.com lc filter design page you cite; that's what got me started down this path in the first place. Unfortunately they don't have an option to specify Q of the parts, but they do give an option to export data to qucs or perhaps other simulator, but that's going to be another learning curve.Learning so far - Q doesn't just impact the in-band insertion loss, but also impacts the shape of the filter, making it "lazy" (poor shape factor) for lack of a better term. Wish I had paid attention in BSEE school but that was almost half century back and 8-bit microprocessors were emerging.As for transient discharge - I do have some GDTs here in junkbox, appreciate the reminder.As for capacitors - was thinking try some of those transmitting ceramics from Johanson (S42E series, 1111 case) however during this initial testing I'm focusing first on the inductors to see if I can get some improvement there. dipped silver mica are problematic to source these days.Also - my primary problem needing help is primarily 50 kW at 1690 kHz about 1.5 kM away from here, so I am happy to compromise on physical size. For this reason, I am going to focus on high pass filter at this point, not band pass. If I can realize sufficiently sharp transition between 1.7 and 1.8 MHz with insertion loss reasonable (<6 dB @ 1.8 MHz) then adding simple low pass filter for image rejection (high IF) will probably be negligible impact; I do not need sharp rejection above 2.0 MHz.

author: A.Z.
author_rank: Super Contributor
timestamp: February 17, 2021, 01:50:28 pm
content: Had a look at the coilcraft inductor you're using, if I'm not wrong it's this onehttps://www.coilcraft.com/getmedia/9b59774c-05d4-40c5-99c9-417bb8e881ea/1008cs.pdfnow, if it's the "822" it may be ok for the taskhttps://www.coilcraft.com/en-us/products/rf/ceramic-core-chip-inductors/1008-(2520)/1008cs/1008cs-822/that said, since you're building the filter for a homebrew receiver, you may consider the idea of putting a pair of buffer stages (JFET or BJT) will do at the input/output of the filter so that it will see the design impedance (ok, almost) and won't be affected from impedance variations from the antenna, such a thing may ease the filter design a bit, just remember that the input buffer shouldn't offer gain (or just a little one) and should be designed to be quite resilient to overload/IMD; a simple common gate JFET stage may probably fit well and allow the filter to see the design impedance

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 17, 2021, 02:17:32 pm
content: Narrowing the focus to high pass filter only at this point - I'm using 1008CS 910 nh and 1.2+0.1 nh (as the HPF recipe called for 900 and 1300 nh, and I don't have 1300 nh in the old designer's kit I'm drawing from.) I also used 4700 nh from the same kit for the bandpass filter, but that effort is shelved until I get at least some rudimentary handle on filter shape vs Q type issues.As for common gate buffer or low gain amplifier to isolate between antenna and filter for impedance matching --- I've been focusing so far on benchtop testing (using a nanovna) until I get to the point where a filter has even the potential of being useful. If I can get a promising candidate as seen in a network analyzer sweep, then antenna/filter isolation would be a logical next step.

author: A.Z.
author_rank: Super Contributor
timestamp: February 17, 2021, 02:25:10 pm
content: Quote from: wb0gaz on February 17, 2021, 02:17:32 pmI see, well, mine was just a suggestion to solve the impedance "issue" and allow the filter to work as designed, nothing else; as for the filter, did you consider starting "simpler" ? I mean, you may use that online tool to design a simpler highpass filter and then test it, at that point, once it will work as expected it won't be difficult to add a lowpass filter (if needed), here's an example (attached), that filter is an highpass with a cutoff calculated for 0.8MHz and should fit the bill

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 17, 2021, 02:34:47 pm
content: Yes you read my mind (!) - I was planning on reducing to a 2- or 3-pole high pass filter and working to get that implementation "sufficiently" close to the expected behavior of a design from the rf-tools.com web designer. Just comparing insertion behavior with a single 1008CS inductor in a simplified filter and a same-value hand wound toroid inductor in the same position should provide some insight.I suspect the plots/data generated at rf-tools.com assume ideal components (how the plots are generated on that page's designer doesn't seem to be described anywhere on their design page.)Also in response to another prior post in this topic (thanks!) - I did download and briefly try (as in "can I install and run on current laptop) the iowa hills tool (as well as the old filter design tool from AADE); I've not tried to use either of those for design at this point.

author: A.Z.
author_rank: Super Contributor
timestamp: February 17, 2021, 02:45:41 pm
content: Quote from: wb0gaz on February 17, 2021, 02:34:47 pmQuoteYes, I believe that starting simple is a good idea, there's always time to add complexity (if needed) also and since we're at it, if you're planning to add input/output buffering to the filter, you may probably want to design it for the impedances it will really see which may be different from the 50 Ohms from the direct antenna input, so consider this too

author: mankan
author_rank: Regular Contributor
timestamp: February 17, 2021, 05:48:32 pm
content: Building something similar to this might work:https://www.sv1afn.com/en/product-category-4/1-8-mhz-high-pass-filter-1500-w.html. Since your case is receive only you can use smaller inductor cores and lower voltage specced capacitors.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 17, 2021, 06:14:11 pm
content: That looks promising - schematic on page (!).The idea of using toroids is a good hint.Thank you!

author: radiolistener
author_rank: Super Contributor
timestamp: February 17, 2021, 06:38:54 pm
content: Quote from: wb0gaz on February 17, 2021, 01:22:59 pmtry this one:

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 17, 2021, 06:44:50 pm
content: Close as possible to the receiver input port/mixer?

author: radiolistener
author_rank: Super Contributor
timestamp: February 17, 2021, 06:50:55 pm
content: Quote from: wb0gaz on February 17, 2021, 06:44:50 pmyes

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 17, 2021, 06:57:35 pm
content: Well, anywhere along the line, assuming it's well-behaved coax.In a one-pass filter, you can convert any branch into one of those, to add a notch to the response -- there is some re-tuning of course -- or as I said before, add a zero. In this case, a few-pole highpass with one or two notches might prove best.Tuning the filter itself is a bit annoying, though easier for few poles; tuning the zeroes is pretty easy though.Tim

author: Co6aka
author_rank: Unknown
timestamp: February 17, 2021, 11:18:37 pm
content: Quote from: wb0gaz on February 17, 2021, 01:22:59 pmOr, you could try a 1/4-wave stub cut for 1690kHz.Only about 100 feet if the VF is 0.66 or so - solid dielectric. Actually, anyone ever try this on 160M? How sharp, or broad, with which coax?Gonna need a notch too because a LPF alone won't cut it enough for 1690. Slightly complicates the design. Do the calculators have the ability to add notches? They don't have to be exactly at 1690; could be slightly lower and still deliver enough attenuation.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 18, 2021, 04:57:46 am
content: One problem with designing 50 ohm LC filters for medium & low frequencies is that the DC resistance of high value inductors starts to become an appreciable proportion of the design impedance, compromising the Q of your finished design.I had a project to build something similar, back in the mid 1980s, from then available filter formulas.All good on paper, but when I built & swept it with a 50 ohm sweeper, it looked nothing like the required response.At the time, a little more research revealed that filters for this frequency range usually were preceded, & followed by, impedance matching sections, so that the Z of the actual filter section was much higher than R.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 18, 2021, 08:32:50 pm
content: Back to this project after some local distractions --- I've reduced the problem to a 3-pole Chebyshev 1.8 MHz shunt-first high pass filter. Insertion loss at/around 1.8 MHz is about 10 db; there is a big dip ("suck out?") around 4.1 MHz, then insertion loss continues to improve, less than 1 dB at about 4.7 MHz. Built two versions - one using Coilcraft 1008CS inductors, the other using hand-wound T50-2 toroids; the toroid-based version is modestly better, but both exhibit the same general behavior including the non-monotonic insertion loss property.PS I appreciate the suggestions for methods to notch out 1.69 MHz; as the broadcast band includes other strong sources (1.69 just being the closest and highest in frequency), I'm seeking high pass property (or band pass, however, I've shelved that until I can get some handle on high pass.)Dave

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2021, 12:09:46 am
content: FYI, I did some playing around in the simulator, and figure that a bandpass with notch is probably maybe the easiest way to go, requiring three inductors and a smattering of capacitors (100p-1.5nF). With component Q of at least 80, insertion loss can be a few dB, flat 1.8-2MHz (or a bit beyond), and a good notch of 30dB or so around 1.6-1.7MHz. Takes a lot of tuning though.You'll probably have to settle for some relatively high amount of attenuation at 1.8MHz, and simply more attenuation by 1.6MHz -- you're not going to get both, clearly in pass/stop bands, just riding a slope from one to the other. It's just too close.Anyway, attenuation shouldn't be a big problem in this range -- atmospheric noise dominates. I'm guessing you have a large antenna (hence the wide bandwidth?), so the receiver sensitivity doesn't need to be all that much to pull in nothing but noise. The difference can be spent on a more compact antenna (hence why AM BCB rx's can have tiny loopsticks), or say, a filter that's relatively lossy at in the desired band.I recall a similar situation in the 2m band, what was it, there's a satellite band, ham band, and other things, and one or the other needs to be selected or rejected very close together (a few MHz -- so, similar Q factors involved). At least that's high enough that a helical resonator isn't hard to make.Tim

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 19, 2021, 04:14:41 pm
content: Thanks Tim et al again for the detailed reply/info!I have some good progress to report (finally!).I switched from the online LC filter design tool at rf-tools.com to the downloaded filter design application from Iowa Hills software; this program lets me specify inductor/capacitor parasitic properties. A 5-pole 1.8 MHz Chebyshev HPF (whose component values differ significantly between Iowa Hills and rf-tools.com) was built from the Iowa Hills design and tested using nanovna; the insertion properties are reasonably close *provided* that I specify an inductor Q of 10 rather than ~30 as from Coilcraft datasheet. I did not adjust the series resistance spec for the capacitors (I'm using garden variety 0805 ceramic capacitors of unknown spec).This is reasonably as expected, compared with filter built from the rf-tools.com circuit (which for the same basic parameters less parasitics, has significantly differing component values and produced strongly non-monotonic result.)Although the insertion loss difference between 1.80 and 1.69 MHz is modest (about 6 db?) and the insertion loss at 1.8 MHz is higher than I'd like (about 6 dB), the overall roll-off headed lower in frequency tracks the Iowa Hills design and transmission somewhat above the transition frequency has insertion loss of less than 2 dB, and the passband ripple is not far from design criteria (1 dB is the largest allowed passband ripple spec in Iowa Hills software, although I'd accept more in trade for sharper roll-off.)Next step is to try the same build with toroid inductors (wound on T50-2 cores) and then with see what impact the capacitors have (I don't have a way to measure series resistance of the capacitors, but do have a few 1111-package capacitors with specifications.)In response to a posting in the thread --- receive signal levels at 1.8 MHz are not as high as might otherwise be the case - the receive antenna is a varactor tuned loop based on Chris Trask design (about 10M circumference) and it's about 20 dB down from a more full-sized wire antenna (the loop improves SNR at the expense of absolute level, so losses in the rest of the receive path are a consideration.) A preamp stage was tried (HMC480 MMIC) but it's gain (about 20 dB) results in something in the receive chain going into saturation and being unusable. Not going to work on that issue until I'm further along with the filter effort.Thanks again for all of the help/advice/learning (it's the learning here that's the most valuable to me - I can buy some commercial product, but I'm much more interested in learning about filter design/implementation, even though it's been 45 years since circuit analysis 101.)Dave

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2021, 08:49:51 pm
content: Ahhh hmm, tuned loop. Depending on how big, that might interface rather poorly with a filter; but also may provide more selectivity all by itself?The article itself seems pretty good:https://manualzz.com/doc/13400124/a-varactor-tuned-indoor-loop-antenna(The original / source URL seems to have fallen offline.) However, it does not discuss distortion -- at least, any further than noting it's not recommended for transmission.Now, varactors are fine with ~mV of signal level, but ~V might introduce a shift in tuning, and IMD. Which might for example, cause images to pop up on the upper side of the band (1.9-2.4MHz?), which would be some mixture of in-band signals with the out-of-band / interfering carrier.And, evidently it's got enough signal that it's somewhere <20dB below the receiver's dynamic range limit. (Or something else is fucky, like the MMIC oscillating, something to check I suppose, but, assuming it's okay.)So if we assume the receiver is blowing out ca. 1V, then that's 0.1V into the MMIC, and if the loop has a Q of 10 or more, that may easily be >1V across the varactor.So let's see if we can calculate some things...What other parameters did you use for the antenna, conductor diameter, tuning network components, values, turns ratios?Tim

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 19, 2021, 09:11:21 pm
content: Much more progress now with 5 pole Chebyshev high pass filter with 1.8 MHz cutoff:Two successive component-type modifications both had helpful effects - from Coilcraft 1008CS SMD inductors to T50-2 toroid inductors, then from no-name chinese 0805 package ceramic capacitors to Johanson S42E (1111 package) capacitors. In all three scenarios, insertion loss well below transition remained about the same (@ 1.3 MHz, about 28 dB loss in all three configurations), however, insertion loss above transition improved (@ 2.0 MHz, from 6.2 dB with 1008CS to 1.2 dB with T50-2 toroid inductors, then to 0.3 dB with S42E type capacitors), and the transition modestly improved (sharper) with each change (@ 1690 vs 1800 kHz, 3.6 dB with 1008CS, 4.9 dB with T50-2 inductors, and 5.2 dB with S42E type capacitors) . Need connect the nanovna to PC so I can capture data.Before changing design (increase poles and experimenting with notch filter), I'm going to wire it into the receiver antenna path and see how it affects receiving result - there are certainly impedance mismatches there...And Tim - regarding the loop --- the loop follows the Chris Trask paper design (I've built a number of these over the years since I found his article online, and the loops are clearly useful receiving adjuct). I made some component changes to his design, but basic concept is not changed (target minimum frequency ~1.7 MHz), loop dimensions (30' circumference RG6), and available components - T1 (the 4:1 impedance balanced autotransformer is on a T68-2 toroid core), T2 and T3 (both use a small SMD mounted RF transformer rather than binocular cores and hand wound transformers), and there are four 1SV149 varactors in parallel for each D1 (shown twice). I've not measured RF voltages anywhere yet. It does provide some selectivity that helps with the strong local AM BC stations. I haven't tried to compare it to any other reduced-size receive antenna to this point.Thanks again for all of the help/advice/suggestions --- this has been a learning experience!Dave

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 20, 2021, 02:34:52 am
content: Seeing as there was large difference going from garden variety 0805 ceramics to Johanson S42E 1111 package, going to use the existing circuit (5-pole Chebyshev HPF 1.8 MHz, 1 dB max ripple, series C, shunt L, populated with toroid inductors wound on T50-2 cores) to informally evaluate two other types of caps in the junkbox here.Tonight substituted (for the Johanson S42E types) 1812 NP0 caps (1000 pF = Kemet C1812C102KDGAC, Q unspecified, ESR = "extremely low"), and 560 pF (no part number, but also 1812 package). These yielded results (insertion loss @ 1300, 1690, 1800, 1840 and 2000 kHz) within 0.1 dB (always to the good) vs. the Johanson S42E (1000 pF & 560 pF).Tomorrow will evaluate the other junkbox option - 12065A-331-JAT2A and 12061A-122-JAT2A 1206 NP0 (AVX parts Q>1000, ESR unspecified, 330 and 1200 pF respectively).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2021, 03:47:48 am
content: C0G are generally quite good; though as you say, those unknown Chinese parts might be anything. (I wonder how much they can cheap out on metallization, before the thing no longer works at all?)KEMET has their info available here,https://ksim3.kemet.com/capacitor-simulationsays that one is 143mΩ ESR at 2MHz (Q ~ 560), more than good enough for here.Tim

author: ogden
author_rank: Super Contributor
timestamp: February 20, 2021, 08:22:53 am
content: I would rather build bandpass preselector because most likely there are hi-freq offenders as well, FM stations and random interferers. Bandpass will allow option of external LNA.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 20, 2021, 02:16:53 pm
content: Thank you Ogden for the suggestion.I started this project out as bandpass filter, however, I was thinking later (maybe not correct, haven't tried to verify) that the sharpness needed for high pass behavior (<1.8 MHz vs. AM broadcast) is far more demanding than the low pass needed (just enough to address images and VHF sources), so once the HPF is worked out sufficiently, the current plan is to cascade the HPF with a simple 2-pole LPF whose transition frequency is high enough to minimize further insertion loss at 1.8 MHz.After this experience with HPF component selection - component Q and ESR vs. in-band insertion loss - I know now to be careful about that.Today I'll try the AVX 1206 parts in the HPF (just as a learning experiment).Dave

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 20, 2021, 05:13:29 pm
content: AVX 1206-case capacitors (part numbers cited previously) with T50-2 core inductors produced results which are very close (within few tenths dB) of the S42E (case 1111) and Kemet (case 1812) capacitors. This exhausts the available junkbox parts.Next step is to build out the 9-pole version (using T50-2 toroid core inductors and the AVX 1206-case capacitors.)

author: A.Z.
author_rank: Super Contributor
timestamp: February 20, 2021, 06:19:33 pm
content: WB0GAZ, just a question, you started this discussion by stating that the filter is for a homebrew 160m receiver, later on you added that you are using an active loop antenna, now... are you sure that your MW signals issue isn't due to the antenna preamp and/or receiver frontend overload causing IMD and, in turn, the "appearance" of those MW signals ? Because, see, if that's the case, I believe you'd better start by lowering the loop preamp gain and/or adding a bandpass filter there

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 20, 2021, 06:43:19 pm
content: A.Z. - I've been using this loop with a Kenwood TS-940 (a 1980's era transceiver which was considered higher-end in it's day and is still well regarded) for some time. The TS-940 accepts signals from the loop without apparent spurious responses with one exception (which I believe is a second harmonic harmonic from another local broadcaster), so I have not investigated the loop as a source of mix products (a previous poster noted the use of varactor diodes in this loop antenna design.) This (filter) project started as a wish to try my hand a homebrew receiver, starting with 160 meters, as my initial experiments on that front did not fare at all well in presence of AM BC signals.Update -Just finished extending the filter from 5 to 9 poles; it works somewhat better than expected (that's a change) - insertion loss across the 1.8 MHz ham band is between 1.7 and 2.0 dB; insertion loss at 1690 kHz is 17 dB, and insertion loss at 1300 kHz is 55 dB.The filter parts were moved and are now assembled and unit tested on the receiver project board (the board itself is just an upconverter/front end at this point so I can evaluate it's behavior feeding the TS-940); I'll do some on-air testing this afternoon and evening.Dave

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 20, 2021, 08:20:14 pm
content: First "on air" test (daytime):Antenna to the receiver circuitry now connected and the loop is tuned to 1690 kHz (normally the loop would be tuned to somewhere in the 160 meter ham band). Daytime (=high power mode, 10 kw) 1690 kHz AM station showing -12 to -10 dBm (up to 100 microwatts) at the input of the high pass filter. Same signal at the output of the high pass filter is -31 to -29 dBm (around one microwatt).Tonight (=lower power mode, 1kw) will compare levels with loop tuned again to the broadcaster, then with the loop tuned to the bottom end of the 160 meter ham band.

author: A.Z.
author_rank: Super Contributor
timestamp: February 20, 2021, 08:35:07 pm
content: could we see the schematic and infos of this latest filter ?

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 20, 2021, 09:03:04 pm
content: (by the way, just connected the long wire thru the tuner, peaked for 1690 kHz, and got +3 dBm on the spectrum analyzer - wish I still had a crystal radio!)I will do some screen captures from the iowa hills software and post those shortly; it includes a schematic, but in the meantime,From one port to the other, just goes like this:Shunt (2.2 uH) - #24 AWG on T50-2 coreSeries (1500 pF) - Low ESR, C0G/NP0 (see discussion in thread)Shunt (1.5 uH) - #24 AWG on T50-2 coreSeries (1500 pF)Shunt (1.5 uH)Series (1500 pF)Shunt (1.5 uH)Series (1500 pF)Shunt (2.2 uH)toroids.info can provide specs to create the needed inductances; the core/wire was based on my junkbox availability.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 21, 2021, 05:09:59 pm
content: Here are screen captures from the design software for this project, as well as the design file itself (the design file is in a ZIP archive as the file type would not attach.)The iowa hills software I used can be obtained here:http://www.iowahills.com/9RFFiltersPage.htmlhttp://www.iowahills.com/8DownloadPage.htmlTwo notes about the software -(1) If you change parameters, be sure to re-check the R Load field (far right of window); it may change in response to other parameter changes (I had to reset it to 50 ohms after making some parameter changes.)(2) You can modify the values of the generated filter by in a spreadsheet-like area below the schematic (this adjusts the plot above). I did not do this as I didn't realize the feature existed until after I finished this task. The toroid inductors I wound are all slightly lower inductance than design (about 5% low per my AADE LC meter). Use of a LC (preferably LCQ) meter is suggested to verify components before build.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: February 22, 2021, 03:06:18 am
content: Now have the 9 pole chebyshev 1.8 MHz high pass filter feeding a 2 pole butterworth 3.0 MHz low pass filter. This has cleared up the spurious signal issues, and also is letting the first gain stage (HMC480ST89 MMIC, about 20 dB gain, ~18 dB P1dB) following the filters operate without saturation. The nearby 1690 kHz AM broadcast station is no longer causing problems, and the only evidence of AM broadcast in the 160 meter band is a (probably second) harmonic from one of the stations which cannot be addressed with filtering.Attached is a screen shot from nanovna-saver of S21 from 500 kHz to 30 MHz. The passband characteristic is highly asymmetric as intended.Thanks again for the help and advice - this has been a good learning experience and the result meets original expectations.Dave

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: March 12, 2021, 04:00:35 pm
content: I use Elsie, the free version is more than adequate for filters in home brew radio kit:http://tonnesoftware.com/elsie.html

author: wb0gaz
author_rank: Frequent Contributor
timestamp: March 12, 2021, 05:08:18 pm
content: Thanks for the pointer! I'm pretty sure the original problem was assumption by the online tool I first used that the components would be ideal (q=infinity, etc.); when I return to this project I'll look up elsie.Dave

author: aristarchus
author_rank: Regular Contributor
timestamp: March 13, 2021, 01:04:05 am
content: Hi,Used thehttp://rfcooltools.com/many times.Broadband Match | S-Parameter File Plotting | Filter Synthesis | S-Parameter Bisection and Complex Rational Polynomial Fitting

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: March 13, 2021, 07:08:55 am
content: Quote from: wb0gaz on March 12, 2021, 05:08:18 pmBeen there, fallen for the “ideal Q” issue. I think Elsie defaults to realistic values, which you can change if you have more accurate data.The only other “got ya” I come across is that HF filters tend to have increased leakage as you sweep HF with your SA or VNA, resulting in a slight hump in the measured plot in the VHF region. For example, my 7th order Chebyshev low pass filters drop quite steeply to around 80dB attenuation, but then gradually rise by typically 20dB in the lower VHF region. Band pass filters have similar results. Plots using Elsie fall away completely as the frequency rises. When used to attenuate transmitter harmonics, the above leakage isn’t usually a problem as the harmonics naturally fall off at higher frequencies.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 13, 2021, 08:31:01 am
content: The zeroes and additional poles show up when you include parasitics in the design -- of course, for a relatively poor layout (which, don't feel bad: at high enough levels, that might mean, anything less than a multi-chambered metal block!), the difficulty of modeling that might range from individual component strays (e.g. a properly detailed SPICE model, or s1p file or other 1-port representation), to the coupling between them (which, good luck even with a field modeling tool? -- SOP for simpler* microwave stuff, however).*By which I mean -- imagine solving for the fields in and around spools of wire, embedded in oddly shaped ferrite and dielectric blocks, laid haphazardly on a fiberglass and copper laminate. Not all of those boundary conditions need to be modeled in detail, but some kind of model of external fields around the components is needed to solve for coupling between them; whereas, the same level of detail for a transmission line filter, is just strips of copper in the plane -- much simpler geometry to operate on, and you have all the specs to hand, you don't have to crack open and measure anything yourself.When very wide stopbands are needed, it's a good plan to add impedance stabilization (shunt R+C or series R||L) and additional poles of higher cutoff components. Tapering a filter in this way, over a few decades of component valuesand sizes-- because size is what's important for parasitics -- allows very high attenuation indeed. That, and good shielding, to keep the input and output ports away from each other in a reasonable manner.Or, well really, it's not so much stopband, as high frequencies, period. For which, lowpass is probably easier than highpass, you can just add more attenuation up there, it's fine.Highpass, every inductor needs to look like a very good bias tee at high frequencies, in order to have very flat passband. Which isn't possible in general over many decades; a component will have extra poles and zeroes up there, which need to be dampened with losses, and extended with additional higher frequency poles.The usual ultrawideband bias tee uses a geometrically tapered chain of inductors, with a bit of R in parallel with the middle ones, or R+Cs to ground. This acts as a crossover from one inductor -- and its paired resistance, which acts as termination for this sub-network -- to the next, keeping the impedance high. The composite inductor thus has dominant resistance at high frequencies, where that resistance is some factor times Zo -- the higher this is, the lower the insertion loss, and higher the return loss. So, for a high order highpass, it could add up to a lot.So heh, I guess it's no accident that highpass filters are usually simple -- DC blockers and bias tees, one or two poles. A simple inline chip cap does most of the work, and that can be made very flat by shaping the transmission line it's in.Tim

author: wb0gaz
author_rank: Frequent Contributor
timestamp: March 13, 2021, 02:46:58 pm
content: OK, thanks for keeping this thread alive - this has been invaluable tutorial from the beginning!Related question:How do I evaluate a known-value (10-2000 pF is my range of interest) SMD (mostly 0805) capacitor for its unlabeled properties, particularly ESR/Q and dielectric type (C0G/NP0 or X7R/etc.)?One of the original gotchas in my initial build attempt (after which I started this thread) was my no-name chinese ebay assortment of 0805 capacitors, which turn out to probably be mostly X7R, even at fairly low pF values (low hundreds of pF). I discovered that many of these capacitors change dramatically in value after soldering, slowly (over the course of some seconds/minutes) reverting mostly back to original value - evidently this is a property of "category 2" dielectrics (which is about as far as I've delved so far). The two changes I made after my initial build attempt boiled down to replacing the inductors (which were originally coilcraft wirewound smd) with small hand-wound toroids, and replacing the capacitors with ones of known characteristics (C0G/NP0); either of these changes made a modest improvement in filter behavior; both together made a dramatic improvement.Thanks again,Dave

author: cdev
author_rank: Super Contributor
timestamp: March 13, 2021, 07:25:37 pm
content: A small test jig using microstrip toallow testing of SMD parts in a test circuit is the way to test compnnts. I have a small testboard, purchased on ebay that has a matrix of machined sockets allowing you to test both series and shunt TH and SMD parts in an impedance appropriate environment. Its probably okay up to several hundred MHz.with regard to LNA choke inductors, we've had a number of discussions about this here. Ive always been curious about the commercial conical inductors and how they would perform. The pointed end has a very thin flying lead that inserts itself into the signal path. The reactance at that end is highest. Its interesting to read the patents..Otherwise, as you said one has to use multiple inductors to get a smooth response for your bias tee.