Increasing the Q of a home made band pass filter for LF - Page 1
Forum section: electronics / rf-microwave

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 03, 2018, 09:39:43 am
content: I am about to try and increase the Q of my home brew BPF for RX on2200 meters. Are polypropylene capacitors good for this application? Ihave been using film caps and wire ended miniature chokes,(the ones that look like 1/4W resistors)so I haveordered some #26 mix cores in 1.3" OD hoping I can get enough turns onthem. I have the required caps in polyester film but am wondering ifpolypropylene are both different and better. I need to achieve circa616nF so have been using 47nF and 560nF in parallel. As physical sizedoesn't really matter I was also wondering if part of the parallelingcould practically be done with multi gang air spaced variable caps soI can trim the thing a useful amount to center it where I want it. Theschematic is athttp://www.chriswilson.tv/filter.jpgThanks.

author: Wolfgang
author_rank: Super Contributor
timestamp: October 03, 2018, 09:49:09 am
content: Hi Chris,the culprits for a low Q are definitely the cheap inductors. Those have a Q of ca. 50. What you need are large, if possible air core coils like in an antenna tuner. The second best idea are large iron powder toroids (Q about 200 possible).

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 03, 2018, 10:06:44 am
content: Wow, fast reply Wolgang, thanks a lot! I think an air cored inductor of 680uH might be a bit TOO big, but I haven't looked at any software to simulate its dimensions i have to admit. I think these #26 iron dust toroids might be a reasonable improvement, what do you think, the specs are attached.

author: Wolfgang
author_rank: Super Contributor
timestamp: October 03, 2018, 11:32:16 am
content: Yeah, why not. Amidon is also popular with radio amateurs, if you want to have a look.Dont forget to measure them and to provide trimmer caps for the fine tuning.

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 03, 2018, 11:59:33 am
content: When you say trimmer caps this is where I am coming from with the multi gang air variables, I don't think trimmer caps come much bigger than about 600pF which will probably make sod all difference across 600 odd nF, but the ability to fine tune would be ideal. I suppose I can add / remove discreet polypropylene caps to get pretty darned close though? I appreciate your time, thanks Wolfgang.

author: Wolfgang
author_rank: Super Contributor
timestamp: October 03, 2018, 12:15:20 pm
content: Its OK. This is who I do it:- Compute everything until it makes sense in a simulator. You have done that.- Wind a toroid and measure inductance. If its off (ferrites have large tolerances) add or subtract a few windings until you are 5% off target or less).- Now the caps. Get standard value caps that leave below 300pF the intended capacitance. Styroflex (PP) are good, stable and have low loss.- Add a variable cap with 500pF or more to fine tune the rest.- Assemble the complete filter and (if you have) use a VNA to tune it to perfection.

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 03, 2018, 12:23:32 pm
content: You have given me some good ideas, thanks. In fact in the meantime I have calculated air cored coils need not be THAT massive and presumably have even less losses? Great forum, thanks for sharing your knowledge. I have a wide band pro noise source and access to a decent (HP8568B) SA, but the SA won't go down that far, so i have used the noise source into my SDR and use the spectrum display to check the bandwidth, best I can do "down there"

author: Wolfgang
author_rank: Super Contributor
timestamp: October 03, 2018, 12:30:53 pm
content: Well, the idea with the noise source is a valid approach.I also tried to work around with noise sources for filters and amps, like here:https://electronicprojectsforfun.wordpress.com/making-noise/a-noise-figure-measurement-solution-for-the-poor/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2018, 02:07:54 pm
content: Mix #26 has a Q of 8 at that frequency, it's hopeless.You need possibly #18 but something between that and #2 (in terms of permeability) will be best.Otherwise, ferrite shape cores can do a fine job at that frequency.You may need Litz wire also.Yes, polypropylene caps are good.Don't forget that filters only work when impedance-matched. The output impedance of your PA is quite low, and this needs to be taken into account. Your antenna is probably far from 50 ohms as well (if you've tuned it to 50 ohms at 136kHz, it may be okay strictly at 136kHz, but the filter needs to do its job at frequencies around there, too, so basically the matching network and antenna are all part of the filter). (If neither of these cases is true, then that leaves using this filter between amplifier stages I guess, which is a much easier task to match up.)Tim

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 03, 2018, 02:16:11 pm
content: It's solely for receive, not for TXI want to get a narrow RX pass band between 135 and 139 kHz Thanks for the core tips!

author: cj
author_rank: Contributor
timestamp: October 04, 2018, 05:57:17 am
content: Another way to increase Q is to use a Q-multiplier circuit. This is an active circuit using positive feedback to compensate for losses in the LC circuit and therefore increases Q. This means you don’t need to optimize coil and capacitor for best Q.If you make the positive feedback adjustable you’ll have adjustable bandwidth but too much feedback will cause the circuit to oscillate.A Q-multiplier works like a charm, I’m using and adjustable one on my SAQ (17.2kHz) receive converter.CJ

author: HB9EVI
author_rank: Frequent Contributor
timestamp: October 04, 2018, 09:19:36 am
content: Since it's for rx only, did you already think about an active filter as addon to a basic passive filter? 2200m frequency is still complying quite well with many inexpensive opamp specs; you could likely get a sharper BPF like that

author: in3otd
author_rank: Contributor
timestamp: October 05, 2018, 07:24:57 am
content: you can check the Q curves athttps://cdn-reichelt.de/documents/datenblatt/B400/Amidon%23AMI.pdf: from there it seems you can get quite a good Q using a T68-15 and 0.2 mm wire, see chart at the top of page 10. Still, you'll need to wind 200 turns so some patience will be needed.IMHO, you should try to see if a different filter topology which gives lower values in inductors is suitable, the ELSIE SW should allow experimenting with that.