single pot variable frequency bandpass / resistor programmable active filter - Page 1
Forum section: electronics / projects

author: cybermaus
author_rank: Frequent Contributor
timestamp: January 22, 2014, 11:00:36 pm
content: Hi allanyone has either a simple schematic for a single pot (or at best a single dual-pot) 2nd order active bandpass filter, the pot to vary frequency from about 10KHz to 120KHz ultrasound.Or alternatives to theLTC1562 "Active RC Quad Universal Filter".That IC is actually pretty ideal for what I want, except it is a quad pack, I only need the one.(oh, and it is a 0.65mm pitch, my poor eyes and fat fingers already scream in frustration)I actually already found some on ebay, so if no-one has alternatives, I may go with that chip anyway.It is to test ultrasonic cleaning tanks for faulty piezo's. I build a device that worked perfectly on the sample cleaner I had which worked at about 30KHz, so I put in a filter from 25000 to 35000 and then I tried it on a 2nd cleaner and it turned out that worked on a combination wave of 12KHz and 100KHz. So I think I need a variable filter.ThanksMaurits

author: papo
author_rank: Regular Contributor
timestamp: January 23, 2014, 02:58:42 am
content: HelloI don't know anything about ultrasonic cleaners. That being said, the easiest way to achieve this, save from a single-chip-solution, is probably aSallen-Key topology. However with just a dual pot you won't get a constant Q. Do you want a constant Q? If so, astate-variable filtermight do the trick. That link gives a nice overview on active filter topologies by the way, maybe there's something else in there that suits your needs better in case what I suggest does not fit.For a more precise answer, you probably have to provide additional characteristics of the filter.RegardsMatt

author: cybermaus
author_rank: Frequent Contributor
timestamp: January 23, 2014, 07:57:19 am
content: So you do not know of chips similar to LTC1562 but in a more convenient package?---Papo : Well, I do not know much about ultrasonic cleaners myself as well. But I know more today then yesterday. (Both illustrated my my silly now rejected assumption they all work on about the same frequency). (Actually, the faulty assumption was that the chinglish manual provided actual info and was not copy/pasted from another manual)I do not think I need a constant Q. In fact, I do not think I need any specific characteristics other then "roughly filter out overtones and noise". The most important characteristics is probably "easy to build". The only frequencies I *must* filter out are any inducted 50Hz line and ringing from the switched power supply (up in the MHz area). So a single wide band filter may well work.But its a semi-hobby project, so I want to see if I can do it nicer. That State-Variable link you provided is interesting. I'll breadboard that just to play with, and while I asked for single pot, I sort of expected that a single dual-gang may be the best I could get (short of the purpose build chip solution)Variable Q may seem unwanted, but if I take that LTC1562 chip: If I only vary R2 and not Rq then indeed Q would vary, but in such a way that it results in a constant f-delta. Which is pretty cool too.---Anyway: so no-one known of chips similar to LTC1562 but in a more convenient package?

author: papo
author_rank: Regular Contributor
timestamp: January 23, 2014, 09:33:57 pm
content: Quote from: cybermaus on January 23, 2014, 07:57:19 amIsn't the LTC1562 also produced in a PDIP package? Not sure how it would get more convenient than that. There's quite a bunch of other chips out there but I don't know of one with just a single filter in that frequency range. (Which does not mean it does not exist!)Quote from: cybermaus on January 23, 2014, 07:57:19 amOut of curiosity, how does one operate these things? Apparently the filtered waveform is some sort of input to the cleaner. But what's the input to the filter? I really rarely ever used ultrasonic cleaners, but whenever I had to, these things sort of were integrated pieces of chemistry lab equipment; there was no need to fiddle with filters. I guess I'm missing something.Quote from: cybermaus on January 23, 2014, 07:57:19 amThere's always variations where you use a voltage input to set the center frequency. You could set that voltage with a single pot. You can have a look at the VCA610 datasheet to get an idea on how to achieve that.Quote from: cybermaus on January 23, 2014, 07:57:19 amYeah the choice of your filter topology really depends on what your requirements are. A nice way to experiment with all of that is to build it up and check the characteristics. Which seems to be what you are doing

author: cybermaus
author_rank: Frequent Contributor
timestamp: January 23, 2014, 10:30:48 pm
content: Quote from: papo on January 23, 2014, 09:33:57 pmSpecsheet says yes. But internet/ebay/digikey/conrad says no, PDIP does not exists.Quote from: papo on January 23, 2014, 09:33:57 pmIt's not to operate, it to diagnose. Or at least to triage if shutting down, opening up and proper diagnostics is called for. This is for a big multi-piezo industrial device.Quote from: papo on January 23, 2014, 09:33:57 pmYes, and the State Variable Filter is working nice. Only you need to carefully ensure gain and Q are low enough. Because even if the band-output is well within rail, if you put Q to narrow Low and High output smash into the rail quite quickly and cause distortion on the band output.

author: papo
author_rank: Regular Contributor
timestamp: January 23, 2014, 11:13:28 pm
content: Quote from: cybermaus on January 23, 2014, 10:30:48 pmOh I see. I thought you could order from LT directly but maybe I'm mistaken. For what it's worth, I'm ordering stuff once to twice a month and I could easily add that TSSOP chip to the order, solder it on a breakout and mail it to you. The last order went out at the beginning of this week though, so it's going to take a bit.Also I think LT has similar chips in SOIC packages, that might be easier to solder. I didn't check but I don't remember having seen single-filter chips, though.Quote from: cybermaus on January 23, 2014, 10:30:48 pmThanks for clarifying. It makes sense I guess.Quote from: cybermaus on January 23, 2014, 10:30:48 pmCool, sounds like you figured it out then.RegardsMatt

author: cybermaus
author_rank: Frequent Contributor
timestamp: January 23, 2014, 11:46:18 pm
content: Quote from: papo on January 23, 2014, 11:13:28 pmOoh, nice offer. Careful man, people may abuse that kindness.But no thanks. While impossibly small, I do have some solder paste and a hot-air gun, so I should be able to get past that hurdle. I already ordered some SSOP to DIP breakout mini-boards.

author: papo
author_rank: Regular Contributor
timestamp: January 24, 2014, 12:03:17 am
content: Quote from: cybermaus on January 23, 2014, 11:46:18 pmI don't think I would let that happen.Quote from: cybermaus on January 23, 2014, 11:46:18 pmIt's not that hard, really. Even with an iron, flux and magnification for inspection it's a breeze once you figured it out. If you go for the hot air method, in my opinion the trickiest part is to apply the proper amount of paste. Since these chips are not the cheapest, you might want to experiment with some 'dummy' chips or something before you go for the real deal. Personally I prefer the iron to the paste and hot air method but everyone has his/her own strategy.Good luckMatt

author: uoficowboy
author_rank: Regular Contributor
timestamp: January 24, 2014, 08:57:10 pm
content: Quote from: cybermaus on January 23, 2014, 11:46:18 pmPersonally, I find it significantly easier to solder TSSOPs with a pencil iron, solder, and lots of flux, than to use hot air and paste. But that's just me.

author: jwm
author_rank: Regular Contributor
timestamp: January 27, 2014, 09:34:50 am
content: Quote from: papo on January 24, 2014, 12:03:17 amYou could always use my dead simple and safe method for prototyping with SMD, just use two small magnets to sandwich the chip onto a breakout board. I used it for voltage references that wouldn't take a hand soldering well and it works great and doesn't hurt the chip for actual mounting on a real board later.I have pictures here:http://notanumber.net/archives/142/simple-reusable-zif-socket-smd-partsJohn

author: Unknown
author_rank: Unknown
timestamp: January 27, 2014, 12:12:59 pm
content: Quote from: jwm on January 27, 2014, 09:34:50 amThat's bloody brilliant!I've been pondering how to do something like that, and had resigned myself to coughing up ridiculous amounts for a couple each of various SOIC /SSOP ZIF sockets. Now all I've got do is find what those damned micro neodymium magnets I bought a while ago have stuck themselves to...