Practical Electronics for Inventors - Narrow Bandpass Filter doesn't add up - Page 1
Forum section: electronics / beginners

author: aa7ae
author_rank: Contributor
timestamp: February 21, 2016, 04:32:16 am
content: On page 679-680 of Practical Electronics for Inventors, there is a description of a narrow bandpass filter design using an op amp. The problem is, I don't know if the author was stoned when he wrote this book or if I'm just not getting it, but no matter how I slice his equations for this circuit, they don't add up. Consider the attached example for calculating resistor R1. If you do the math just as it's shown, it doesn't equal 79.6 no matter how I've tried to slice and dice this thing. 2 x pi x 2000 x .01 = 125.66, then divide that by 50 gives me 0.3978 which is not 79.6. So the next thing I assumed is you need Farad's not microfarads. 2 x pi x 2000 x .00000001 = .0001256. Divide that by 50 and I get 398089. Again, not 79.6 or even 79,600. I looked up errata on the web to see if there was an error, didn't find one for this chapter and the author didn't print what base these numbers are supposed to be, but I would assume Ohms, Farads, not Kohms or Microfarads. What am I not seeing here?

author: ataradov
author_rank: Super Contributor
timestamp: February 21, 2016, 05:53:14 am
content: "Practical Electronics for Inventors" sounds just like a thing any unrecognized overunity inventor needs.This teaches practical approach to equations - you can use arbitrary units, as long as they produce a result you want. Just don't tell anyone, huge corporations are trying to hide the truth.

author: Dave
author_rank: Super Contributor
timestamp: February 21, 2016, 06:02:19 am
content: Interestingly enough, R1comes to 397.9k, close to 400k.\( R_2 = \frac{397.9k\Omega}{2*(50)^2 -1} = 79.6\Omega \)

author: vk6zgo
author_rank: Super Contributor
timestamp: February 21, 2016, 06:15:23 am
content: It's just a plain old typo--The answer to the first formula should be "near as dammit" to 400kUsing that as the numerator in the second formula gives,again "near as dammit",79.6 Ohms.Just as inDave'sposting.Back in the day,typos weren't unusual in Maths Books,& kids would sometimes get their homework marked wrong,due to the teacher just checking against the answers in the back of the book.Typos in Text books weren't as common,but were not unknown.

author: JoeO
author_rank: Frequent Contributor
timestamp: February 21, 2016, 01:15:26 pm
content: Quote from: ataradov on February 21, 2016, 05:53:14 amAoE has its share of errors also.

author: uncle_bob
author_rank: Unknown
timestamp: February 21, 2016, 01:31:36 pm
content: HiEqually to the point, if you wire up the circuit and try it out .... the resistors that get it on frequency / right bandwidth with real capacitors are going to be further away than a few percent from the calculations (if it can be done at all).Bob

author: MrSlack
author_rank: Frequent Contributor
timestamp: February 21, 2016, 01:35:11 pm
content: They're both chock full of errors. Some of them are quite jarring.I wish someone did a decent structured content e-reader that you could actually read textbooks on and we got books delivered how we get software delivered i.e. Practical Electronics for Inventors version 2.1, 2.2, 2.3 etc. It's really not beyond the publishing industry to do this. It's not like we're using linotypes anymore.Kindle doesn't cut it before anyone mentions that...plus DRM - yuck.

author: aa7ae
author_rank: Contributor
timestamp: February 21, 2016, 06:29:50 pm
content: Quote from: uncle_bob on February 21, 2016, 01:31:36 pmOk, yes, that was going to be my next issue. I found another website,http://www.bowdenshobbycircuits.info/opamp.htmin which I did the calculations for 60 Khz because I no longer trusted the book. I ordered the specific resistors from DigiKey. When I wired the thing up on my breadboard, it didn't behave like I expected at all. Also, the circuit would go into self oscillation at something like 35 Khz. So I substituted a couple of variable resistors for R2 and R3 and started trimming the circuit up at 60 Khz. When I got the trimmers to the spot where it worked as expected, it seemed miles away from my original calculations, which left me even more confused. What good is a formula to calculate this is it's miles off when you go to use it? I used two different kinds of op-amps in my experiment, a TL081 and LM741 which both worked exactly the same in behavior that I observed with the scope.Original calculations for a Q of 50 and unity gain at 60 Khz (according to my notes):R1 = 13KR2 = 1 OhmR3 = 26KWhen I measured the trimmers after getting the expected behavior, the values were actually:R1 = 100 OhmR2 = 200 OhmR3 = 1KStill, the weird self oscillation thing happens if I push the trimmer past a certain point close to where the signal is peaking from the signal generator.The original purpose of this was to use it in a circuit to isolate the WWVB time clock signal to use for a Nixie tube clock project.

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: February 21, 2016, 07:26:50 pm
content: I'm only on page 144 of the book thus far. In the process of doing all the calculations I am currently capable of doing, I have already found at least 4 mathematical errors. I made a note on one of them, but will start flagging every one I find.Later, after I refresh my calculus abilities, I will go back and run thru those as well. I forget nearly everything about calculus - after all it has been 11 years since the last time I used it...

author: JoeO
author_rank: Frequent Contributor
timestamp: February 22, 2016, 12:54:21 am
content: I was the technical editor for a book that was written by a friend.The publisher does not give you much time to complete it. I could only do about 4 hours a day because it was mentally exhausting. And of course every day extra that the author to write the book is a day less that you have to edit the book.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 22, 2016, 01:18:48 am
content: Quote from: aa7ae on February 21, 2016, 06:29:50 pmMany of the simple Op-Amp filter circuits you will find online are designed for audio frequencies.If you simply scale them up to around 60kHz,they become more difficult to "tame".You can either go to LC tuned circuits,or search for better active filters designed for the frequencies you are interested in.

author: LvW
author_rank: Frequent Contributor
timestamp: February 22, 2016, 10:36:42 am
content: Quote from: vk6zgo on February 22, 2016, 01:18:48 amI think, it will be a problem to find "better active filters" because the critical path is not the topology of the filter but the opamp capabilities.For a pole frequency of app. 60 kHz you need opamps having a GBW of at least several MHz (10...20 MHz) and - most important - a sufficient slew rate.

author: aa7ae
author_rank: Contributor
timestamp: February 22, 2016, 09:49:21 pm
content: Quote from: LvW on February 22, 2016, 10:36:42 amAccording to what I understand about GBW, I only need a higher GBW if I'm seeking gain in the circuit. If this is a unity gain circuit, GBW should match 60 Khz. But if I want gain, GBW needs to match the gain factor, example, gain of 10, would need 600 Khz GBW. To require a 10 Mhz GBW in your example, I would need a gain of at least 166. One of the example circuits published on the web that I've considered trying, uses a 60 Khz signal going into a crystal filter op-amp design using a instrumentation op-amp, which only has a GBW of 800 Khz and a slew rate of 0.9V/us (INA118). Article on example circuit:http://www.creative-science.org.uk/MSF_crystal_filter.htmlDo I understand, or am I missing something?

author: vk6zgo
author_rank: Super Contributor
timestamp: February 23, 2016, 03:43:01 am
content: Quote from: aa7ae on February 22, 2016, 09:49:21 pmThis is very different to the active filter you started with,as all the filtering is done by the crystal.You could just connect the crystal in series with your input & get quite good filtering.The circuit relies on balancing out any "feed-through" of signal via the crystals stray capacitance,to enhance the selectivity.Apart from that,the Op-Amps are only there for impedance matching & to contribute a bit of gain.

author: aa7ae
author_rank: Contributor
timestamp: February 23, 2016, 08:59:50 pm
content: Quote from: vk6zgo on February 22, 2016, 01:18:48 amThe original circuit I was working on did not have this. This was another design I found that I thought about trying.My next question is, do I really need to use relatively expensive instrumentation op-amps, or could another substitute work just as well? I have several 60 Khz crystals I got from EBAY that I was going to use for experimentation. I already have a couple of WWVB clock receiver modules I got from pvelectronics in the UK, but I wanted to build my own WWVB receiver in order to learn something from this. The antenna I'm using is the one supplied by pvelectronics, loopstick antenna. I also have several ferrite rods I picked up fro Russia that I was going to use to make my own loopstick antenna as well. Since the op-amps are high impedance input, is there any special way to couple the loopstick antenna to the op amp?

author: uncle_bob
author_rank: Unknown
timestamp: February 25, 2016, 12:09:45 am
content: Quote from: aa7ae on February 23, 2016, 08:59:50 pmHiIf you have ever looked at how tight spec active filters are actually used in a production environment, there is *always* an alignment process involved. This is just as true at 240 Hz as it is at 2.4 KHz or 24 KHz. The same is true for narrowband L/C filers and almost all crystal filters. There are frequency limits to each technology where the alignment and performance degrade enough that you move on to something else. Learning what the limits of each technology are is part of the design process. Real world filter design is not just a plug in the numbers and buy some parts thing *if* you have tight specs.Bob

author: Cerebus
author_rank: Super Contributor
timestamp: February 27, 2016, 01:43:22 am
content: Quote from: MrSlack on February 21, 2016, 01:35:11 pmTo you I say "etaoin shrdlu".

author: MrSlack
author_rank: Frequent Contributor
timestamp: February 27, 2016, 11:16:30 am
content: Quote from: Cerebus on February 27, 2016, 01:43:22 amI'm enough of a typography geek to actually get that. Not sure if I should be ashamed or not

author: Cerebus
author_rank: Super Contributor
timestamp: February 27, 2016, 01:43:55 pm
content: Quote from: MrSlack on February 27, 2016, 11:16:30 amDepends whether you're ashamed of loving the smell of hot lead.

author: MrSlack
author_rank: Frequent Contributor
timestamp: February 27, 2016, 01:50:10 pm
content: I'm not sure anyone should love the smell of hot lead. Loose link here but my grandfather lost a couple of toes to a typograph due to pouring molten lead on his feet. You wouldn't think printing was so hazardous historically!