is there a correlation between a filter passband width and drift characteristics - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 01, 2014, 04:52:27 pm
content: So as far as I understand a bandpass unity gain filter is not really unity gain (although it is very close), and component drift/aging will somewhat change this gain, in a miniscule amount.Is there a correlation to the pass band width and the effect of component drift ? For some reason I have a gut feeling that a wider band filter will be less sensitive, but I don't know if this is true, or if its even universal between topologies (mfb/sallen key/biquad/voodoo), so I don't even know where to start.I am not interested in the initial conditions, as the filter is in an input stage that later is finely corrected digitally, I only care about the stability of the initial conditions. Is there anything to it other then using low drift parts?if i made a bw 1000 and a bw 100 filter using same drift ppm parts

author: David Hess
author_rank: Super Contributor
timestamp: May 02, 2014, 12:15:29 am
content: Higher Q filters and filters with higher Q sections are more sensitive to tolerances if only because the passband is smaller or the cutoff is more critical so a shift in frequency is more significant.

author: LvW
author_rank: Frequent Contributor
timestamp: May 02, 2014, 05:36:59 pm
content: It is a well-known fact that the sensitivity of filters against parts tolerances primarily is determined by the chosen filter topology.As you know, there are many different topologies for realizing filters.As far as I know, the topology that is superior to all others (as far as tolerance sensitivity is concerned) is the double-terminated ladder structure, which can be realized passive or active (FDNR technique).

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 02, 2014, 05:59:27 pm
content: Quote from: LvW on May 02, 2014, 05:36:59 pmso the evil gyrator rears its ugly head again...the only problem is that those FDNR circuits seem to have a bad reputation for being cranky and the fact that there are no good design tools for them.if Ti made a tool I would try them.I was familiar with the advantages of a zero DC offset FDNR low pass filter. I did not know that the bandpass or high pass filters also offer superior performance against temperature.

author: David Hess
author_rank: Super Contributor
timestamp: May 02, 2014, 07:59:48 pm
content: Quote from: LvW on May 02, 2014, 05:36:59 pmWhich explains why it was popular for high order antialiasing filters in digital audio before oversampling converters became available. The other filter topologies were just way to sensitive at high orders.Quote from: SArepairman on May 02, 2014, 05:59:27 pmI remember Burr-Brown publishing detailed application notes on FDNR filter design.

author: LvW
author_rank: Frequent Contributor
timestamp: May 02, 2014, 08:03:35 pm
content: Quote from: SArepairman on May 02, 2014, 05:59:27 pmI think, "filter solutions" supports FDNR structures.

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 02, 2014, 08:50:36 pm
content: ok as soon as i setup a non networked computer I will have filter solutionscannot trust the russians that much...;.im surprised all the linear component manufacturers don't just buy out all these filter programs and make some awesome free filter suite so everyone and their grandma can make awesome analog filter designs.. i mean they spend several billion on making some nice op amps but then you need to spend several billion hours studying stuff you don't like to use them right... its almost like they want analog to be inaccessible voodoo, Ti is on the right track with their free design program, but its still pretty damn lack luster when you consider the amount of $ they spend on semiconductors.... some of those chips must be a billion in up front costsedit: we are in business and i must SAY DAMN it feels good to be a internet gangsta and save $ !!!!!by this i mean i wrote my own program in russian that mimics filter solutionsso which of the filters most stable?BiquadsAkerberg-Mossberg Biquads,Positive Gain SAB's (Sallen & Key),Negative Gain SAB's (MFB),GIC,Leap Frog,and Parallel Filters.I assume the GIC is the FDNR type of filter? but I read leap frog is good too... what should I go for ultimate stability?GIC ladder or leapfrog?

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 03, 2014, 01:14:41 am
content: I am really having trouble believing that ladder is better for high order filter structures because well, it uses four times as many op amps.can the design of a 4th order BPF implemented in the GIC ladder configuration (8 opas) really beat out a BPF implemented with 2 op amps in the sallen key 4th order configuration?this website does a conversion from 2 opamps to 1 op amphttp://www.millertechinc.com/pdf_files/MTI%20TN094%20Zero%20DC%20Offset%20LPF%20and%20the%20D%20Element.htmso, the program gives me these daisy chainedso I take the values that the program gives me for the Z's,.pop them in herethen set the output of that equation equal tohttp://www.millertechinc.com/pdf_files/MTI%20TN094%20Zero%20DC%20Offset%20LPF%20and%20the%20D%20Element_files/image010.gifthen choose a nice capacitor valuethen solve the equation for Rfso i have thishttp://www.millertechinc.com/pdf_files/MTI%20TN094%20Zero%20DC%20Offset%20LPF%20and%20the%20D%20Element_files/image009.gifthen I can make a chain of those, instead of a chain of the original, and it should work fine?how does the RF resistor size effect the circuit? how much noise from a large resistor will transfer into the output of my D element filter?I assume it is the resistor noise bandwidth high pass filtered by the capacitor.how much better is it likely to be? I mean, it doubles my op-amp count (assuming my idea works), is the drift at least twice as low? its kind of expensive to implement...

author: LvW
author_rank: Frequent Contributor
timestamp: May 03, 2014, 08:24:06 am
content: Quotes SArepairman:so which of the filters most stable?BiquadsAkerberg-Mossberg Biquads,Positive Gain SAB's (Sallen & Key),Negative Gain SAB's (MFB),GIC,Leap Frog,and Parallel Filters.Question: What do you mean with "stable"? Stable (a) against passive tolerances or (b) against active opamp caused tolerances or (c) against unwanted phase shifts within the loop (i.e. against tendency to oscillate)?I am really having trouble believing that ladder is better for high order filter structures because well, it uses four times as many op amps.I think, the question "which topology is better ?" is the most challenging problem a filter designer has to solve.What means "better"? In most of all cases he has to find a trade-off between several conflicting requiremnents: Number of opamps, type of active elements (opamp, OTA, CC), sensitivity against passive and active tolerances, sensitivity to parasitics, tuning requirements, input/output impedances, economical aspects (cost), power conditions,.....Summary: The §best" choice is governed by the particular requirements for a specific application.

author: LvW
author_rank: Frequent Contributor
timestamp: May 03, 2014, 08:29:09 am
content: May I add a short comment to FDNR applications:In most cases, the FDNR is used in conjunction with the BRUTON transformation (very simple!) which is applied to a passive RLC ladder structure.This mathematical tool transforms each L into a resistor, each resistor into capacitor and each capacitor into an FDNR (to be realized with the shown 2-opamp circuit invented by Antoniou as a GIC).

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 03, 2014, 08:39:15 am
content: Quote from: LvW on May 03, 2014, 08:29:09 amgain and phase stability against component drift due to temperature is my primary concern.I don't care if it has a initial gain of -0.7623 or 1.24 and a phase shift of 44.21 or 15.93 degrees so long it stays that way. What is an active tolerance, CC and OTA?What would tuning do? Is it possible that the drift of a circuit will be enhanced if it is not tuned? Tuning sounds to me like something that is prone to drift, I would rather have it imperfect but rock solid. I can adjust phase and amplitude digitally. It could have a pass band that looks like the devils golf course for all i care*, so long its stable.http://en.wikipedia.org/wiki/Devil%27s_Golf_Course, as its only dealing with a single frequency (which will be by far the most stable thing in the system)and, is there any disadvantage to using the 1 op amp circuit rather then the two op amp circuit? as shown in my above link.http://www.millertechinc.com/pdf_files/MTI%20TN094%20Zero%20DC%20Offset%20LPF%20and%20the%20D%20Element_files/image009.gif*i had the gut feeling that a butterworth filter would experience the least drift, but this could be complete bullshitand there is no great need for phase shift linearity across frequency, i just want things to stay constant. I would put phase shift and amplitude shift with temperature at equal importance. The frequency will change in EXTREMELY small amounts with temperature, it is negligible compared to the rest of the system.

author: LvW
author_rank: Frequent Contributor
timestamp: May 03, 2014, 12:38:49 pm
content: Hi SArepairman,I must confess that I really don´t know how to answer your last post.Are you looking for a specific filter circuit topology for some specific requirements?What is the meaning of the repeatedly mentioned numbers (-0.7623 or 1.24,phase shift of 44.21 or 15.93 ) ?Quote:What is an active tolerance, CC and OTA?Are you asking for the meaning of these terms? Or do you need real values?When speaking of 1.opamp and 2-opamp circuits you certainly know that there are many different filter circuits (with 1 or 2 or even more opamps).In your first post you have mentioned a "bandpass unity gain filter" Which topology?Since you have mentioned Butterworth filters: Are you mixing lowpass approximations methods with filter topologies?Sorry for my questions - perhaps they are caused (partly) by my bad knowledge of the english language?RegardsLvW

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 03, 2014, 06:57:54 pm
content: Quote from: LvW on May 03, 2014, 12:38:49 pmwell its probably caused by me being un specific, I tend to be a bad communicatorI should describe my application better. There is a sensor that outputs a frequency that I measure the magnitude of in order to make sense of the sensors reading. There is ALOT of gain so I need to filter my sensor output before gain, other wise the amplifier willsurelybe saturated due to noise pickup on the sensor and the sensor wire. The frequency output of the sensor is EXTREMELY stable.I am trying to find which topology of filter will drift the least with temperature, because any gain drift in the filter (like due to resistor and capacitor temperature coefficient and aging) will be multiplied by alot in the gain stage. I have my my gain stage pretty damn stable using very low temperature coefficent resistors. I feel like the capacitors in my filters are the worst offenders (all my filter resistors are like 5ppm/degree C).I was trying to get at the point that I don't care about the initial phase shift or gain of the filter, because I am doing my own calibration. I don't want people to be confused and think I am interested in meeting some certain specification of the filter, I don't have a piece of paper that says "the gain of the bandpass filter must be 1 and the phase shift must be 90 degrees by using 1% resistors"So, if my gain is 20, or my gain is 0.1, I don't care, because I am going to have very fine digital gain adjustment. I can easily change my gain stage after the filter to be 1002, 908 or even 5(random numbers i made up!). So the numbers -0.752 and 1.24 were just random numbers, to show that I don't really care what happens to the amplitude of my sensor output when it goes through the filter, as there is ALOT of gain adjustment I can do later down the signal processing chain.Like wise for the phase shift, the numbers I gave you (44/15) were just random numbers that I came up with to indicate that I don't care. I know some people will try to design filters so that their output phase shift fits some kind of specification, but I can adjust the phase of the sensor output digitally very finely as well. I can adjust phase by 16 bits (360 degrees / 16bits = 0.00549 degree phase adjustment resolution)I just want a filter topology that will be the most stable in a wide temperature range since I can have a very fine calibration adjustment capability that will be stable with temperature.If the filter changes the phase and gain as the temperature changes then my finely tuned digital calibration will be useless, unless I attach a thermometer to my unit that adjusts the gain/phase with temperature, but this seems like a desperate measure.Are you asking for the meaning of these terms? Or do you need real values?I am asking what the terms mean, I never heard of them before.and by 1 or 2 op amps, I mean doing the transformation of the Antoniou GIC (5 impedance's and two op amps) into this circuithttp://www.millertechinc.com/pdf_files/MTI%20TN094%20Zero%20DC%20Offset%20LPF%20and%20the%20D%20Element.htmThe last image on that page appears to be a implementation of the Antoniou GIC with one op amp and three impedances, instead of two op amps and 5 impedances. Are these two circuits equivalent or is there a draw back from the "simplified" Antoniou circuit.

author: David Hess
author_rank: Super Contributor
timestamp: May 03, 2014, 07:07:50 pm
content: If you have independent access to the frequency source that the sensor uses, then synchronous demodulation which is how a lock-in amplifier works will solve this problem nicely without a finicky filter. If you do not have access to the reference frequency, then a phase locked loop may be used to extract it in the presence of noise for use in synchronous demodulation.

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 03, 2014, 07:08:57 pm
content: Quote from: David Hess on May 03, 2014, 07:07:50 pmah I know , I am still interested in the finicky filter though.I need to know at this point. I can't sleep at night thinking about itand besides, would you rather gain AC or DC? I have a beef with 1/f noise.Even for a lock in, the less DC gain you do the better, unless the filter is so bad that the 1/f noise high gain DC amplifier is a better trade off, which may be true. I am curious to investigate this. Unless its done digitally, which is probably better, but I don't really find that to be interesting, analog has my curiosity..I think it will be good information to have on the forum regardless, no one ever goes into detail about filters like this. I just want (to at least know depending on price) the best. I don't think Arnold Schwarzenegger ever thought about bringing his body to some kind of specification..

author: David Hess
author_rank: Super Contributor
timestamp: May 03, 2014, 08:29:16 pm
content: Quote from: SArepairman on May 03, 2014, 07:08:57 pmI have done that. Sometimes I just get up and start hacking away.QuoteThere is nothing to prevent using a bandpass filter before the lock-in amplifier to reduce the crest factor preventing overload and since all of the lock-in amplifier gain is AC, 1/f noise is well removed and will be rejected by the demodulator anyway.Page 4 of Linear Technology application note 3:http://www.linear.com/docs/4100QuoteThere are a lot of books on the subject.Another alternative I would consider is switched capacitor filters but nothing is going to work as well as a lock-in amplifier which while not being digital is still a sampled system.

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 03, 2014, 08:34:06 pm
content: Quote from: David Hess on May 03, 2014, 08:29:16 pmright, the band pass filter should still be stable, as it will effect things. The output of a lock in amplifier DC will be similar to the RMS of the "buried" AC input IIRC (from some app note written by stanford research, meaning if your initial signal is not gained and only band pass filtered, you will have a extremely low level DC, which will probably need a fair bit of gain for your ADC.I think it would be best to filter it, amplify it so your noise peaks are almost the full scale of the multiplier input, then while your signal of interest is still buried under noise your locked in signal will be of a higher value and less DC gain is required, so your 1/F noise is less. Maybe it is better to make some kind of super low noise DC gain stage using parallel LT1028's or such instead of messing with a band pass pre filter and AC gain for a lock in amplifier. It all depends on the filter drift, as the AC gain stage could be made extremely stable like the DC gain stage, and it would be more simple too as 1/f noise and DC offset is not such a problem on AC coupled AC signals.*I think the app note you linked is doing just that on page 4. Later on I would assume the circuit designer to add a DC gain stage to set the full scale to whatever he desires.any recommended reading?but in the end I feel that the only way to evaluate it is to design the most stable bandpass filter possible and then compare its drift + ac gain to the best DC gain stage achievable. Hopefully LvW has the answers, however, no matter what, you will have lower final DC noise in the signal the more AC gain you have, even if there is a temperature gain nonlinearity.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 03, 2014, 09:19:13 pm
content: What frequency? Do have have a choice in frequency?High Q filters are just by nature sensitive, and especially annoying at low (audio) frequency. Some topologies are worse than others, but they are all bad.++ on using a lock-in amplifier. If you absolutely need to, a simple low-Q bandpass filter (Q~10) is usually enough. A digital lock-in rarely even needs a pre-filter, and doesn't suffer from 1/f noise because the baseband processing is all digital. If you don't have a phase reference, you can also use a DSP filter, which again won't need a pre-filter if it has a good ADC.If you signal is higher in frequency, you could use a crystal filter, which would be extremely high Q and very stable.QuoteLock-in amplifiers generally have some (potential a lot) of built-in gain for exactly this reason! For instance the SR830 from stanford research can do 1 volt / 2 nanovolt (although that won't be useful unless your time constants are very long: 100s of seconds or more usually).

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 03, 2014, 09:25:43 pm
content: do you know which simple Q filter topology is the best? The sallen key filter I was going to use as a pre filter for the AC gain was actually only a Q of 7 or so, so we are on the same page. I am not expecting that much noise close to my output frequency.or if LvW is right, how much better the ladder GIC filter is then say a sallen key?do you think that LT spice (or some free spice program thats better?) will give me useful comparison results, or should I build both of them and throw them in a oven?=as a refresher the program I have lets me design these:BiquadsAkerberg-Mossberg Biquads,Positive Gain SAB's (Sallen & Key),Negative Gain SAB's (MFB),GIC (ladder),Leap Frog,and Parallel Filters.so for stability @ low Q (<10), what is king?leap frog filters are really op-amp consuming, GIC are somewhat more op amp consuming... I'd like to at least have a hunch of what to do before I put in a 50$ digikey precision resistor/capacitor order and etch boards to compare them.I would also have to hope that the toplogies do not develop demons as they grow big, for example it would be a nightmare to find out that the leap frog filter is better until order >4, then the GIC is better... Its too expensive to figure that out for me experimentally.

author: LvW
author_rank: Frequent Contributor
timestamp: May 03, 2014, 09:54:22 pm
content: Quote from: SArepairman on May 03, 2014, 09:25:43 pmHi SArepairman,in my reply post#8 I have mentioned some aspects regarding the "best" filter.With the aim to approach an answer to your problem I must state that the most important information from your side are still missing:*Center frequency*Bandwidth*Ripple within the passband?*Order of filter (sideband attenuation)

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 03, 2014, 09:59:07 pm
content: Quote from: LvW on May 03, 2014, 09:54:22 pmhello LvWcenter frequency is 50khz, Q would be 5-10, ripple in passband I don't think I care about (as there is only one un changing frequency), and I think that higher orders are better but I don't really have a set requirement.Can you offer some "regime" options for different filter orders? I guess I need to know how much the drift changes vs filter order, so I can pick some kind of happy middle ground.a higher Q would be better, but, again, I don't know how to optimize between Q and drift. I know lower Q is better for drift, but I have no equation to go by.

author: fcb
author_rank: Super Contributor
timestamp: May 03, 2014, 10:35:57 pm
content: Is it possible to occasionally switch the front end of your system to (precision voltage reference derived) oscillator and perform a calibration of your filter every X minutes or hours?

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 03, 2014, 11:04:35 pm
content: Quote from: fcb on May 03, 2014, 10:35:57 pmoh yea that is defiantly on the table. I have a DDS driven by a VREF that I could use exactly for this purpose. But still, in addition to this, I would also like to chose an excellent filter design.i will def implement this.

author: LvW
author_rank: Frequent Contributor
timestamp: May 04, 2014, 08:37:21 am
content: Let me summarize the filter specification:* Bandpass with center frequencyFc=50 kHz*Q=5...10(can you give a reason for this requirement?)Because you didn´t give a certain bandwidth BW I assume you wantBW=Fc/(5...10)* To me, this implies that you speak about a bandpass ofsecondorder (lowest order), otherwise the bandwidth wouldn´t be given via Q, correct?

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 04, 2014, 08:57:26 am
content: Quote from: LvW on May 04, 2014, 08:37:21 amwell the reason for this requirement is that people are saying don't make Q high or drift is high, but I don't know the numbers! it just seems good. I can't give you a "good" reason until I know some kind of general proportion, like, an equation of PPM drift / degree C vs Q factor.and I thought Q was unrelated to order? I thought order just means how many DB per decade of decay you have?i calculatedFo/bandwidth = qand order = how many db of loss per decade in the non pass bands, unrelated to Q ?I would like Q and order to be infinite... (or i guess it doesnt matter if Q is infinite lol)again, to choose order I would need to have some kind of knowledgeorder 2 = x ppm/ degree corder 4 = z ppm/ degree C********MADE UP NUMBER EXAMPLE STATEMENTS AHEAD********I know that you cannot give me exact numbers, but even knowing something like "well a order of 2 will probably drift x ppm degree C while a Q of 4 will drift two orders of magnitude higher PPM degree C" and "a sallen key filter has ~3 orders of magnitude more drift then a GIC ladder filter using equal PPM/degree C parts" would help me make a educated decision.oh yea, and I am also interested in phase shift, so the same goes, "filter x has 0.0000x degree phase shift per degree C while filter Y might have...."do you think simulating this stuff in a SPICE program would give me meaningful results?I can't tell you much because I don't even know if the differences between the filter drift due to Q, order and topology is orders of magnitudes or minor differences that I would not even care about. for all I know the difference is so small that I don't care, or so big that my current design is absolutely retarded, people seem to think there is a fair bit of difference, but again without at least some basic proportions im flying blind

author: fcb
author_rank: Super Contributor
timestamp: May 04, 2014, 03:21:44 pm
content: Any (analogue) filter characteristics are going to be determined by the R/C/L values - these are going to be subject to temp & age based effects. The exact effect can be determined by modelling it (LTSpice etc) - I can't think why any analogue filter topology would be more or less affected by this drift?If it really matters then you can use ultra low tempco parts, regulate or compensate for temperature, try switched capacitor filters, mix the outputs of couple of filters with complimentary topologies, swap between two channels (calibrate one whilst using the other), etc...No magic bullet.

author: LvW
author_rank: Frequent Contributor
timestamp: May 04, 2014, 03:54:10 pm
content: Quote from: fcb on May 04, 2014, 03:21:44 pmYes - it is a good and helpful advice to use suitable simulation hardware for further inverstigations.On the other hand, I must repeat again that some filter topologies are more sensitive to component variations (aging, tolerances, temperature) than other topologies.In this respect, it has been shown that the passive RLC structures (double terminated) are best.@SArepairman:Some remarks regarding QThe usage of the quality factor Q makes sense for 2nd order filters (lowpass, bandpass) only because this figure Q is derived from the position of a pole pair in the complex s-plane. That means: Filter Q is identical to the pole Q.For bandpass functions with an order larger than "2" (more than one single pole pair) it makes no sense to use such a figure. Instead, we define bandwidth and filter order independently.Example: A very sophisticated bandpass with Fc=50 kHz, a bandwidth of 5 kHz and rather steep transitions to the stop bands (higher order transfer function) would be described with Q=10 only. Such a figure doesn´t say anything about selectivity (as is the case for a second order function).

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 04, 2014, 06:13:40 pm
content: Quote from: LvW on May 04, 2014, 03:54:10 pmwhat simulation software would you recommend? LT spice? how close do you think my results will be to real life?

author: LvW
author_rank: Frequent Contributor
timestamp: May 04, 2014, 07:40:00 pm
content: Quote from: SArepairman on May 04, 2014, 06:13:40 pmI work with PSpice - but each of the known simulation packages will do the job (including LTSpice).More important are the MODELS which are used to simulate the circuit - in particular the temperature properties.Regarding the filter topology (active, passive, filter order,..) I cannot recommend anything to you because the specification is rather unclear.As far as small temerature sensitivities are concerned, a simple RC bandpass (without inductors) would be best, for my opinion. However, the selectivity is rather poor.

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 04, 2014, 10:46:12 pm
content: Quote from: LvW on May 04, 2014, 07:40:00 pmwell i cannot make a specification if I don't know the trade offs between Q and order and topology. I need at least a super vague rule of thumb. I want an active filter of course, but I cannot pick Q, order and active filter topology because I have no idea how much different drifts will be, if I tell you I want a order 6 filter with a Q of 20 and it turns out that an order 4 Q 10 filter is 15 times more stable then I will feel like an idiot.The better the filter is the more gain I could have, I need some kind of equation to determine the "sweet spot" between the interference reduction and gain. I have absolutely no proportions to work with, so I cannot make a specification.can we compare a order 4 salen key band pass filter with a Q of 7 filter to a order 4 GIC-ladder filter with a Q of 7? that seems like it would work reasonably well.and how much worse would order 8 equivalents of the same filters be? anything would help. if you tell me "double, triple, some where between 2-10, order of magnitude, half, 0.1," it would all help me make a decision .

author: LvW
author_rank: Frequent Contributor
timestamp: May 05, 2014, 09:13:54 am
content: Quote from: SArepairman on May 04, 2014, 10:46:12 pm1.)SArepairan, I have a general question: It seems that you need a bandpass filter.But WHY? There must be something in the usable frequency band you want to suppress.If you have such a desire you must be able to sayWHICH part of the spectrum should be suppresed by how many dB. Otherwise you do not need any filter at all.And these requirements lead immediately to a specification which defines filter order and path bandwidth.2.) I recommend to forget about Q at all. Why?Because I have tried to explain that a Q value can be allocated to asecond-orderfunction only. It makes no sense to speak about a 4th order filter with a Q of "7".3.) There are no "trade offs between Q and order and topology" and also no "super vague rule of thumb".The only rough information I can give is to use a filter as simple as possible (but not simpler).4.) But why do you think that an ACTIVE filter is needed? Up to now I was of the opinion that you don´t care about gain/attenuation of the filter?

author: qno
author_rank: Frequent Contributor
timestamp: May 05, 2014, 09:34:00 am
content: An old trick is to use different types of capacitors. with different tempco's.If you need say 100 pF you use 47 pf with a positive tempco of 1 ppm/°C and a 47 pf with a negative tempco of 1 ppm/°C.Obviously the tempco is never the same so you have to adjust the capacitor value accordingly.This way they used to stabelize FM radio recievers of 100 MHz to within a few 100 Hz

author: fcb
author_rank: Super Contributor
timestamp: May 05, 2014, 11:28:26 am
content: Quote from: SArepairman on May 04, 2014, 06:13:40 pmLTSpice, because it's free and sorted.How close to real life? Depends entirely on the accuracy of your models - but VERY close. And the nice thing is you can do thermal simulations.

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 06, 2014, 01:49:21 am
content: Quote from: LvW on May 05, 2014, 09:13:54 amI don't understand, again, what you mean with Q. Why can it only be allocated to a second order? I am sorry it is frustrating to understand.I was taught in school thatOrder 1 = 20db/decadeorder 2 = 40db/decadeorder 3 = 60db/decadeorder 4 = 80db/decadeorder n = 20*n db/decadeAnd, that Q isSo, if I have 50 KHz Fo, and a 7 KHz pass band (the area between the 3db points of High and Low pass)50000/7000 = 7.142 = QSo, why does it not make sense to say I have a 4th order filter with a Q of 7 and a 2nd order filter with a 8 of seven???The pass band will look the same, but the slope is steeper. We must have a different definition of Q.http://upload.wikimedia.org/wikipedia/en/thumb/9/94/Roll-off_graph_multiple.svg/800px-Roll-off_graph_multiple.svg.pngWith all those orders the pass band width is the same, or almost the same. The exact location of the 3db point changes I guess, but the variation in usable bandwdith is pretty small, so I disregard it, and treat all three of those curves to have the same bandwidth., maybe this is a bad "primitive way" to look at it.but anyway, my demands for an active filter is a damn good question, I don't know the answer, perhaps I should just put a RC CR chain, that should work and its the most stable tempco right?I had it in my mind attenuating the already small signal seemed like blasphemy. I need to quickly run the numbers to see how much attenuation I will have from RC CR network, it may actually be a viable option, I never ran the numbers, I assumed that there would really be alot of loss. I also wanted to avoid inductors and keep the gain AROUND unity, like 0.8 or 1.2 is ok, but 0.1 or 0.05 is not ok due to gain constraints in my design. \... or is it...... hmmmmmmmso long my gain is not undoing my filters work, I guess for some reason I just had it in my head that active is better.

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 06, 2014, 07:24:21 am
content: has anyone here measured the gain/phase drift of a active bandpass filter in PPM?I am just going to put my original salen key filter and my modified design GIC filter on little pcb and do a basic temperature drift test.. , i will post results soon enough, i can draw this out in MS paint , etch and assemble it faster then dealing with thisdamnSPICE program, i have a nice quad op amp test subject right here and a e96 resistor booki will get to the bottom of this madness.. unfortunately I don't have a good method of measuring phase drift.

author: LvW
author_rank: Frequent Contributor
timestamp: May 06, 2014, 07:36:40 am
content: Quote from: SArepairman on May 06, 2014, 01:49:21 amYes - it seems so. The definion you have given applies to a second-order bandpass (example: RLC tank) only. There is another definion of the Q value which is generally applicable to lowpaas, highpass, bandpass and bandstop filter - all of second-order: Q is defined using the pole position in the complex s-plane (Qp=1/(2*cos(phi)), with phi=angle between the negative real axis and the connection line to the pole). And it turns out that for a bandpass we have Qp=Q, which means: The above definition of Qp gives the same value as the definition as given by you. But this applies to second-order only because the pole pair used for the definition of Qp belongs to a second-order function.Quote from: LvW on May 05, 2014, 09:13:54 amYes - as I have mentioned already: A simple RC-CR connection has a bandpass response (pure selectivity), however, because its simplicity and because no inductors are used it has very good temperature properties. And if you - at the same time - are using different tempco´s (as proposed earlier in this thread) it should work for your application. Don´t worry about gain: Such a filter will have a gain which only is slightly below unity (depending on the bandwidth, which is determined by the difference of both corner frequencies).

author: LvW
author_rank: Frequent Contributor
timestamp: May 06, 2014, 07:39:20 am
content: Quote from: SArepairman on May 06, 2014, 07:24:21 amFor further discussions it is perhaps helpful if you could post the circuit diagrams for both filters?