Professors Help Me Please - Page 1
Forum section: electronics / projects

author: Shenandoah
author_rank: Regular Contributor
timestamp: July 09, 2013, 05:01:01 pm
content: HeellooHow can I apply the same circuit on a signal multiple times without having to cascade the same circuit? For instance I want to use the differentiating op amp circuit to get the 4th derivative? Is there anything like this in the EE world?Thanks.

author: Paul Price
author_rank: Super Contributor
timestamp: July 09, 2013, 05:38:24 pm
content: Do you offer a schematic and some waveforms in and out, frequency, precision of results, etc?

author: Shenandoah
author_rank: Regular Contributor
timestamp: July 09, 2013, 05:44:35 pm
content: Just a simple differentiating amp circuit.

author: JackOfVA
author_rank: Unknown
timestamp: July 09, 2013, 05:54:02 pm
content: Differentiators are similar to high pass filters and hence emphasize noise, so by the time you cascade four of them you may find the output has more noise than useful fourth derivative. All depends on the S/N of the input signal and a few other factors, but it must be kept in mind.

author: KTP
author_rank: Frequent Contributor
timestamp: July 09, 2013, 06:10:21 pm
content: I suppose *maybe* you could use a delay line for some types of signals to feed back the output of the circuit to the input where the length of the delay is longer than the time it takes for the circuit to reach steady state? Might work for some short pulses or something?

author: Shenandoah
author_rank: Regular Contributor
timestamp: July 09, 2013, 06:46:05 pm
content: Thanks...damn noise!Is there a circuit that does derivatives but without using noisy OpAmps? Only using transistors?

author: edavid
author_rank: Super Contributor
timestamp: July 09, 2013, 06:52:19 pm
content: Quote from: Shenandoah on July 09, 2013, 06:46:05 pmNo one said anything about the op amps being noisy.Can you give an example of a real world signal, where taking the 4th derivative would be meaningful?

author: Shenandoah
author_rank: Regular Contributor
timestamp: July 09, 2013, 07:18:48 pm
content: QuoteSo they don't make noise, all ideal? Excellent.QuoteI cannot call something meaningless just because I cannot do it. O0Oh differential equations???

author: CodyShaw
author_rank: Contributor
timestamp: July 09, 2013, 07:39:22 pm
content: Quote from: Shenandoah on July 09, 2013, 07:18:48 pmUm, are you trying to make some sort of hardware calculus machine?I think edavid was asking what type of conventional electronics system would require a 4th derivative of a specific signal.

author: madshaman
author_rank: Frequent Contributor
timestamp: July 09, 2013, 07:44:42 pm
content: You can't really do what you want without storing your output (either digitally in a micro, or as discrete stored charges as in a line delay device) and feeding it back through your network and recapturing it.I'd go with the micro because you'd probably need one to drive the capture/resend stuff if you used a line delay.If you're going to do that (digitise your signal) you might as well do the math discretely in a micro and not even bother with any analog other than the ADC/DAC hookups and trimming.

author: Shenandoah
author_rank: Regular Contributor
timestamp: July 09, 2013, 10:08:48 pm
content: I see...but now I'm thinking the OpAmp has to retire and we should find a better approach to analog signal processing.

author: KTP
author_rank: Frequent Contributor
timestamp: July 09, 2013, 10:16:35 pm
content: Quote from: Shenandoah on July 09, 2013, 10:08:48 pmthe faster digital processors get and with sampling rates increasing in DACs, I think the better approach in some cases to analog signal processing is DSP

author: westfw
author_rank: Super Contributor
timestamp: July 09, 2013, 10:50:34 pm
content: QuoteIf you convert your signal to digital, you can easily do calculus on it. Look for a book on "Numerical Methods."I don't recall there being any way to do a forth derivative other than running a differentiation algorithm four times, but the last time I did any of that was a very long time ago.(Whether you can do this in "real time" or not is dependent on ... lots of factors. Back when I was learning things, we only cared about doing it in "affordable time.")

author: w2aew
author_rank: Super Contributor
timestamp: July 10, 2013, 12:18:05 am
content: Quote from: Shenandoah on July 09, 2013, 10:08:48 pm...not likely...But, if you have an aversion to op amps and want to use transistors, then simply use four high pass passive filters, with simple single transistor buffers between each stage.

author: GK
author_rank: Super Contributor
timestamp: July 10, 2013, 12:36:19 am
content: The standard technique in the days of analog computers was to always avoid differentiation where possible and reformulate your mathematical model in terms of integral equations and solve with integrator stages instead.See chapter 1, here:http://www.analogmuseum.org/library/handbook_of_analog_computation.pdfWarning: 24Mb file.

author: Shenandoah
author_rank: Regular Contributor
timestamp: July 10, 2013, 02:32:58 pm
content: Very interesting book, from the good old vintage time.I could go with DSP but the point is I want to see if the effect of repetitive application of some circuit to a signal is possible, without having to cascade the same circuit, by using some sort of feedback...unfortunately I could not find anything like this.

author: IanB
author_rank: Super Contributor
timestamp: July 10, 2013, 03:16:32 pm
content: Quote from: Shenandoah on July 10, 2013, 02:32:58 pmLook up super regenerative radio receiver.

author: jpb
author_rank: Super Contributor
timestamp: July 10, 2013, 03:38:49 pm
content: GK is right about differentiation - avoid it at all costs. Even doing it numerically with extended precision. Curves which look wonderfully smooth look horribly wiggly when differentiated from measured data.Your main question is about feeding back a signal without mixing it up with the original. For a continuous signal this must be impossible because you can't use the same circuit for two different signals at the same time.But if you can time slice your signal so it is in a finite packet and have some sort of packet switch arrangement with a delay line then you might be able to contrive a circuit but it would be a lot more complicated than just repeating the circuit.The fourth derivative of sin(w.t) is (w^4).sin(w.t) so if you can filter out each frequency of interest separately you need only weight them by a fixed amount and then recombine them (digitally I suppose you could use an FFT though you'd need fast processing and a slow signal)!

author: ejeffrey
author_rank: Super Contributor
timestamp: July 10, 2013, 04:15:52 pm
content: You can't really 'reuse the same circuit' 4 times, at least not in a way that is simpler than adding extra stages. You may be able to make an opamp filter that is a second order derivative with a single opamp. Take a look at the Sallen-Key and the multiple feedback filter topologies and see if they can be adapted to do a second derivative.The noise concerns still remain.