Signal Reconstruction - Page 1
Forum section: electronics / projects

author: Chet T16
author_rank: Frequent Contributor
timestamp: July 26, 2012, 02:40:59 pm
content: Sorry, i'm not even sure if thats the right term in the title. Once again i'm creating problems as a way to learn about solutions.Given the original signal on the bottom and the received signal on the top, how can we reconstruct the original signal.I think a comparatormightwork but i'm not sure how the timing might be affected.I found this, isthis. But i'm not sure if its suitable for this.

author: hlavac
author_rank: Frequent Contributor
timestamp: July 26, 2012, 04:42:15 pm
content: Input seems to be drifting all over the place, fixed threshold comparator will not work.I would use a derivator to detect if the signal is going up or down followed by a comparator to output high if the slope is positive and low if negative.Maybe add a bit of hysteresis to the comparator to keep the noise out and provide stability when input is idle.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 26, 2012, 04:44:29 pm
content: if its in software, i think its easy, but not sure about what hardware that can do this. +ve dv/dt = high, -ve dv/dt = low, 0 dv/dt = no change

author: hlavac
author_rank: Frequent Contributor
timestamp: July 26, 2012, 04:46:33 pm
content: There's an opamp circuit for that

author: Chet T16
author_rank: Frequent Contributor
timestamp: July 26, 2012, 05:14:07 pm
content: Is a derivator the same as a differentiator?The signal as shown is at 2400bps, if the slope could be detected a faster rate could be used.

author: IanB
author_rank: Super Contributor
timestamp: July 26, 2012, 06:06:20 pm
content: What hlavac said.Differentiate the input signal: if the slope is downwards the output is low, if the slope is upwards the output is high. If the slope is close to +/- zero the output latches where it is.The trouble with differentiating is that it takes a certain amount of time to establish a good measurement. You can't do it instantly (this is basically the uncertainty principle). So by taking the necessary time to get an accurate measure of the signal gradient you will lose resolution on the timing of the output edges. There is no good solution to this except for software post processing and interpolation.

author: Chet T16
author_rank: Frequent Contributor
timestamp: July 26, 2012, 06:10:32 pm
content: I guess I can test it and see. This isn't a real problem, just something I've made on the bench out of interest.

author: hlavac
author_rank: Frequent Contributor
timestamp: July 26, 2012, 06:19:28 pm
content: Here is acomplete simulation

author: Chet T16
author_rank: Frequent Contributor
timestamp: July 26, 2012, 06:53:21 pm
content: Quote from: hlavac on July 26, 2012, 06:19:28 pmHa, brilliantNot that i understand the rest yet but what are you doing with the two diodes at the end?

author: JackOfVA
author_rank: Unknown
timestamp: July 26, 2012, 08:06:14 pm
content: Reconstructing the signal after RC low pass filtering, or, for that matter any filtering, is sometimes called "slicing" and can be done with a comparator.the trick is setting the level of the comparator reference to avoid distortion. One traditional way to do this is to base the comparator reference on the average of the input signal. Or, take the peak upward voltage excursion and the peak downward excursion of the last few time periods and determine the mid point voltage and use that as a reference.The details of how to derive the comparator reference depend on the input waveform - will it be idle at one voltage extreme or the other for extended times? Is there a maximum number of consecutive 1's or 0's, etc.Jack

author: hlavac
author_rank: Frequent Contributor
timestamp: July 26, 2012, 08:36:20 pm
content: Quote from: Chet T16 on July 26, 2012, 06:53:21 pmThey simulate the builtin input protection diodes on the input of the inverter. Comparator outputs +15/-15V, these trim it to the +5/0V required for the gate.

author: hlavac
author_rank: Frequent Contributor
timestamp: July 26, 2012, 08:48:48 pm
content: Quote from: JackOfVA on July 26, 2012, 08:06:14 pmToo tricky. The average will be moving slowly, and with the slight slopes on the input signal you will find your edges are moving and are not where they are supposed to be, resulting in massive jitter/distortion...Thats why I suggested the differentiator/derivator design - it detects the edges correctly and where they are supposed to be, and does not depend on the shape of the input signal or its average, just the direction of change (up/down).It knows the signal is supposed to be rectangular, and knows where the edges are, so it can reconstruct the signal nicely.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 30, 2012, 10:04:06 am
content: Old style---Feed it into a bi-directional clipper,clipping it hard,so just the centre section gets through,then through agoodamplifier,so that your rise time is only slightly increased.Then,feed it into a Schmitt trigger.ORHave a look at an Oscilloscope triggering circuit,or a TV sync separators for some other ideas.

author: Chet T16
author_rank: Frequent Contributor
timestamp: August 01, 2012, 04:53:26 pm
content: Here is my output (top) and input (bottom) using hlavacs designSo, its almost there, its just not idling high