How to combine multiple AC signals over DC line? - Page 1
Forum section: electronics / projects

author: antixK
author_rank: Newbie
timestamp: November 27, 2015, 05:41:11 am
content: Hey,I am involved in a project in which I need to combine multiple AC signals (output from a Wien bridge oscillator) with a 24V DC line one-by-one. Thus, the voltage measured at the DC line must have an oscillating signal at 24V at the frequency generated by the oscillator. I need a method to implement this.Basically, the following figure explains what I want exactly. The circuit which I require is shown by "?"Things I have already tried:1) Resistive T network - I got good results, but the DC level was reduced due to the voltage divider (Note that the DC line will power the subsequent oscillator, so, the DC level should not reduce much)2) RC circuit - No output3) LR circuit - A Large inductor was connected in series with the 24V DC line at the beginning and a resistor R, (R< |XL|) was used to combine. This method also did not yield satisfactory results.Kindly suggest me a failproof method to do this. I heard about coupling transformers used for this purpose, May I know how that is done? If there is any error in my methods kindly point it out and how to rectify them.Any help appreciated.Thank you

author: IconicPCB
author_rank: Super Contributor
timestamp: November 27, 2015, 06:05:09 am
content: Sketch the signals that need to be combined and then show the end result of combining them.This is to help understand the problem.Youmight want to show this in a spread sheet format so that there is no confusion.

author: Ian.M
author_rank: Super Contributor
timestamp: November 27, 2015, 06:49:42 am
content: Key parameters that need to be known:* Frequency or frequency range of each oscillator* Desired amplitude of each signal superimposed on the line* DC current, or current range in the line.* Impedance of the 24V source at DC and at the frequency ranges above* Maximum source impedance that the device eventually supplied by the line will tolerate.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 27, 2015, 11:25:54 am
content: If you don't mind all the AC on the line being presented to each driver, then transformers can be good.The equivalent circuit might be, voltage sources driving transformer primary windings; secondaries connected in series, with the DC source on one side, and each DC load strung along.All the DC loads need to be filtered, otherwise they load down the outputs.Signal at the end is the sum of all oscillators.If you need them filtered individually, consider a diplexing filter. You will need two for each node: one for DC (also known as a bias tee), and one for the AC signal. The DC filter has a high-pass low-pass characteristic, while the AC signal part will have a notch-bandpass characteristic.Diplexing filters are designed as normal filters are (i.e., go find a filter calculator for e.g. Butterworth designs), but adjusted slightly: instead of both ends being matched impedance, one end is high impedance. The Hi-Z ends of multiple filters can be connected in parallel, and if the filters are complementary (i.e., one passes frequencies that the other one cuts), then the total response will be correct (when driven by a matched impedance). (At least, that was what I read. I don't know a proof of this method.)For extracting DC from a combined AC and DC line, you simply pair high-pass and low-pass filters together. Or, since you don't need to split the AC at that point, even just the LPF is enough. Easy!Tapping AC is going to be harder, because you need a diplexer designed for each oscillator frequency, and each diplexer needs to be different.If you can tell us more about your application, we may be able to greatly simplify your circuit. Typically, signals are not combined at full power, but combined at a small signal level (where the attenuation of a resistor divider, for example, is easily compensated with an amplifier), then boosted to power output levels with a general purpose power amplifier.Tim

author: nctnico
author_rank: Super Contributor
timestamp: November 28, 2015, 03:45:12 pm
content: The easiest way is to modulate the current in the DC line. A current sensing circuit can then be used to extract the signal. There are various transmission systems which work by modulating the voltage and current to create two way communication over power lines.

author: DanielS
author_rank: Frequent Contributor
timestamp: November 28, 2015, 09:00:36 pm
content: My first thought would be to couple the signals into the DC line using transformers. As others have already said, this means having to add LC filters to every load to remove AC components from the power input to individual units and reduce loading on the signal generators. You will also need an inductor at the DC source on both power and return so it does not short out AC signals. If you are attaching the receiver near the DC source, you will also need a capacitor at the end of the line between supply and return to close the signal loop. An even better way to sense the signal at the source would be to use a differential transformer for power and return with a third winding providing the recovered signal.