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Newby Question on Signals

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linguist

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Just wanted to verify some newbie queries.

If I have a Sinewave that is 100mA peak to peak but the lowest amplitude of the signal starts at 20mA & ranges to 120mA.

The signal is all positive voltage from +20mA to +120mA so it is not an AC signal but more like a big DC ripple.

What Terminology is used to describe the signal, is it just a DC signal.

With a signal as mentioned, is the Peak to Peak terminology correct or is that term only used for AC signals where the signal is above & below 0V.

Is the rms value still calculated the same as an AC waveform or do you average out the signal strength somehow.
Do I just calculate the rms from the peak of 120mA even though the signal itself is 100mA Peak to Peak due to the 20mA floor.

Not sure if any of it makes sense.
Cheers
 
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Here is what LTSpice says. Note that you can think of the Current source as a 70mA dc current source in parallel with a 50ma sinosoidal current source. (Superposition theorem)

However, if you connect it to a 1Ω load, and have LTSpice compute the average and RMS values, it comes up with 70ma and 78.419mA, respectively.

Now, we know that this source should produce as much heating in the resistor as (Irms)^2/1 or 0.078419^2/1 = 0.0061945W.

If we ask LTSpice how much power is in the resistor, it plots V(A)*I(R1), the dark blue trace. Note the weird wave-shape, which results from the product of two sine waves. When I ask LTSpice what is the average of the blue trace, it comes up with the value in the second box, namely 6.1495mW... Not bad, huh.
 

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MikeMl,

Thanks for the reply, I have a couple of questions.

How did you calculate the 70mA dc current source in parallel with a 50ma sinosoidal current source to start with to enter it into LTSpice.

Do the normal Equations apply for rms & Average values, my figures seem out a little.

I rms = 0.707 * Ipeak
= 0.707 * 100
rms = 70.7mA

I av = .637 * Imax
= 0.637 * 120
Iav = 76.4mA

I tried the circuit mentioned in LTSpice,

How do you get the mW table on the right hand side, I get mA on the left & seconds on the bottom.

Also how do you get it to show the waveform I(R1) box like you have, I am only new to all this, could you give me a run down on how you did it please & maybe the asc so I can have a look.
I would like to be able to learn how to do it.

In LTSpice I have the current source, resistor etc & have the I(R1) waveform from 20mA to 120mA but that's about it at the moment.


Cheers
 
First, the RMS calculation must include the DC component. The RMS calc of your function would involve an integral. I modeled it just as you described it. I could have as easily used two current sources, as shown in this sim.

Once you have a trace in an LTSpice plot pane, place the mouse cursor over its name at the top of the plot pane. Now CTRL-LEFT CLICK the mouse button, and LTSpice computes the average and RMS of the displayed waveform. If the waveform is periodic, the interval over which the average/RMS is computed must close on an exact number of cycles, otherwise the values will be skewed by the incomplete cycle....

You know the trick of placing the mouse cursor over a component, then LEFT MOUSE CLICK, and LTSpice will add a trace showing the current through the component to the plot pane. If you ALT LEFT CLICK, then a plot of POWER gets added to the plot pane. Then you convert that to AVERAGE power by the method described above.

A lot of short cuts and good stuff about LTSpice is shown here.
 

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