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Amplitude Modulation Source in LTspice

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crutschow

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LTspice does not have a generator for amplitude modulation so here's a way to do that.
Below is a simulation of using a sine function to vary a resistance with time, along with an AC current-source, to generate an AM signal and a DSB suppressed-carrier modulated signal.
The function works to generate a simulated negative resistance for the DSB generation. This provides a carrier phase reversal for alternate half-cycles of the modulation signal, which is characteristic of a DSB modulated signal.

The modulation frequency is (2pi*time*X), where time is the simulation time and X is an arbitrary constant. Thus sin(2pi*time*1k) gives a sine modulation frequency of one cycle per ms or 1kHz.

The peak output voltage is equal to I1/1000, thus giving a peak value of 1V for the shown value of 1000A.

Note that the output impedance of the circuit varies with time (from 0 to 1mΩ in this example) but that should not cause a significant error in most circuits. If necessary, a high impedance buffer circuit can be added.

To generate just one function, eliminate the resistor for the other function and ground the current source.

Of course other arbitrary time functions can be used to generate outputs besides sine modulation or a sine carrier.

upload_2016-4-25_10-37-14.png
 

Attachments

  • AM & DSB Source.asc
    668 bytes · Views: 674
LTspice does not have a generator for amplitude modulation
Actually it does. It's the 'modulate' device in the Special Functions folder.
 
Actually it does. It's the 'modulate' device in the Special Functions folder.
I stand corrected although I don't think it will do DSB modulation.
Where does it show how to connect and use that device? I didn't find it in the Help file.
 
This is what the 'Help' says :-
"The MODULATE device is a voltage controlled oscillator. See the example schematic .\examples\Educational\PLL.asc. The instantaneous oscillation frequency is set by the voltage on the FM input. The conversion from voltage to frequency is linear and set by the two instance parameters, mark and space. Mark is the frequency when the FM input is at 1V and space is the frequency when the input is at 0V. The amplitude is set by the voltage on the AM input and defaults to 1V if that input is unused (connected to the MODULATE common)."
Here's a simple demo
Modulate.PNG
 
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Okay, thanks.
I see the Help info for that was buried under the Special Functions listing.
And it will do DSB modulation if the AM input is a sinewave with no offset.
 
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Here is what I was working on while alec was posting:
 

Attachments

  • Draft368.asc
    590 bytes · Views: 565
Another way to get a modulated waveform is to use a behavioral voltage source with a function such as "V=sin(2*pi*50*time)*sin(2*pi*1k*time)"
 
This is a very useful post. Wish I had LTSpice on my iPhone. :confused:
 
It would be fun trying to draw a schematic in LTspice on an iPhone.:rolleyes:
 
It would be fun trying to draw a schematic in LTspice on an iPhone.:rolleyes:
Maybe just a schematic and sim viewer would be nice. That way I could view what Mike posted. Currently I'm stuck in the VA hospital so all I got is my iphone. I will be sure to revisit this post when I get out of here As I'm anxious to try this new info out.
 
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It would be fun trying to draw a schematic in LTspice on an iPhone
With my eyesight and fat fingers it would be fun doing almost anything on a tiddly iPhone :).
 
With my eyesight and fat fingers it would be fun doing almost anything on a tiddly iPhone :).
You would probably like the iPad to accommodate your fingers and eyesight.
 
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