LT Spice For Radio Amateurs: PART 2

Ok meat and potatoes time. Lets look at some simple circuits that are electronics fundamentals to illustrate the basics of using LT Spice.

FILE: Voltage-Divider

So you built the voltage divider circuit above, or if you were lazy you downloaded the one I have linked above. Either way, you now have a complete circuit that can be simulated. We have a voltage source providing 12 volts to the circuit, we have R1 and R2 forming a voltage divider, we have a ground and we have 2 Net Labels, Out-A and B.

In this instance the net labels are not being used for their proper purpose, but rather to provide a convenient place to probe the circuit under simulation. So, lets run this thing and see if it does what we expect, in proving Ohms Law is true and working HEHE.

So if you assembled the circuit yourself, right click the schematic and select RUN. This will bring up the simulation command window. We will be performing a Transient Analysis, plug in some values as I have above, and click ok. The window will now split and show you the simulation window.

If you move the mouse around in the schematic window you will notice that the cursor will change from cross hairs to a probe when you hover over OUT-A and B and will look like a current meter when you hover over the voltage source. So we can measure the voltages at points A and B and the current flowing in the circuit. And if you look in the above image, you will see 2 traces, the green one is 12v from out-a and the blue one is 6v at out-b, just what we expected for a voltage divider with 2 10K resistors.

Change the value of the resistors and then run the simulation again, notice what effect that has on the voltage at out-b, its going to change, by how much will depend on the value you change it to.

FILE: Pi-Attenuator

Another simple resistor only circuit is the pi attenuator, I think off the top of my head that this attenuator is -3db. Load it up and run it and see if i am right.

This time we are using a voltage source as a signal. Select a sine wave, give it an amplitude of your choice, its value is Peak to Peak, set the frequency of the signal, I gave it 7 megahertz. Now, this is important, all signal sources need to have their impedance set. The Series Resistance box will set the signals impedance, and being that we like to have 50 ohms impedance everywhere, lets set the series resistance to 50R. Also, all circuits need a load impedance also, if you look at the schematic you will note there is a 50R resistor to ground after the circuit under test, this sets our load impedance to 50R also. 50R in, 50R out, with known impedances we should get accurate results.  Click ok, then run the simulation using the transient analysis numbers shown in the above schematic. Probe points A and B and you should have something like the image below.

The green trace is our 1v p-p 7mhz input signal, the blue trace is our attenuated now 700mv p-p signal. And now you know enough about how to use LT spice to test simple circuits, using both DC voltage sources and AC signal sources. Change the values about and see what happens, the good thing is none of this costs any components or solder. Just your time and some self learning.

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