LT Spice For Radio Amateurs: PART 3

Flow control is something we need to do in radio circuits, we want to keep things out, let only some things pass and all that. We certainly do not want the output of our amplifiers being sent out the power jack into the house wiring and turning the house into a giant antenna for our oscillator circuit. Nor do we want DC being passed into our amplifier circuits either, because that adds a DC offset to the signal that is often not desirable. So, what we do is use DC and AC blocking. So lets knock up a circuits that demonstrates these principles. No gimmies this time, you are on your own.

In the above schematic we have a 7mhz signal source consisting of a 1v p-p sine wave with a 5v dc offset. Signal impedance is set to 50 ohms and a 50 ohm load is used.

So when this circuit is run, out-a should show the 1v signal with 5v offset and when probing the other side of the DC blocking cap, all we have left is the 1v p-p signal as the cap blocks the 5dc from passing on in the circuit.

An inductor does the opposite of the dc blocking cap, it blocks the ac signal and allows dc to pass through. The green trace is out-a and the blue trace is out-b. So from the above 2 simulations we have a good demonstration of the DC blocking and AC blocking action of capacitors and inductors. Add in a voltage divider and a transistor to both of these and we have a simple amplifier.

And with that, you now know a bit more about electronics fundamentals and a good grounding in some of the fundamental aspects of using LT Spice in using signal sources, voltage sources, dc offsets and making complete circuits. Have a play with values in the above circuits and see what the outcomes might be. As this will demonstrate why certain values are often chosen and used in different designs.


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