Chuck asked about radio receiver simulations:
? ? "1.How do I add a antenna. Or can I use a 27Mhz voltage source."
SPICE doesn't do antennas.? Yes, you can use a signal source, connected to the appropriate point in your circuit.? I'd recommend a voltage source in series with a resistance, because I'm sure the antenna doesn't behave like an ideal voltage source, and it would likely mess up your receiver circuit if driven directly by a voltage source.
? ? "Need to modulate it with 1K sig. How do I modulate the 27Mhz."
There are a lot of ways to do that, depending on how you want it to be modulated.? AM?? FM?? PM?
LTspice has a most helpful circuit element, called "Modulate".? It is an AM and FM modulator.? I strongly urge you to use that, rather than any other method for making an AM or FM signal for simulations.? Find it in the Components menu, in the [SpecialFunctions] folder.? You'll have to scroll to the end to find it.? The "Modulate2" component is the same except that it has quadrature (Sine and Cosine) outputs.
To use them, connect the 1kHz voltage source to either the AM or FM input (not both).? The other input can either be connected to a suitable voltage, or left floating.? Then right-click on the Modulate symbol, and add these two parameters to the Value field:
? ? Mark=27MEGHz? Space=27MEGHz
The Space parameter value will be the output frequency when the FM input voltage is 0V.
The Mark parameter value will be the output frequency when the FM input voltage is 1V.
Note that SPICE requires "MEG".? If you use "MHz" you'll get milliHertz.? Can be just "MEG" because SPICE ignores what comes after it.
If you want the signal to be AM, you can set both Mark and Space to 27MEGHz and leave the FM input disconnected or grounded.
If you want AM, the voltage that you apply to the AM input pin needs to be suitably offset, so that its value never goes negative.? When the voltage at the AM input just reaches 0V on the negative peaks, you are at 100% modulation, so anything more than that would be overmodulation.? Yes you could do that, but then you don't have a proper AM signal anymore.
The output of the Modulate device has a 1 ohm output impedance.? So you probably should add a series resistor between it and your receiver circuit.? How much resistance, well that depends on what sort of antenna you have.? You might also want to attenuate the signal too -- either that, or drive the AM input with a very small voltage.
? ? "How can I tune a tank circuit? Variable cap? Model somewhere?"
Do you want to change the tuning during a simulation?? It's probably better to run a simulation, then change the tuning and run another.? Use the .STEP command to automate this process.
If you do want to vary a capacitor during a simulation, be careful because it's not just a matter of changing its capacitance.? That would violate conservation of charge.? LTspice lets you overcome that problem by specifying the charge instead of the capacitance.? See the Help page for capacitors.
If you run into problems with your simulations, consider uploading the circuit you've done to the "Temp" folder, and send a message with your questions.
Regards,
Andy
