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Trying to simulate a 27Mhz receiver. Couple of questions
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Actually a BUNCH of questions. LOL ? 1.How do I add a antenna. Or can I use a 27Mhz voltage source. Need to modulate it with 1K sig. How do I modulate the 27Mhz. Was able to model a sine wave source at 27Mhz but sticking the 1K signal on that is what is got me stumped.? Trying to simulate a "toy" receiver? How can I tune a tank circuit? Variable cap? Model somewhere? Can't use the PC out of the toy. Want to be able to turn VCC on and off to save battery life.? Brand new to spice and not the sharpest pencil in the drawer, so I really need some help.? Thanks Chuck |
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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 |
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Interposted -
*Plain Text* email -- it's an accessibility issue () no proprietary attachments; no html mail /\ <> On 2018-11-21 7:28 p.m., chuck@... [LTspice] wrote: Actually a BUNCH of questions. LOLUse a 27MHz voltage source with a Modulate device. (F2 or [Edit]->[Component]) -> SpecialFunctions -> modulate) Apply your modulation signal to the AM or FM input as required. Trying to simulate a "toy" receiverInsert a capacitor. Set its value to "{varCapVal}" (right-click on the cap symbol and type {varCapVal} into the Capacitance box) Set the capacitor value in a .param statement (e.g. ".param varCapVal=1u" (without the "" marks)) You can use a list for the varCapVal (Help is very useful for syntax help) to get a variety of capacitances, or enter a formula so that the capacitance varies with time (then use the oscilloscope to look for whatever characteristic is important, and use the time at the max/min of that characteristic to figure out what capacitance is desirable.) Can't use the PC out of the toy. Want to be able to turn VCC on and off to save battery life.I don't understand this question. The simulation has all the energy in the universe available; the PeeCee uses a little to simulate your circuit and calculate the answer. It takes very little time and/or battery. If you need the simulated Vcc to turn on/off, right-click on the V symbol you put on the schematic and set the off voltage, on voltage, delay time, on time, ... Brand new to spice and not the sharpest pencil in the drawer, so I really need some help. |
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Another thing comes to mind: It could be useful to do an .AC analysis rather than (or in addition to) .TRAN analysis.? .AC analysis involves sweeping the frequency, so it is helpful for looking at tuned circuits.? To do an .AC analysis, you should remove the Modulate component and substitute an ordinary voltage source (keeping the series resistor), with its "AC" value set to 1V.? In SPICE .AC analysis is a linear analysis, so you get the same results whether signals are microvolts or megavolts; therefore it is often convenient to set the signal source's amplitude to 1V. I might start with .AC analysis until I get the tuned circuits where I want them, then change to .TRAN analysis with the modulated signal source. An .AC analysis doesn't tell you anything about circuits that are nonlinear.? Therefore, it would be meaningless for the detector portion of your receiver.? Likewise, RF mixers in a heterodyne receiver would not simulate correctly in .AC analysis.? But it's useful for checking the linear RF circuits. Regards, Andy |
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Robert Thomas
Re your antenna question --
Antennas can be very simple or very complex. But for example -- A simple center-fed half-wave dipole (about 17 feet long in your case) operated at its resonant frequency, and in free space or well away from everything else looks like a pure resistance of about 75 ohms. Incoming signals can be represented by a voltage source (modulated or otherwise) in series with this. A quarter-wave whip antenna (around 8.5 feet long in your case) on an ideal ground plane acts like half a dipole, so ideally looks like a pure resistance of around 37 ohms. These simple antennas have a useful bandwidth of a few percent; outside this range, they develop significant series reactance, and the resistance also changes. Depending on your situation, these simple antennas may be practical. If you want to look further afield, EZNEC? (eznec.com) is? a good antenna analysis program -- it comes in demo and paid versions, with a good manual and examples. As a general comment -- Making a small antenna is very easy. Making a good antenna is easy (see above) Making a good, small antenna can be very difficult. Hope this is useful Bob --- This email has been checked for viruses by AVG. |
¿ªÔÆÌåÓýThe real question in my mind is what an antenna model would do in simulation.?1. ?Do you want it to represent the actual terminal impedance of some real antenna? That, in itself, is challenging. EZNEC, referenced below, might help. 2. Do you want it to provide a signal to the receiver? If so, you can simulate that with a standard voltage (or current) source. 3. Do you want it to provide many signals over some band that you can ¡°tune in¡±? If this is what you want, there is a real problem ¡°tuning¡± the receiver during run time.? 4. Do you want many signals that you can use to test for ¡°adjacent channel¡± rejection? If so, you can simulate that with several standard voltage (or current) sources. Jim, KA7EHK James Wagner Oregon Research Electronics
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