That might be important when using real-life parts. You might also want to swap the values of R1 and R2 to lower the current. You need to reverse your varactor so that it is reverse biased so it can act like a varactor instead of a simple diode. Select/Click the diode then click the gear icon or simply double click the diode. If the diode that you want to use is fully configurable its parameters can be accessed in the Configuration pane. That means that your varactor is forward biased. Hi BigUn, I am a free subcription user of Multisim live so 1N4002 is not available for me. Select the Use Custom Setting radio button and click on the Customize button. If not: Select Simulation » Interactive Simulation Settings. Check to see if the simulation now works. I see that you are using a PNP transistor. Select Maximum Time Step (TMAX) and change it from 1e-005 to 1e-003. 001 uf ceramic for 100 MHz, 0.1 uf at 1 MHz are good values. That would also increase modulation bandwidth a lot.Īlso, I don't think Spice cares, but in real life you should have a bypass capacitor, something like. You are only going to be modulating the capacitance across the tank circuit by (probably) hundreds of fempto farads so you can safely couple your varactor with 10 pf or so. When you get up to high power Mica capacitors are superior. Ceramic capacitors are most common in RF circuits. In real life the 15 uf capacitor would probably work but for these frequencies we don't use such large capacitors because the construction of such large capacitors makes them not suitable for high frequency use -too much stray inductance is the main issue. You can adjust that after you get the RF oscillator working. The larger capacitor(C2, 300pf or so) along with the 100 k resistor will cause a high frequency roll off of your modulation signal at about 5 kHz. You also need to add that 100k resistor between the modulation source and the varactor. Either you need to make your tank circuit resonate around 100 MHz or change the cap (C2) to 300 pf. The capacitor from the emitter to collector that I suggested (5 pf) is good for 60x that frequncey but around 1 Mhz the impedance is probably way too high. I have learned many software such as Solidworks, Matlab, RSLogix 5000, Minitab, and NI Multisim and I also completed many projects such as a Bomb Disposal Robot, Line following robot, a 3D model of a water filter and a Manipulator, Traffic Light Control, and a distance measuring wheel. The RF oscillator is not oscillating.Īt what frequency do you want this to oscillate? Right now it is around the top of the AM broadcast band in most regions -1.6 MHz. All I see is 100 Hz which is your modulation. I would expect the L/C circuit to oscillate at about 1.6 MHz. I didn't look closely at the waveform and only took your word for it.