• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Very Basic Questions about Sine-Wave Oscillators

Status
Not open for further replies.

Sukoshi

New Member
I had in mind a project to design a superhet receiver for US "AM" radio signals (fairly simple, I thought). So I started reading about oscillators, and I found the literature online about common oscillators such as the Hartley, Colpitts, and Clapp Oscillators. I tried simulating some of these myself in LTSpice, but they only worked for certain frequencies, and playing with certain capacitor values succeeded in making the oscillator fail in most situations.

Articles in this forum refer to special capacitor values, and biasing transistors, and other such things. I've heard of transistor biasing, and know it is used to have a signal fit into a transistor's threshold, but how is it used in oscillator design? Is there any book/document that describes the theoretical basis on which oscillators work, or can anyone give me an explanation? Thanks, and sorry for the basic questions.
 

MikeMl

Well-Known Member
Most Helpful Member
Wiki Oscillator.

Oscillators are very difficult to model in Spice; there are lots of parasitic capacitances in a real circuit that are not accounted for in a Spice schematic. A practical oscillator needs noise to start it; Spice is too damn perfect.

There has been a lot of spice modeling of L-C and crystal oscillators on the LTSpice forum on Yahoo.
 

Roff

Well-Known Member
An oscillator can usually be coaxed to start by putting a .IC v(X)=v directive (using .op) in the schematic, where X is the name of a node, and v is a voltage (0 will often work). This forces the circuit to settle to it's quiescent state after the sim starts. The changes in voltages and currents will usually cause the oscillator to start. Post a schematic if you have specific questions.
 
I had in mind a project to design a superhet receiver for US "AM" radio signals (fairly simple, I thought). So I started reading about oscillators, and I found the literature online about common oscillators such as the Hartley, Colpitts, and Clapp Oscillators.
Forget about using Spice for that. The best way to design these is empirically. For an AM BCB xcvr, the frequencies aren't extreme, and you can do the job on a solderless prototyping board. For low to medium frequencies, I'd use the series tuned Colpitts (aka Clapp) oscillator.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top