quartz oscillator

[yalect]

Hi, I have circuit of quartz oscillator as it is shown on attachment

Please, I want to know how it works and can I replace the coil of collector L1with resistance and what

The roll of C1?

 thank you

[johnsold]

Have you done any research? A quick on-line search for "crystal oscillator circuits" produces several links to sites describing circuits similar to yours. Before you build such a circuit, make sure the crystal can handle the voltages which will be applied. That circuit tends to operate the crystal at higher power levels than some other configurations.

Lynn

[yalect]

Hi, thank you for your reply,  you mean by crystal can handle the voltages that applied, transistor prevents the voltages of crystal because it's between collector and tbe base or you have another Idea ?

thank you

[johnsold]

The collector-base voltage can get large, so if you use that circuit configuration be sure the crystal can handle the power. That circuit typically operates with the crystal at series resonance. Other circuits operate at parallel resonance with the crystal in the base circuit where it operates at lower voltages.  Choosing the appropriate oscillator configuration and getting the design to work is the subject of complete books.

Lynn

[yalect]

Hi

You mean Generally that circuit can oscillates but not good way, in fact I made small change that capacitor was connected parallel but I think that doesn’t affect   

thank you 

[johnsold]

That circuit does work and is used in practice. I just wanted to warn you that it does have the possiblity of delivering higher power to the crystal, so that you did not damage a crystal by using one specified for a different type of circuit.  This is one of the kinds of things which a simulation probably will miss or may not model accurately.

Lynn

[yalect]

thank you for your reply,sorry if may Our discussion  takes long

In fact I tried this circuit on multisim but it didn’t work  

  About the inductor of collector is it for protection prevents frequencies go throw line of power or has a main roll in oscillation system ? some make it with a nucleus and other no  

[johnsold]

The inductor is an essential part of the oscillator. The impedance of the inductor results in an AC voltage drop across it during oscillation. If the crystal and the collector were tied directly to V1, there would be no feedback to make the oscillator work.

It is often difficult to get simulators to produce the start up behavior of oscillators. In real (physical) oscillators a small amount of noise gets amplified and filtered by the resonant circuit so that feedback occurs at the oscillation frequency. The amplitude builds until some non-linearity prevents further increase. At that point the oscillator is running in its steady state. Probably.

Simulators do not have the noise found in real oscillators. Yes, they can do a noise analysis, but the noise is not present during DC or transient analysis which are used to simulate the oscillation. Sometimes you can get the oscillator to start by applying an initial condition.  Try starting the simulation with about 0.5 V on C1.

Lynn

[yalect]

Hi

Actually, almost  it’s not possible simulation of an oscillator on multisim or other simulator.

 when we talk about miltusim, I observed that even when I apply a pulses to simple LC or quartz circuit from signal generator with serial resistor and it doesn’t react

 in previous reply You said try start simulation with 0.5v on C1 means I decrease Vb to 0.5 or connecting source voltage to C1? And about the inductor of collector some replaced with resistor I think that it has not to be inductor what do you say ?

thank you

[johnsold]

I am not sure how initial conditions work in Multisim, but standard SPICE simulators allow you to set initial conditions on components such as capacitors. Here is the section from the manual for the simulator I use:

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4.2.2 .IC: Set Initial Conditions

General form:

     .IC V(NODNUM)=VAL V(NODNUM)=VAL ...

Examples:

     .IC V(11)=5 V(4)=-5 V(2)=2.2

The .IC line is for setting transient initial conditions. It has two different interpretations, depending on whether the UIC parameter is specified on the .TRAN control line. Also, one should not confuse this line with the .NODESET line. The.NODESET line is only to help DC convergence, and does not affect final bias solution (except for multi-stable circuits). The two interpretations of this line are as follows:

1. When the UIC parameter is specified on the .TRAN line, then the node voltages specified on the .IC control line are used to compute the capacitor, diode, BJT, JFET, and MOSFET initial conditions. This is equivalent to specifying the IC=... parameter on each device line, but is much more convenient. The IC=... parameter can still be specified and takes precedence over the .IC values. Since no DC bias (initial transient) solution is computed before the transient analysis, one should take care to specify all DC node voltages on the .IC control line if they are to be used to compute device initial conditions.
    
2. When the UIC parameter is not specified on the .TRAN control line, the DC bias (initial transient) solution is computed before the transient analysis. In this case, the node voltages specified on the .IC control line are forced to the desired initial values during the bias solution. During transient analysis, the constraint on these node voltages is removed. This is the preferred method since it allows SPICE to compute a consistent DC solution.

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It does not mean changing the circuit or adding a voltage source.

Replacing the inductor with a resistor will change the DC operating point and likely change the gain at the oscillation frequency. The reactance of the 100 uH inductor at 25 MHz is >15000 ohms. If you use a resistor of that value, the transistor would be saturated. If you use a much smaller resistor, the gain will be low.

Lynn