Colpitt-Oscillator PSpice-Simulation does'nt work

Hello,

i tried to design a colpitt oscillator (10mhz) in pspice, but there are some "start" problems. i got the data of the crystal oscillator (RLCC-network) of the german wikipedia (Schwingquarz) and the dimension of the parts from this site:
Transistor Crystal Oscillator Circuit :: Electronics and Radio Today

the only think i cannot realize is the additional capacitor below the crystal (placing it causes an node-error). does anybody recognize why this does not work?
and i have further question : can anybody tell me how i can dimension the parts for other frequencies? (i finally need a colpitt oscillator @ 16,3Mhz (local oscillator for a fm receiver))

thanks for help

halunke

The node error (assuming it is a floating node) is usually caused by a node with no DC path to ground although I cannot see one in your circuit. A very large resistor (eg 1G ohm) from the node to ground usually fixes it.

Keith

Start-up of oscillators mostly needs special measures in circuit simulation. Crystal oscillators are worse because of their high Q (e.g. about 25000 in the present equivalent circuit). Standard LC oscillators can be started by simple kick-off circuits, for crystals, these have to be well considered for a startup within a limited simulation time. Limited numerical resolution of the circuit simulator also requires a kick-off above it's numerical "noise" level.

The most simple way is to connect a VPULSE source in series with the crystal series resonator circuit and apply a sufficient voltage step (it's several 100V up to kV for the present crystal load). Then the oscillator is working at least with LTSPICE. With a suitable start pulse, you see reasonable voltage levels immediately, but steady-state will need many ms in any case.

The crystal load capacitance of about 90 pF seems unusually high and will pull the crystal to a frequency slightly below it's specification.

C16=0.01pF ??? so 10fF !!
This oscillator never works..

This oscillator never works..

Click to expand...

It's not said very clearly in the original post, but obvious from the extreme capacitance and inductance values: It's a crystal equivalent circuit, the values are reasonable, and the oscillator can work if you put in a 10 MHz crystal with respective parameters.

If you are not familiar with crystal equivalent circuits, you should at least wonder: "Where's the crystal?"

i think it should be possible without a kick-off-circuit, i already found one (but with a frequency of 10khz). i think it's a problem with the dimension of parts. anyone an idea?

i think it should be possible without a kick-off-circuit

Click to expand...

With LTSPICE, the oscillator reaches it's steady state amplitude of about 500 mVpp after 15 ms without any special startup tricks and only 8 ms when skipping the initial operation point analysis. But I didn't check with PSPICE.

halunke86 said:

Hello,

i tried to design a colpitt oscillator (10mhz) in pspice, but there are some "start" problems. i got the data of the crystal oscillator (RLCC-network) of the german wikipedia (Schwingquarz) and the dimension of the parts from this site:
Transistor Crystal Oscillator Circuit :: Electronics and Radio Today

the only think i cannot realize is the additional capacitor below the crystal (placing it causes an node-error). does anybody recognize why this does not work?
and i have further question : can anybody tell me how i can dimension the parts for other frequencies? (i finally need a colpitt oscillator @ 16,3Mhz (local oscillator for a fm receiver))

thanks for help

halunke

Click to expand...

Hi,

If i remember correctly, for oscillation, you can do either of these ways :
1) Decrease the maximum timestep and try again. usually you have to decrease it too much to work,so it will increase your simulation time and your data file size.

2)use some Initial conditions.(For EX.: C5& C6). if you want to set initial condition for any element, double click on it , in Property Editor you'll see an empty box "IC", set a value. remember for capacitors it's capacitor initial voltage and for inductors it's inductor initial current. usually this will do the trick!

Good Luck!

2)use some Initial conditions

Click to expand...

As explained below, the special problem with crystal oscillators is the large Q of the crystal. To create initial conditions with oscillation energy "in" the crystal, you have to apply IC to the crystal oscillator elements. I think, it doesn't work easily for the present circuit without adding additional elements. The suggested VPULSE source is the most simple way to achieve it and should work with any SPICE variant. IC gives good results with standard LC oscillators, also "skip initial operation point analysis" mostly does it.