Hey all,

My goal is to create a variable oscillator for a superheterodyne radio receiver. Since I am using this oscillator to move the wanted station into an IF of 455kHz, my oscillator range is about 1Mhz to 2Mhz. I know there are many options for oscillators, but for my convenience I am trying to convert a state variable filter into an oscillator.

Here is an image of the circuit: http://en.wikipedia.org/wiki/File:State-variable-filter.png

Since there is no source other than opamp rails, ignore Vin and Rg. Basically, I set Rq to zero so that the Q of the circuit goes to infinity, which to my understanding should make it unstable and cause it to oscillate as long as there is an impulse to get the thing started. Thi frequency should be = 1/(2*pi*Rf*C) if Rf1=Rf2 and C1=C2.

I picked LM118 for my opamp because the high slew rate would allow the kind of frequencies I am going for. The problem I am having is that when I simulate this in PSpice, I cant get the frequency to go above .65 MHz and I can't figure out why.

I have it set so that one of the capacitors has an initial condition to get the circuit started. The oscillations grow exponentially until the opamps hit their rails (which I set as + and - 5v) giving me three square waves with 3 different phases (nodes marked HP, BP and LP). By changing the Rf's and C's the frequency will change, but not past .65 MHz. Could this be the op-amp model (I'm using a generic LM118 model that comes with PSpice) or am I missing something?

I did this in netlist, so if anyone would like to look at it let me know. Sorry for the lengthy post.

Thank you for your help,
Dave

 

At 650kHz the gain of an LM118 is only about 30 so your filter works poorly.
Why not use a newer faster opamp?

 

At 650kHz the gain of an LM118 is only about 30 so your filter works poorly.
Why not use a newer faster opamp?

My problem is that I have to simulate this on PSpice using a netlist. I have access to all models that come with the Cadence package and that doesn't include a lot of the newer op-amps. And all I know about the models that I do have are their names, so if I were to search for one that would meet my specs, I'd have to google datasheets for each one (this would take a very long time).

And a question about your comment: if I set the RC value to have a frequency higher that 650kHz, shouldn't the output just be a dampened version of the wave at that RC frequency? Or maybe it wouldn't even oscillate? I ask this because no matter what I set RC above 650kHz, I get the same 650kHz squarewave with the same exact amplitude (peaks at about 3.9v with 5v input rail). The only thing that changes is the transient stage which lasts between 1-10 microseconds depending on the RC values.

Do you know some common op-amps that would meet my spec that are popular enough to likely be modeled? Or perhaps a database of PSpice models for new op-amps?

 

I think your simulation program doesn't know if it is coming or going.

 

I found a nice model for LT1190 and now it's working as it should. I see now that LM118 was not an appropriate choice, and on top of that the model for it appears to be faulty.

Thanks for the help