Cannot get calculated resonance frequency in FFT in LTspice

Thread Starter

alimurad00

Joined Jul 27, 2020
1
Hello AOA, I am trying to build a Collpits Oscillator that has a resonance frequency of 433Mhz. However, the peak I see in the FFT spectrum is at 27Mhz Mhz rather than 433Mhz. I will be thankful if someone can tell me why this happens and how can I solve this problem to get the highest peak at 433Mhz.
FFT result.PNGLt1222 design.PNG
 

Papabravo

Joined Feb 24, 2006
14,205
You are almost 90 dB down at 433 MHz. Did you not listen when we told you you can't do this with an opamp. Yes the GBW says 500 MHz, but that was evaluated at 1MHz.
Do you know the story of horsepower? It goes like this: an average draft horse can lift 550 lbs. a distance of 1 foot in 1 second. That is the definition of 1 Horsepower. Now a question: do you know of any horse anywhere that can lift 1 lb 550 feet in 1 second? I didn't think so. In order to use an opamp effectively you must be well below the GBW limit. In any case you won't get a gain of 20 (=1000/50) at 433 MHz regardless of what you do.

I also suspect that you calculated or applied the component values incorrectly.

Edit: I thought I would investigate the AC performance of the LT1222. Using a gain of 20 = 1K/50. I get the following dismal picture. I think a 433 MHz. oscillator with this part is just completely out of the question, but I could be wrong and I may live to see a horse lift 1 lb. 550 ft. in 1 second.

LT1222_20.png LT1222_20_Plot.png
 
Last edited:

LvW

Joined Jun 13, 2013
1,078
I am afraid, in reality (with a real opamp and real capacitances) the situation will be different if compared with simulation...
 
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