I am trying to simulate a 150MHz crystal in LTspice, but I don't get any oscillation out of it. I have an equivalent circuit of a 32MHz crystal, even with that I don't get anything out of it. What am I doing wrong?
Electronic – How to simulate a 150MHz crystal oscillator in LTspice
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Hmm in meetings all day but maybe later or perhaps someone smarter will read this and give a good explanation hint hint. Basically the oscillator circuit is a little inverting amplifier with the input and output attached to the crystal. At time zero nothing is oscillating so it needs to be a little imbalanced to start oscillating. The 1Meg feedback resistor creates a little instability tying the output to the input. This starts the oscillation and then the crystal gets going.
Check out this link too
If you use a series mode crystal in a parallel mode oscillator the frequency will be slightly higher than shown on the package. As shown in this graph the reactance of the cystal varies rapidly with frequency and the frequency of oscillation the actual frequency will depend upon the characteristics of the amplifier making up the oscillator and will be such that the normal oscillation criteria are met (i.e. 360 deg phase shift and unity gain).
Even though it is a series crystal you should use the capacitors to ground at each end or it may not oscillate, I would use something like 22pF. You are probably lucky that the stray capacitance to ground is enough to provide the phase inversion needed for the circuit to work.
If you look at simplified schematic you can deduce the type of oscillator. An oscillator for a parallel resonate crystal will be inverting (i.e. have 180 deg phase shift) and uses the crystal and the associated capacitors to provide the additional 180 deg phase shift needed for oscillation.
As far as I know all oscillators built into MCUs are designed for parallel crystals.
Also if the data sheet specifies any capacitors from the MCU pin to ground that also implies an oscillator intended for parallel crystals; series resonant oscillator circuits do not need them as they need a low impedance input and output from the oscillator.
Example oscillator:
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Best Answer
The comments from the other posters are critical to getting any oscillator to simulate but I also question your choice of components.
The BC547 has rather a low FT for 150MHz oscillation. Its worst case is 150MHz with a typical of 300MHz - this is too low.
Your values for C3 and C4 seem much too large - I would expect values of ~22pF.
Lastly, although it may be a 150MHz crystal it is more likely to be a 30Mhz crystal intended to be operated at the 5th overtone - in order to achieve that it is common to include a tuned circuit in the oscillator to direct which overtone to operate at. The crystal will tend to operate at any odd overtone so the gain of the amplifier at frequencies other than the intended one need to be suppressed.
As the frequency of operation of a crystal is raised the physical thickness required becomes smaller. It is normally impractical to grind a crystal for operation above 30-50Mhz as they become too fragile. To sidestep this overtone operation is used.
Crysal Oscillator Fundamentals