Could someone please help me understand this ?

If the signal looks like like a slightly clipped 453MHz sine wave, most of the energy is at 453MHz, much less energy at twice that. I'm sure you can visualise what it would look like if he 2 frequencies had equal energy.

NT

I'd be happier if it gave 450 MHz. Try feeding it a square wave at least that way you can recognise all the harmonic components easily.

I suspect what you are seeing is some 3rd harmonic distortion from the clipping of the sine waveform at nominally 400MHz x3 = 1.2GHz.

You might find it enlightening to display your raw waveform with a perfect sine wave of the fundamental frequency subtracted from it.

If it looks anything like a sine wave, the FT should show a big spike at the obvious frequency. Maybe a bug in the FT code?

LT Spice does FFTs for you.

The first thing I look at in an oscillator simulation is the FFT. Bad oscillators are visible immediately.

Perhaps the OP can explain why that's not good enough?

Clifford Heath.

LT Spice doesn't simulate oscillators very well in time domain.

At the default settings, it doesn't simulate a sine wave generator very well.

is data - please see above.

Works fine for me.

I wonder how John Larkin has managed to mangle his default settings.

Class C oscillators do depend on turning a single transistor on briefly, on ce per cycle, and the Gummmel-Poon model for transistors (which is what LTS pice mostly runs - if you could get the parameters for VBIC model you could run that but I've never been able to) isn't great in the relevant region.

My variant of the Wien Bridge simulates fine, but gains stay more or less t he same all the way through the cycle.

The following is the output from the FFT program with the 450 MHz test signal and 0.1 nanosecond signal sampling time. 'out1' is the text file in which it dumps the power spectrum. ./fftnew test450MHzsignal out1 0.1

Clearly, the 450 MHz peak is huge compared to the others.

I totally agree with you that FT is the best way to analyze the oscillator output. The output for the

Clearly the magnitude of the 450 MHz signal is huge compared to the others, and the program therefore correctly identified the frequency of the test signal.

Right, but that's immediately visible in the LTSpice FFT spectrum display. There was no need to write your own program.

Well, I use HSpice at work, and Ngspice at home, both of which use a test file input netlist.

The C language FT code is a straightforward application of the transform, and does any features such as radix 2 etc., Over the years I have learned the hard way that often writing one's own code gives very tight control over the problem at hand and avoids bothersome moments like sitting in front of the terminal, staring at some strange results and wondering what could be going wrong.

If you sim an oscillator in time domain, you can look at the waveform and see the frequency. Cursor a full cycle or so and LT Spice will show the period and its reciprocal. If your private FFT shows a different frequency, it's wrong.

If phase noise matters, it's a pain to sim a high-Q oscillator in time domain. You need a tiny dT and many, many cycles to get to steady-state and have a good FFT. Crystal oscillators are really hard to sim in time domain. It's better to analyze in frequency domain, to maximize loop gain/freq slope, but then that misses the amplitude-limiting nonlinearities.

Nowadays I just buy XOs.

Yes, crystal oscillators are notoriously difficult to simulate, but there is the 'kick start" method, which, although brute force always works(as expected). It consists of a damped high amplitude(e.g., 750 V) sine wave signal placed in between the series capacitor and inductor of the RLC leg of the crystal. I have used it with HSpice at work a few years ago to analyze a 60 MHz crystal oscillator. HSpice also supports advanced tools as Periodic Steady State, Harmonic Balance, Periodic AC etc., All SPICE queries that post on this newsgroup are related solely to my work at home using Ngspice. damped

You can set the initial conditions inside the crystal model so that it starts at full swing. Zoom the peaks to see if the amplitude is constant, and tweak a little. But you still need a tiny time step to even sim the steady-state accurately.

HP made a digital delay generator that kick-started an XO when an external trigger arrived. It was ugly.

Our DDGs kick start an LC oscillator at trigger time, which is much easier. That simulates well enough.

Switched DC on the crystal capacitance works too.

Cheers

Phil Hobbs