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UCSB ECE 145b - Osc Sim

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Rev. 4/9/2008 Prof. S. Long/ECE/UCSB Small Signal AC analysis of oscillator - open loop Here is an example using a JFET in a CG configuration. AC small signal analysis is used to evaluate the startup condition. The loop is broken between the source (V1), the input to the loop and the capacitor tap, V1p, the output of the loop. The output is loaded with a resistor that emulates the loading seen at the source of the JFET, 1/gm. Any additional load resistance could also be added here – for the input to a buffer stage, for example. R1 is derived from the unloaded Q of the inductor: 111/1/uBLRQGRLωω== = For higher accuracy, the CGS of the JFET should be added in parallel with C2 for this simulation. This will reduce the resonant frequency slightly.Rev. 4/9/2008 Prof. S. Long/ECE/UCSB Here we see the result of the frequency sweep. The peak loop gain occurs at 179 MHz as does the 0 degree phase shift point. The oscillator should start up at this frequency. Note that the loop gain is rather low for reliable startup. The tapped capacitor impedance transformer should be revised for higher small signal loop gain.Rev. 4/9/2008 Prof. S. Long/ECE/UCSB Transient Analysis of Oscillator The same JFET Colpitts oscillator is now simulated in a closed loop condition using transient analysis. This uses a large signal device model and can predict compression and harmonic generation. Note the pulse current source. This is needed to start the oscillator because there will be no noise in the simulation that would jolt the circuit out of metastability. The current source is used here because it will give an infinite impedance after it delivers its current pulse and therefore will not load down the tank. Transient analysis proceeds from an initial DC solution incrementally with small timesteps. A stop time is required to limit the length of the simulation. Sufficient time should be allowed for the simulation to reach a steady state condition. Large stop times generate large data files and require a lot of simulation time. V1 is shown below. Note that the oscillator is starting up and reaching what appears to be a steady state condition.Rev. 4/9/2008 Prof. S. Long/ECE/UCSB Output spectrum from a transient analysis simulation. The fft (fs function) is used to display frequency spectra. The format of the function is: fs(Vout, min. f, max f,,,,,start time, stop time) The start and stop times are intended to window out any initial startup transients so the spectral display will be more accurate. In this example, a 4 us simulation was run and the last 2 us were used to calculate the frequency spectrum. You need to add the 5 commas (count them) after max f to use this function. The wide noise floor is an artifact of the fft conditions. The time domain waveform is resampled at equal time steps. The dynamic range of the simulation depends on the number of points used. Below, the amplitude of V1 and V2 are compared. We can see that the average voltage at V2 is VDD, 5V. The peak voltage exceeds VDD. The device must be able to handle this peak voltage without breaking down.Rev. 4/9/2008 Prof. S. Long/ECE/UCSB Harmonic Balance Simulation Harmonic balance simulation can also be used for closed-loop oscillator analysis. HB has be advantage that it calculates the steady-state solution directly and is relatively fast compared with transient analysis (when it converges). To simulate an oscillator, the ADS OscPort must be inserted into the signal path. The arrow must point in the right direction. It is important to give HB a starting frequency that is close to what you expect to be the frequency of oscillation. Specify sufficient harmonics so that harmonic distortion can be calculated at the desired integer multiples of the fundamental frequency. The harmonic balance controller must be activated for oscillator analysis in order to recognize the OscPort.Rev. 4/9/2008 Prof. S. Long/ECE/UCSB Information from the ADS help file on OscPort is included below, although its recommendations seldom help achieve convergence. The output from a HB oscillator simulation can be either a frequency spectrum or a time domain representation of any voltage or current in the circuit. You can see that theRev. 4/9/2008 Prof. S. Long/ECE/UCSB spectral representation is much cleaner than the FFT generated one from Transient simulation. This is because HB calculates only the harmonic frequencies. The time domain is generated using the ts function. This is an inverse Fourier Transform or something similar that is used to convert the harmonic frequency amplitudes and phases into a time domain sequence.Rev. 4/9/2008 Prof. S. Long/ECE/UCSB OscPort (Grounded Oscillator Port) Symbol Available in ADS and RFDE Parameters V = initial guess at fundamental voltage Z = initial value for Z0, in ohms (default: 1.1 ohms) NumOctaves = number of octaves to search (default: 2) Steps = number of steps per search octave (default: 10) FundIndex = fundamental number for oscillator (default: 1) Harm = harmonic or fundamental for oscillator (default: 1) MaxLoopGainStep = maximum arc length continuation step size during loop-gain search Notes/Equations 1. This is a special device used for an oscillator analysis. Do not use more than one oscillator port in a circuit. 2. NumOctaves specifies the total number of octaves over which the oscillator search is done. Half of the octaves are below the initial frequency and half are above. For example, if NumOctaves is 2, then the frequency search goes from Freq/2 to Freq2. Steps sets the number of frequency points per octave that are used in the search. For a high-Q oscillator, a large number of steps might be required. 3. If fundamental voltage V is not specified, the simulator first performs a small-signal AC analysis to determine the actual frequency and oscillation voltage. If V is specified, it represents an initial guess at the fundamental oscillator voltage at the point where the OscPort is inserted. The initial guess for V should be as close to the actual value as possible. An inaccurate value increases the simulation time and might prevent convergence. If it is not known, don't specify it.Rev. 4/9/2008 Prof. S. Long/ECE/UCSB 4. Provided the circuit produces at least one complex conjugate pole pair in the right-half-plane over the frequency range tested, the analysis will determine the oscillation waveform and amplitude.


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