cross coupled oscillator design problem

Hello everyone,

I'm currently working on the design of a cross-coupled oscillator, based on the circuit architecture shown in the attached figure (from a referenced paper). The resonator has a center frequency of 2.49 GHz, with C0 = 0.98 pF.

One of the key challenges I’m facing is ringing, which appears to be caused by the negative capacitance effects introduced by Csp and CHPF. In order to suppress this ringing, I attempted to ensure that the loop gain is less than 1 at the ringing frequency. However, this makes it difficult to simultaneously satisfy both of the following conditions:

• Loop gain > 1 at 2.49 GHz

• Loop gain < 1 at the ringing frequency

Simulation Setup:

1. For loop gain analysis, I used the STB (stability) analysis with an iprobe placed at the gate of M2.

2. For analyzing negative impedance and capacitance, I ran AC simulations by placing a current source across the output nodes and calculating the real and imaginary parts of the resulting impedance.

   

I would appreciate any suggestions or insights on simulation techniques and design strategies that could help with this issue.

source : B. Bahr, D. Griffith, A. Kiaei, T. Tsai, R. Smith and B. Haroun, "Class-C BAW Oscillator Achieving a Close-in FOM of 206.5dB at 1kHz with Optimal Tuning for Narrowband Wireless Systems," 2022 IEEE Radio Frequency Integrated Circuits Symposium (RFIC), Denver, CO, USA, 2022, pp. 311-314, doi: 10.1109/RFIC54546.2022.9863092. keywords: {Wireless communication;Phase noise;Performance evaluation;Micromechanical devices;Radiofrequency integrated circuits;Oscillators;Tuning;BAW;MEMS;Oscillator;resonators},