The effects of hot carrier stress on CMOS voltage-controlled oscillators (VCO) are investigated. A model of the threshold voltage degradation in MOSFETs due to hot carrier stress has been used to model jitter and phase noise in voltage-controlled oscillators. The relation between the stress time which induces the hot carrier effects and the degradation of the VCO performance is presented. The VCO performance degradation takes into consideration decrease in operation frequency, increase in jitter and phase noise and decrease in tuning range. The experimental circuits have been designed in 0.5 μm n-well CMOS technology for operation at 3 V. It is shown that when the MOSFET threshold voltage, increases from 0.4 V to 0.9 V due to the hot carrier effect, for the single-ended ring oscillator, the oscillation frequency changes from 538 MHz to 360 MHz, and the phase noise changes from -104 dBc to -105 dBc at 1 MHz frequency offset with a power dissipation of 0.37 mW. For the current-starved VCO, the tuning range changes from 72 MHz - 287 MHz to 65.4 MHz - 201 MHz, and the phase noise changes from -109 dBc to -107 dBc at 1 MHz offset from the center frequency, 200 MHz; for the double-ended differential VCO, the tuning range changes from 32 MHz - 983 MHz to 26 MHz - 698 MHz, and phase noise changes from -86 dBc to -87 dBc at 1 MHz offset from the center frequency, 700 MHz.
KEYWORDS: Signal to noise ratio, Electronics, Oscillators, Interference (communication), Receivers, Telecommunications, Field effect transistors, Orthogonal frequency division multiplexing, Performance modeling, Systems modeling
Phase noise may be regarded as the most severe cause of performance degradation in OFDM systems. Hot carriers (HCs), found in the CMOS synchronization circuits, are high-mobility charge carriers that degrade the MOSFET devices’ performance by increasing the threshold voltage required to operate the MOSFETs. The HC effect manifests itself as the phase noise, which increases with the continued MOSFET operation and results in the performance degradation of the Voltage-Controlled Oscillator (VCO) built on the MOSFET. The HC effect is particularly evident in the short-channel MOSFET devices. The MOSFET instability will impact on the OFDM system performance. The relationship between the OFDM system performance and the hot carrier effect can be analyzed in terms of a crucial parameter, the MOSFET threshold voltage. In this paper, we derive a general phase noise model for OFDM systems based on the Hot-carrier effect and the corresponding drifted threshold voltage in differential ring oscillators. The expected OFDM performance degradation due to the hot carrier effect is provided through our simulations. We show that the OFDM BER performance evaluation using the existing phase noise models can be upto three orders of magnitude different from the results obtained by using our phase noise model.