Full-custom integrated circuit design for a pulse frequency modulator.

Pulse frequency modulation (PFM) is one of a number of pulse time modulation (PTM) techniques which is suitable for the transmission of video or TV signals through optical fibre. Although several PFM transmission systems have been reported over the past few years, those available commercially usually consist of several components. This thesis describes the design of a novel single chip PFM modulator implemented in 2.4 pm CMOS technology. Various design approaches are considered together with a review of circuit and silicon implementation techniques. The fabricated integrated circuit (IC) matches or exceeds the performance of PFM modulators built with commercially available components.The performance of the design is strongly dependent upon its IC layout. A theoretical analysis of the relationship between the folding grade of a transistor and its parasitic capacitances has been undertaken. Novel equations are developed which enable to trade-off design shape against parasitic capacitance. The lowest drain/source capacitance is always achieved at a folding grade of two regardless of the transistor width to length ratio. The equations developed also show that even folding grades generally achieve a lower parasitic capacitance than odd folding grades.Performance tests on prototype ICs have shown that the measurement equipment introduces significant capacitive loading. A novel approach of calculating this capacitive loading from transient measurement results is described.The designed IC was extensively tested in a practical transmission system comprising modulator and demodulator and results of both quantitative and qualitative measurements are reported. At a carrier frequency of 32 MHz and a modulating input signal amplitude of 1 V, the circuit achieves a harmonic and non-linear distortions of -37.87 dB and -56 dB, respectively. These values compare favourably with published results of PFM systems. Further in system tests confirmed that the IC developed is well suited for the transmission of high quality still and moving pictures.The IC was only compared with the VCO circuits available commercially (Plessey SP1658 and TI SN74S124) since VCOs are the main building block of a PFM modulator and single chip versions are not currently available. Results of the comparison show that the performance of PFM modulators, based on those VCOs, is inferior to the designed circuit whilst requiring a much higher power dissipation. The power dissipation of the designed circuit is 47.5 mW compared to 165 mW and 525 mW for the SP1658 and SN74S124, respectively. Due to the much reduced power dissipation, almost no heat is dissipated from the designed circuit. When operated at its maximum frequency (~ 40 MHz) the temperature of the IC was found to be only 3°C above room temperature.