In this paper, an analysis method of the start-up characteristics of CMOS crystal oscillators is described. First, the origin causing oscillation t o start just after d.c. power supply is switched on has been described and confirmed by experiment. Then, this analysis method has been outlined which can calculate the overall start-np characteristics of the amplitude of oscillation. Finally, experimental results have been given to verify the usefulness of this analysis method.
Introd iic t.ionCrystal oscillators are in wide use in commiinication and measuring eqiiipments as sources of standard frequency or of clock of the eqnipments. According to the development of mobile communication, switching on-off of the power of the equipment is made very much frequently for power saving. Accordingly, the analysis of the start-up behavior of the oscillators is desired. In one of the publications of IEC an approximation formula has been considered to calculate the start-up time[]].Start-up behavior of the common Colpitts crystal oscillator using a bipolar transistor has been investigated[2]. In the present paper, the overall start-up behavior of CMOS crystal oscillators which are in a i d e use as clock sources is investigated.In the first section, we discuss the start-up behavior, especially the origin causing oscillation to start, i.e., oscillation triggering. In the next section our analysis method of the overall start-up characteristics is outlined. In the last section, experimental results to verify the usefiilness of the present analysis method are described.
St.art.-up Re havior of 0scilla.t orThe oscillator circiiit configuration considered in this paper is the common Colpitts crystal oscillator using a CMOS INVERTER, and is shown in Fig.1. When the input voltage is in the low level, the hTOS-FET(FET1) on the side of the d.c. power supply conducts and the one(FET2) on the side of the ground is open circuited. Then, when the input voltage jumps to the high level, F E T l is made open circuited and FET2 conducts. A high resistance R, is connected to produce the d.c. bias of the ChlOS circiiit a t V, = VD = Vcc/2, and the CMOS circiiit is used as an inverting amplifier of the crystal oscillator. The upper side of the terminals is the crystal resonator shown in the form of an eqiiiralent circuit. I C A I Fig.]. ChIOS crystal oscillat,or circuit. The overall start-up behavior of oscillator in this circuit has heen measured t,o see how the behavior is. The measured result of the oscillator voltage V across t,he resonator is shown in Fig.2.448 0-7803-0476-4192 $3.00 0 1992 IEEE
Although crystal oscillators are used widely in such electronic devices as communication equipment and measuring apparatus, it is not possible to determine if the circuit oscillates or how the adjustment must be made until the crystal is connected to the circuit. To remedy this situation and to improve the design method for the crystal oscillators, a method has been developed for measuring the two‐terminal impedance of the active circuit seen from the crystal terminal or the equivalent capacitance when negative resistance of a specific value is obtained. In principle, a known resistance is connected in series with the crystal terminal so that negative resistance is cancelled. Then the resultant pure capacitance is measured with a bridge circuit. A measurement system consisting of a bridge circuit which is controlled by a computer has been constructed. Representative oscillator circuits have been measured. It is found that a usable frequency range can be found from the measured frequency characteristics of the excitation current and the equivalent capacitance, and that the static capacitance of the active circuit seen from the crystal terminal can be adjusted to a desired value via an adjusting capacitance.
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