The Pierce circuit is designed to oscillate at the fundamental frequency of the crystal. During the time the pierce oscillator circuit oscillates at the crystal frequency, it comsumes around 2.3 ma current from the 12 V DC supply.Īt this time, without any load the RF output signal amplitude is 6.2v RMS (in the shown circuit this was implemented at 7 MHz frequency). The circuit starts oscillating the moment switch S1 is switched ON.Ĭapacitive output coupling is supplied by means of the capacitor C1, which ensures that that the external load impedance be high enough so that it does not overload the circuit and destroy the oscillations. The 2.5-mh RF choke (RFC1) doesn't typically tune the circuit, however it assists just to keep the RF energy away from the DC supply. In this set up, the quartz crystal (XTAL) is driven between the FET's drain/output and gate/input stages. The following figure demonstrates how to build a Pierce oscillator circuit using a single 2N3823 (or 2N3821, 2N3822) field-effect transistor. Using an FETĪ benefit of the Pierce crystal oscillator circuit is that it does not require any tuning adjustments. Alternatively, when the frequencies is lower than a few 100 kHz you can select C1 and C2 values a little bit larger.Īn AND CMOS gate is indicated in the diagram, it is also possible to use a buffer CMOS gate, such as from the IC 4050. If the frequency is just a few MHz then you may want to decrease the value of C1 and C2 relatively so that you can sustain the oscillation correctly. ![]() With these values the circuit must oscillate without any problems within a extensive range of frequencies. TC1 can now be removed and the crystal can be hooked up directly across R1.Ĭapacitors C1 and C2 can be seen having the values of 470 pF each in this pierce circuit design. TC1 is included to tailor the circuit's oscillation frequency to match the crystal's nominal frequency, but anyway, this specific function could be omitted in case it is not required. Consequently, we may find a 180 degree phase shift by means of the two amplifier and the crystal, and also with a positive feedback. The crystal as a result behaves like a form of transformer working in the series resonance mode, using its a pair of connections operating in antiphase. ![]() However, this might not be true in reality, because C1 and C2 create a capactive centre tap around the crystal, where the centre tap can be seen grounded. With series resonance it might look as if the crystal is supplying the negative feedback to the amplifier. This is because, the CMOS amplifier's input and output work in the antiphase mode. Needless to say, a positive feedback hasn't been used here between the output and input of the circuit. This proposed circuit is designed to operate with the series resonant frequency of the crystal. A crystal can be seen connected between the input and the output of the pierce circuit through the trimmer capacitor TCI. ![]() The single CMOS inverter is biased to form a linear amplifier by means of R1.
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