The major effects are a higher oscillation frequency and a higher crystal output signal to the amplifier. A basic schematic is shown in Fig. The crystal is driven by as low a source resistance as possible RI,. The 5 to 1 capacitive voltage divider and, to a certain extent, using a FET input stage provide a linear input impedance to the crystal that will not overload and put a short circuit directly across the crystal over a part of the waveform cycle, as the Colpitts circuit does.
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The major effects are a higher oscillation frequency and a higher crystal output signal to the amplifier. A basic schematic is shown in Fig. The crystal is driven by as low a source resistance as possible RI,.
The 5 to 1 capacitive voltage divider and, to a certain extent, using a FET input stage provide a linear input impedance to the crystal that will not overload and put a short circuit directly across the crystal over a part of the waveform cycle, as the Colpitts circuit does.
The FET does overload as a gate-to-drain diode clamp to the power supply bus at the positive peak of the input waveform, but the 5 to 1 voltage divider hides it from the crystal. Low capacitance load circuit: a using an inverting amplifier, b using a noninverting amplifier. The oscillator works equally well with a noninverting amplifier, as shown in Fig.
The noninverting amplifier consists of a cascaded FET source follower and a transistor emitter follower, with a total amplifier gain of 0.
Additional gain is obtained by resonance effects between the inductive crystal and the series capacitors Csmalr and 4C,,r, and between 15, and C,. The overall loop gain is controlled by two ratios: the ratio of the impedance of L, to RI and the ratio of the impedance of 4C,rr to the impedance of L1. L, is made relatively small and partially reduces the frequency increase caused by the low capacitance load Csmall on the crystal. Pretty much the same thing that happens when a low capacitance load is used.
The major effects are a much higher oscillation frequency and a larger crystal output voltage to the amplifier. The circuit is very similar to the low capacitance load circuit in Fig. The crystal is driven by as low a resistance as possible, RI,.
The 5 to 1 voltage divider provides a linear impedance to the crystal that will not put a short circuit across the crystal over a part of the waveform cycle, as the Colpitts circuit does. The FET overloads as a gate-to-drain diode clamp at the positive peak of the input waveform, but the 5 to 1 voltage divider hides it from the crystal.
The input capacitance of the FET amplifier in Fig. The amplifier in Fig. The oscillator will work equally well with a noninverting amplifier, using a circuit similar to that shown in Fig. This circuit has several good design characteristics: It uses a common base amplifier, which is unconditionally stable at all frequencies and has a very wide frequency response.
The emitter source and load resistances can be varied over a wide range to provide a suitable crystal load over a wide range of crystal resistances.
The basic circuit works well over a frequency range of kHz MHz. Referring to Fig. The lower the load resistance on the crystal, the more loop gain is reduced, and the larger the gain that Qi has to provide to maintain oscillation. The gain of Q, is proportional to the ratio of its collector and emitter resistors R,IR2. Common base and common gate oscillator circuits. At medium frequencies, replacing the transistor Q1 with a FET will give the crystal a higher load resistance that is more appropriate to the medium resistance of crystals at these frequencies.
It is important to note the diode amplitude clamp in Figs. The purpose of the clamp is to limit oscillation amplitude and thereby keep both Q1 and Q2 operating in their linear regions over the complete waveform cycle. At low frequencies i. To obtain this, the crystal is moved to a higher impedance part of the circuit, as shown in Fig. Here, the crystal is tied between collector and base. The two emitters are tied together and use a common emitter resistor. Qz should be a high-gain transistor in order to maximize both its base input resistance and the biasing resistor R3 in parallel with the base of Q2.
Q2 acts as an emitter follower driving Q1 as a common base amplifier. The gain of Q1 is controlled by the ratio of the collector and emitter resistors RllRz. The circuit in Fig. It uses a straightforward two-stage amplifier, with a FET for the input stage. This will give good in-circuit Q and good short-term frequency stability.
The two-inverter circuit shown in Fig. The circuit is series resonant and uses two cascaded digital inverters for an amplifier.
CRYSTAL OSCILLATOR CIRCUITS
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CRYSTAL OSCILLATOR CIRCUITS MATTHYS PDF