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Design Ideas: August 18, 1994

Oscillator lowers distortion and phase noise

Giovanni Stochino,
Ericsson Telecommunicazioni Spa, Rome Italy

In addition to its normal role of controlling oscillation frequency, the crystal in Fig 1 also acts as a high-Q, self-tuned output filter, which rejects the noise and harmonics of its input voltage, VB. The result is a clean output-frequency spectrum. With an output frequency of 10.7 MHz, second harmonic distortion is -76 dBc, and third-harmonic distortion is -72 dBc. Other harmonics are less than 80 dBc, and output power is +7 dBm at 50 Ohms. C1 and R4 counterbalance the undesirable effects of the parallel capacitance CP of the crystal, which represents a side path through which the highest harmonics of VB can reach the output.

The circuit is basically a Clapp oscillator except that a four-transistor voltage follower (Q1 to Q4) replaces the single common-collector transistor. Also, instead of grounding the crystal, this design connects it to a virtual ground, VG. Therefore, the circuit forces the crystal current, IX, to flow through R5, thus producing the output voltage VOUT= -R5IX. The series resonance of the crystal mostly determines the oscillation frequency according to

fO[wavy=]'1/(2pi(LSCS)½).

Compared with the common single-transistor design, this modified Clapp oscillator has a more symmetrical and linear transconductance (g(IE)=IE/VB) over a wide range of VB. R1, R2, and R3 help control the excess of the oscillator loop gain, and, hence, the amplitude and wave shape of crystal voltage VB. With the values specified, the oscillator is stable under all operating conditions and produces a fairly sinusoidal waveform at node B.

For best performance, the impedance at node G must be low over a wide frequency range. Thus, the circuit uses a wideband current-feedback amplifier that has a built-in low open-loop impedance at its inverting input. R5 further reduces the impedance at node G to fractions of an ohm over a wide frequency range. Due to the high Q of the crystal filter, the circuit also rejects all out-of-band noise produced by both the four-transistor voltage follower and IC1, which results in low phase noise.


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