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, V_B. 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 C_P of the crystal, which represents a side path through which the highest harmonics of V_B 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, V_G. Therefore, the circuit forces the crystal current, I_X, to flow through R₅, thus producing the output voltage V_OUT= -R₅I_X. The series resonance of the crystal mostly determines the oscillation frequency according to

f_O[wavy=]'1/(2pi(L_SC_S)^½).

Compared with the common single-transistor design, this modified Clapp oscillator has a more symmetrical and linear transconductance (g(I_E)=I_E/V_B) over a wide range of V_B. R₁, R₂, and R₃ help control the excess of the oscillator loop gain, and, hence, the amplitude and wave shape of crystal voltage V_B. 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.