In certain oscillator applications, more than others, it is necessary to control modes of operation.
These modes of operation can be undesirable like harmonic, resonant, and spurious modes, or they can be
standard modes like fundamental, third overtone, fifth overtone, etc.

Undesired modes generally have a higher frequency than desired modes within a few hundred kHz. When
working with pull able crystals it is necessary to deposit large electrodes on the surface of the blank to allow
for pullability. This is a common cause of promoting spurs and can result in a crystal oscillating on a spurious
mode. Undesirable modes are usually specified in resistance or the ratio of resistance of the undesired mode
to that of the desired mode over a frequency bandwidth. A separation of 3dB between desired modes and
undesired modes is usually adequate to avoid operation in a spurious mode.

Listed below are two methods for controlling fundamental modes in third overtone circuits:

The inductor-less third overtone circuit (diagram A) is similar to a fundamental mode circuit except the
feedback resistor value should be made much smaller than normal, typically the value will fluctuate between
2kW and 6kW. Because of the resistor value, the component of inductive admittance is greater than the
admittance of the load capacitance at the fundamental frequency, thus preventing oscillation. Meanwhile,
the inductive admittance at the overtone is less than the admittance of the load capacitance therefore
enabling oscillation at third-overtone.

The tuning tank L/C overtone circuit (diagram B) requires an additional inductor and capacitor to select
the 3rd-Overtone mode, while suppressing or rejecting the fundamental mode. You will need to choose
inductance and capacitance values to satisfy the following 3 conditions:

1. The Lc/Cc section forms a series resonant circuit at a frequency below the fundamental
frequency, which makes the circuit look inductive at the fundamental frequency. This condition
does not favor oscillation at fundamental mode.

2. The Lc/Cc and C2 components form a parallel resonant circuit at a frequency about half-way
between the fundamental and 3rd-Overtone frequencies. This condition makes the circuit
capacitive at the third overtone frequency and favors oscillation at the third overtone mode.

3. Locating the Lc tank at the output of the inverter is ideal because it helps to clean up all
unwanted modes before signal goes through the crystal.

A PDF version of these notes is available here.

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