Extra Class question of the day: Piezoelectric crystals and MMICs

September 19, 2012 Dan KB6NU

Piezoelectric crystals are used in several amateur radio applications. They are called piezoelectric crystals because they use the piezoelectric effect, which is the physical deformation of a crystal by the application of a voltage. (E6E03) The equivalent circuit of a quartz crystal consist of motional capacitance, motional inductance and loss resistance in series, with a shunt capacitance representing electrode and stray capacitance. (E6E10)

Perhaps the most common use for a piezoelectric crystal is as the frequency-controlling component in an oscillator circuit. To ensure that a crystal oscillator provides the frequency specified by the crystal manufacturer, you must provide the crystal with a specified parallel capacitance. (E6E09)

Piezoelectric crystals are also used in crystal filters. A crystal lattice filter is a filter with narrow bandwidth and steep skirts made using quartz crystals. (E6E01) The relative frequencies of the individual crystals is the factor that has the greatest effect in helping determine the bandwidth and response shape of a crystal ladder filter. (E6E02) A “Jones filter” is a variable bandwidth crystal lattice filter used as part of a HF receiver IF stage. (E6E12)

Monolithic microwave integrated circuits, or MMICs, are ICs that are made to perform various functions at high frequencies. Gallium nitride is the material that is likely to provide the highest frequency of operation when used in MMICs. (E6E11)

The characteristics of the MMIC that make it a popular choice for VHF through microwave circuits are controlled gain, low noise figure, and constant input and output impedance over the specified frequency range. (E6E06) For example, a low-noise UHF preamplifier might have a typical noise figure value of 2 dB. (E6E05) 50 ohms is the most common input and output impedance of circuits that use MMICs. (E6E04)

To achieve these specifications, great care is taken in building and using an MMIC. For example, microstrip construction is typically used to construct a MMIC-based microwave amplifier. (E6E07) The power-supply voltage is normally furnished to the most common type of monolithic microwave integrated circuit (MMIC) through a resistor and/or RF choke connected to the amplifier output lead. (E6E08)

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