Augusto
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Location: San Diego, CA
| PCM and PPM are encoding formats. They are both FM modulated. In other words the transmission method is the same but the information transmitted is different (Same phone call different languages).
Radio crystals are part of an oscillator circuit. They are called crystals because they are an actual quartz disc cut in a certain angle that optimized the frequency resonating band and to a certain size and thickness that determines the exact resonating frequency.
In a quartz crystal, a thin slice of quartz is placed between two electrodes. An alternating voltage applied to these electrodes causes the quartz to vibrate, or oscillate, at a particular frequency. The frequency is a function of the thickness of the crystal. By carefully preparing a crystal, it can be made to oscillate at any frequency. In our RC frequency applications most crystals are of the "AT cut" type.
Even though two crystals have the same resonating frequency they have additional characteristics that make them fit for a certain oscillator circuit.
The fact that there are two electrodes closely spaced together makes any crystal a capacitor. The crystal capacitance value then becomes part of the circuit's parameters to determine the oscillating frequency.
Two crystals with the same resonating frequencies can have different capacitances because they can have different disc sizes and different electrode areas. This explains why even though you might have two crystals with the same resonating frequencies one will work in a circuit while another one won't work in the same circuit.
As a matter of fact if you want to get really precise a crystal is a whole circuit in itself. As such it has an equivalent circuit.
For those technically inclined here's an explanation of the parameters that determine the equivalent circuit's component values:
The equivalent circuit consists of L1 (motional inductance), R1 (resistance), and C0 (shunt capacitance). All the parameters can be customized through design but are tied together so that a change to one parameter changes other parameters.
L1: The motional inductance of the crystal is determined by the mechanical mass (of quartz) in motion. The lower frequencies (thicker and larger quartz wafers) tend to run at a few henries where higher frequencies (thinner and smaller quartz wafers) tend to run at a few millihenries. The L1 and C1 are related by Thomson's formula:
L1 = 1/ (4p2f C1)
It is preferable to have the customer specify C1 (if necessary). Then, the L1 will follow the above formula.
C1: The motional capacitance of a crystal is determined by the stiffness of the quartz (which is constant), the area of metalization (electrode size) on the face of the crystal and the thickness and shape of the wafer. At lower frequencies, the wafer must be shaped (contoured or beveled) to improve the performance of the device. This will lower the C1 of the device. The C1 for fundamental mode crystals can range from approximately .005pF to .030pF. As a general rule, if a fundamental design is used on an overtone, the C1 will divide by the square of the overtone (i.e., 3rd overtone will be 1/9 of the fundamental).
C0: The shunt capacitance of a crystal is due in part to the thickness of the wafer. This is the measured capacitance while not vibrating, Shunt capacitance ranges from 1-7pF. It is not typical to exceed 7pF due to compatibility with the oscillator circuit.
So to recap, you either have two different oscillator circuits that need different types of crystals or you might just have a deffective crystal.
Augusto
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