MC13175 Datasheet, PDF (12/17 Pages) Motorola, Inc – UHF FM/AM TRANSMITTER

MC13175 MC13176
Figure 20. Input Data Waveform
Figure 21. Frequency Deviation
Figure 22. Modulation Spectrum
–10
– 20
– 30
– 40
(dBc)
Figure 23. Unmodulated Carrier
(dBc)
Reference Crystal Oscillator (Pins 8 and 9)
Selection of Proper Crystal: A crystal can operate in a
number of mechanical modes. The lowest resonant
frequency mode is its fundamental while higher order modes
are called overtones. At each mechanical resonance, a
crystal behaves like a RLC series–tuned circuit having a
large inductor and a high Q. The inductor Ls is series
resonance with a dynamic capacitor, Cs determined by the
elasticity of the crystal lattice and a series resistance Rs,
which accounts for the power dissipated in heating the
crystal. This series RLC circuit is in parallel with a static
capacitance, Cp which is created by the crystal block and by
the metal plates and leads that make contact with it.
Figure 24 is the equivalent circuit for a crystal in a single
resonant mode. It is assumed that other modes of resonance
are so far off frequency that their effects are negligible.
Series resonant frequency, fs is given by;
fs = 1/2π(LsCs)1/2
and parallel resonant frequency, fp is given by;
fp = fs(1 + Cs/Cp)1/2
Figure 24. Crystal Equivalent Circuit
L3
Cp
R3
C3
the frequency separation at resonance is given by;
∆f = fp–fs = fs[1 – (1 + Cs/Cp)1/2]
Usually fp is less than 1% higher than fs, and a crystal exhibits
an extremely wide variation of the reactance with frequency
between fp and fs. A crystal oscillator circuit is very stable
with frequency. This high rate of change of impedance with
frequency stabilizes the oscillator, because any significant
change in oscillator frequency will cause a large phase shift
in the feedback loop keeping the oscillator on frequency.
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MOTOROLA RF/IF DEVICE DATA