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When low noise is required in the microwave
(or higher) frequency range, SAW oscillators and dielectric resonator
oscillators (DROs) are sometimes used.
When compared with multiplied-up (bulk-acoustic-wave) quartz oscillators,
these oscillators can provide lower noise far from the carrier at the expense
of poorer noise close to the carrier, poorer aging, and poorer temperature
stability. SAW oscillators and DROs
can provide lower noise far from the carrier because these devices can be
operated at higher drive levels, thereby providing higher signal-to-noise
ratios, and because the devices operate at higher frequencies, thereby
minimizing the "20 log N" losses due to frequency multiplication by
N. L(f) = -185 dBc/Hz noise
floor (at 400 kHz from the carrier) has been achieved with a 400 MHz SAW
oscillator. Of course, as is the case
for high-frequency bulk-wave oscillators, such noise floors are realizable
only in environments that are free of vibrations at the offset frequencies of
interest.
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Shown above are the phase noises (with no
vibration) of three low-noise oscillators multiplied to 10 GHz: a 5 MHz
bulk-acoustic-wave (BAW) oscillator, a 100 MHz BAW oscillator and a 500 MHz
SAW oscillator. The SAW oscillator is
better at far from the carrier; the BAW oscillators are better close to the
carrier.
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As shown on the next page, in the presence
of vibration, it is the vibration induced phase noise that dominates the
phase noise (except very far from the carrier). The device that has the lower acceleration
sensitivity will have the lower phase noise, independent of the effects of
frequency multiplication.
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G.K. Montress &
T.E. Parker,”Design and Performance of an Extremely Low Noise Surface
Acoustic Wave Oscillator”, Proc. 1994 IEEE Int’l Frequency Control Symposium,
pp. 365-373, 1994.
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