168 research outputs found
The Measurement of AM noise of Oscillators
The close-in AM noise is often neglected, under the assumption that it is a
minor problem as compared to phase noise. With the progress of technology and
of experimental science, this assumption is no longer true. Yet, information in
the literature is scarce or absent. This report describes the measurement of
the AM noise of rf/microwave sources in terms of Salpha(f), i.e., the power
spectrum density of the fractional amplitude fluctuation alpha. The proposed
schemes make use of commercial power detectors based on Schottky and tunnel
diodes, in single-channel and correlation configuration. There follow the
analysis of the front-end amplifier at the detector output, the analysis of the
methods for the measurement of the power-detector noise, and a digression about
the calibration procedures. The measurement methods are extended to the
relative intensity noise (RIN) of optical beams, and to the AM noise of the
rf/microwave modulation in photonic systems. Some rf/microwave synthesizers and
oscillators have been measured, using correlation and moderate averaging. As an
example, the flicker noise of a low-noise quartz oscillator (Wenzel 501-04623E)
is Salpha = 1.15E-13/f, which is equivalent to an Allan deviation of
sigma_alpha = 4E-7. The measurement systems described exhibit the world-record
lowest background noise.Comment: 39 pages, 22 figures, 8 tables, 21 references, list of symbol
On the 1/f Frequency Noise in Ultra-Stable Quartz Oscillators
The frequency flicker of an oscillator, which appears as a 1/f^3 line in the
phase noise spectral density, and as a floor on the Allan variance plot,
originates from two basic phenomena, namely: (1) the 1/f phase noise turned
into 1/f frequency noise via the Leeson effect, and (2) the 1/f fluctuation of
the resonator natural frequency. The discussion on which is the dominant
effect, thus on how to improve the stability of the oscillator, has been going
on for years without giving a clear answer. This article tackles the question
by analyzing the phase noise spectrum of several commercial oscillators and
laboratory prototypes, and demonstrates that the fluctuation of the resonator
natural frequency is the dominant effect. The investigation method starts from
reverse engineering the oscillator phase noise in order to show that if the
Leeson effect was dominant, the resonator merit factor Q would be too low as
compared to the available technology.Comment: 20 pages, list of symbols, 1 table, 6 figures, 43 reference
The effect of AM noise on correlation phase noise measurements
We analyze the phase-noise measurement methods in which correlation and
averaging is used to reject the background noise of the instrument. All the
known methods make use of a mixer, used either as a saturated phase detector or
as a linear synchronous detector. Unfortunately, AM noise is taken in through
the power-to-dc-offset conversion mechanism that results from the mixer
asymmetry. The measurement of some mixers indicates that the unwanted
amplitude-to-voltage gain is of the order of 5-50 mV, which is 12-35 dB lower
than the phase-to-voltage gain of the mixer. In addition, the trick of setting
the mixer at a sweet point -- off the quadrature condition -- where the
sensitivity to AM nulls, works only with microwave mixers. The HF-VHF mixers
have not this sweet point. Moreover, we prove that if the AM noise comes from
the oscillator under test, it can not be rejected by correlation. At least not
with the schemes currently used. An example shows that at some critical
frequencies the unwanted effect of AM noise is of the same order -- if not
greater -- than the phase noise. Thus, experimental mistakes are around the
corner.Comment: 16 pages, list of symbols, 8 figures, 27 reference
Applications of the optical fiber to the generation and to the measurement of low-phase-noise microwave signals
The optical fiber used as a microwave delay line exhibits high stability and
low noise and makes accessible a long delay (>100 microseconds) in a wide
bandwidth (about 40 GHz, limited by the optronic components). Hence, it finds
applications as the frequency reference in microwave oscillators and as the
reference discriminator for the measurement of phase noise. The fiber is
suitable to measure the oscillator stability with a sensitivity of parts in
1E-12. Enhanced sensitivity is obtained with two independent delay lines, after
correlating and averaging. Short-term stability of parts in 1E-12 is achieved
inserting the delay line in an oscillator. The frequency can be set in steps
multiple of the inverse delay, which is in the 10-100 kHz region.
This article adds to the available references a considerable amount of
engineering and practical knowledge, the understanding of 1/f noise,
calibration, the analysis of the cross-spectrum technique to reduce the
instrument background, the phase-noise model of the oscillator, and the
experimental test of the oscillator model.Comment: 23 pages, 13 figures, 41 reference
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