Interferometric Noise Measurement Using Frequency Offset
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Solution Overview
Problem
Existing methods for measuring phase and/or amplitude noise at microwave frequencies are limited by the noise of the mixer, particularly DC voltage drift, which affects the baseband signal at small frequency offsets, reducing sensitivity and accuracy.
Innovation Solution
An interferometric apparatus that generates a third signal with a carrier frequency offset, using a bridge configuration with carrier suppression and amplification, and mixing means to produce a signal characteristic of phase and/or amplitude noise, while employing a lock-in amplifier to translate noise to a higher frequency, reducing the impact of mixer noise and DC drift.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a double-balanced mixer is used to compare the reference signal with the modified signal, then a baseband signal representative of phase or amplitude noise is produced, but the mixer's noise dominates the baseband signal when the device is sufficiently low noise
Solution Approach 1:
The patent translates the noise measurement from the baseband frequency region (where mixer noise dominates) to a higher frequency offset region by modulating the reference signal with a dither signal. This frequency translation moves the measurement to a dimension where thermal noise dominates and mixer noise (particularly DC voltage drift) is significantly reduced, improving measurement sensitivity for low-noise devices.
Solution Approach 2:
The patent introduces a dither signal as an intermediary modulation signal that shifts the noise measurement to a different frequency domain. This intermediary signal acts as a carrier that transports the phase/amplitude noise information to a frequency region where the measurement is not contaminated by mixer noise, enabling accurate measurement of low-noise devices.
2Measurement precision
If carrier suppression is used to amplify noise sidebands and reduce mixer noise influence, then measurement sensitivity improves, but DC voltage drift still influences the baseband signal at small frequency offsets
Solution Approach 1:
The patent moves the measurement away from the baseband (low frequency offset) region where DC voltage drift affects the signal, to a higher frequency offset region determined by the dither signal frequency. This dimensional shift in frequency space eliminates DC voltage drift influence while maintaining the benefits of carrier suppression for noise sideband amplification.
Solution Approach 2:
The patent applies preliminary modulation of the reference signal with a dither signal before the mixing process. This preliminary action pre-shifts the frequency content of the signals, ensuring that when mixing occurs, the noise measurement is automatically translated to a higher frequency region where DC voltage drift does not contaminate the measurement.
3Measurement precision
If the reference signal is modulated with a dither signal to translate noise to a higher frequency, then mixer noise and DC drift impact is reduced, but device complexity increases
Solution Approach 1:
The dither signal generator and modulation circuitry can be integrated into existing signal generation equipment, allowing the same hardware to serve both as the primary reference signal source and as the dither modulation source. This multi-functionality approach reduces overall device complexity while maintaining the measurement sensitivity improvements.
Solution Approach 2:
The patent combines the dither signal generation and modulation function with the existing reference signal generation path. By merging these functions into a unified signal generation architecture rather than adding completely separate systems, the increase in device complexity is minimized while still achieving the frequency translation benefit for reduced mixer noise impact.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution significantly improves measurement sensitivity by reducing mixer noise and DC drift, maintaining thermal noise limits even at low frequency offsets, achieving an improvement of over 10 dB at 1 Hz Fourier offset frequency compared to prior art.
Implementation Method 1
first mixing means responsive to the third signal and to the carrier suppressed signal to produce a signal characteristic of phase and/or amplitude noise of the device under test at an offset frequency
Implementation Method 2
signal processing means comprising lock-in amplifier means arranged to produce the offset signal and to operate at the frequency of the offset signal
Data Source
AI summary
An interferometric apparatus (10) for producing an output signal characteristic of phase and/or amplitude noise of a device under test (22), an input signal being provided to the interferometric apparatus (10), comprising a signal generation means (36,38) arranged to produce a third signal having a carrier frequency offset from that of the input signal; a first bridge (12) having first (14) and second (16) arms, the first (14) and second (16) arms having input thereto first and second signals, respectively, produced from one of the input signal or the third signal; the device under test (22) being provided in one of the first (14) or second (16) arms of the first bridge (12); a carrier suppression means (24) connected to the first (14) and second (16) arms of the first bridge (12) to produce a carrier suppressed signal; a first amplifier (32) arranged to amplify the carrier suppressed signal; first mixing means (34) responsive to the third signal and to the carrier suppressed signal to produce a signal characteristic of phase and/or amplitude noise of the device under test (22) at an offset frequency.