Variable Frequency Circuit for RF Attenuator Noise Reduction
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Solution Overview
Problem
Conventional negative voltage generators in electronic systems introduce spurious tones, which increase noise and can lead to functional failures by reducing noise margins and failing stringent noise specifications.
Innovation Solution
A variable frequency circuit with a dithering circuit that generates a sequence of numbers to adjust the frequency of the output, spreading spurious tones randomly or pseudo-randomly through the frequency domain, thereby lowering the noise floor, using a variable oscillator or programmable frequency divider controlled by a dithering circuit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional negative voltage generator uses a fixed frequency oscillator, then the circuit structure is simple, but spurious tones are concentrated at specific harmonics causing high noise floor
Solution Approach 1:
The patent applies dynamics by making the oscillator frequency variable instead of fixed. A dithering circuit generates a pseudo-random sequence that modulates the oscillator frequency, causing spurious tones to spread across the frequency spectrum rather than concentrate at discrete harmonics. This dynamic frequency modulation resolves the contradiction by accepting increased circuit complexity (adding dithering circuitry) to achieve the benefit of reduced noise floor through spectral spreading.
Solution Approach 2:
The patent changes the frequency parameter of the oscillator dynamically. By modulating the oscillator frequency according to a pseudo-random sequence generated by the dithering circuit, the spurious tones are distributed across a broader frequency range. This parameter change approach transforms the static frequency into a time-varying parameter, effectively lowering the noise floor at any given frequency point while maintaining the overall signal integrity.
2Object-affected harmful factors
If spurious tones are reduced by downstream filtering, then the noise floor is lowered, but the circuit area increases and signal integrity may be compromised
Solution Approach 1:
The patent applies preliminary action by addressing the spurious tones at their source (the oscillator) rather than attempting to filter them downstream. The dithering circuit modifies the oscillator frequency in advance to prevent spurious tone concentration, eliminating the need for additional filtering stages. This source-level intervention reduces circuit area while effectively lowering the noise floor.
3Object-affected harmful factors
If the oscillator frequency is modulated to spread spurious tones, then the noise floor is reduced, but the frequency stability may be affected
Solution Approach 1:
The patent uses periodic action through the dithering circuit that generates a pseudo-random sequence with specific statistical properties. This sequence modulates the oscillator frequency in a controlled manner, spreading spurious tones while maintaining predictable statistical characteristics. The periodic nature of the modulation ensures that frequency stability is preserved in terms of statistical behavior, even though the instantaneous frequency varies.
Data Source
AI summary
Apparatus and methods for distributing spurious tones through the frequency domain are disclosed. One such apparatus can include a dithering circuit configured to generate a sequence of numbers that exhibit statistical randomness and a variable frequency circuit configured to adjust a frequency of an output based on the sequence of numbers so as to spread energy of spurious tones in a frequency response of the output to lower a noise floor. In one example, spurious tones can be reduced in a negative voltage generator of a radio frequency (RF) attenuator.