Patent Application
20060214706
Embodiments in accordance with the invention are related to frequency dividers, and particularly to reduced phase noise dividers.
An ideal signal source generating a single frequency should produce a sine wave output. Mathematically, such an output would be described as:
V(t)=V0 sin 2πf0t
where V0 is the nominal amplitude and f0 is the nominal frequency. Unfortunately, such signal sources are not realizable. Actual signals are better described mathematically as:
V(t)=(V0+ε(t)) sin (2πf0t+Δφ(t))
where ε(t) represents amplitude fluctuations versus time, and Δφ(t) represents phase fluctuations versus time.
The time varying quantities, ε(t) and Δφ(t), can be broken into two components, spurious signals and noise. Spurious signals, also called spurs, are discrete signals appearing as distinct spectral lines in a spectral density plot. These signals are usually related to known phenomena such as unwanted mixer products, leakage of other signals, and the like. Noise is random in nature. Sources of random sideband noise in a signal. source such as an oscillator include thermal noise, shot noise, and flicker noise in the components making up the source. A signal source such as a phase-locked-loop digital synthesizer will typically contain both components in it's amplitude modulated (AM) and phase modulated (PM) spectra.
Phase noise in a signal may be measured and described as the ratio of the power at an offset f Hertz away from the carrier frequency to the power at the carrier frequency. This measurement is known in the art as L(f).
A common approach to reducing phase noise from a signal source is to divide the signal down, commonly by a power of two such as 2, 4, 16, or 256 for example. An ideal divider should reduce phase noise 6 dB per octave, and of course, not introduce any noise itself.
Unfortunately, ideal dividers are not realizable. Non-ideal dividers introduce noise into the signal they are processing.
A frequency divider with reduced phase noise converts a single-ended electrical input signal to differential form, feeds the differential signal to one or more differential frequency dividers, and finally to a differential limiting amplifier. The input stage of one of the frequency dividers may be used for the single-ended to differential signal conversion.
Input stage 110 may be a separate stage, or it may be a part of divider 120. In achieving the best noise performance, it is important to provide a low phase-noise signal to input 100. Many signal sources achieve their lowest noise levels at their highest output power. It may be necessary to use an attenuator between the signal source and input 100 to stay within the signal handling range of input stage 110. In the embodiment shown, the maximum signal input level is on the order of +6 dBm.
Divider blocks 120 and 130 contain flip-flop dividers plus selection circuitry responsive to select inputs 125 and 135 which select divide ratios of 1 (pass-through), 2, 4, 8, or 16, corresponding to one, two, three, or four divide stages internal to the divider block. Other divider configurations are also possible, so long as the final divide stage is a divide-by-two, providing an output signal with a 50% duty cycle. As an example, an arbitrary divide-by-N block could be followed by a divide-by-two. With two such divider blocks in series, divide ratios in powers of two up to 256 may be achieved. Fully differential ECL topologies are used in the signal paths of divide blocks 120 and 130.
In the embodiment shown, input stage 110 and divider blocks 120 and 130 are constructed using Gallium Arsenide heterojunction bipolar transistors (GaAs HBT), and typically demonstrate performance up to 15 GHz. Other ECL families or similar forms of current-mode logic may be used if this frequency range is not needed.
Differential limiting amplifier 140 takes differential output 138 of divider stage 130 and produces complimentary (differential) outputs 145 and 148. Only one output may be used, or both outputs may be used, providing a differential output, or two complimentary outputs. A limiting amplifier takes an input signal and provides an output signal with a full-range voltage swing given a wide range of inputs. In the embodiment shown, a differential limiting amplifier using GaAs HBT transistors and having a single-ended gain of about 28 dB and a maximum frequency response of 15 GHz was used.
Practitioners of the electronic arts will appreciate that in making measurements at these low noise levels, and designing equipment to achieve these low noise levels, careful attention must be given to circuit board layout including characteristic impedance of signal lines, adequate grounding and shielding, proper bypassing of power supplies, providing stable, low-noise power supplies, and the like. Cables and connections must be high quality and secure, as noise may be induced by environmental factors such as vibration and/or temperature fluctuation.
As will be understood by those familiar with the art, the noise level of the signal source dominates in the region up to approximately 100 KHz from the carrier; it is difficult to distinguish among the different dividers in this region.
Line 210 shows the phase noise performance of a commercially available high-performance divider such as the Agilent Technologies model 70429A Option K90 divider. This divider provides very good phase noise performance, on the order of −145 dBc/Hz. Line 220 shows the performance of a 70429A Option K90 divider under optimal operating conditions, dropping the phase noise outside 100 KHz to around −155 dBc/Hz. Line 230 shows the phase noise performance of a divider according to the present invention, producing phase noise values on the order of −160 dBc/Hz.
While the embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to these embodiments may occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.
