Oscillators – With distributed parameter resonator
Reexamination Certificate
2000-12-08
2002-04-16
Mis, David (Department: 2817)
Oscillators
With distributed parameter resonator
C331S034000, C331S175000
Reexamination Certificate
active
06373344
ABSTRACT:
TECHNICAL FIELD
The invention relates to microwave synthesizers. In particular, the invention relates to a microwave synthesizer that has a dual YIG tuned oscillator.
BACKGROUND ART
Signal sources, sometimes referred to as frequency sources, are used to generate signals for use in many electronic systems. Signal sources can be divided into two main types known as the so-called “free-running” or “unlocked” sources and the “locked” or referenced sources. In general, there are two classes of unlocked signal sources: fixed frequency sources and tunable sources. Fixed frequency signal sources operate at a single frequency and can be made very stable, spectrally pure, and accurate by using so called high-Q resonators. High-Q resonator based sources include crystal oscillators, dielectric resonator oscillators, and surface acoustic wave (SAW) oscillators. While fixed frequency, high-Q resonator based sources offer high stability and state-of-the-art frequency accuracy, these high-Q resonator based sources generally produce a single, fixed frequency.
Tunable sources, on the other hand, provide an output that can be varied or tuned over a range of frequencies. The tunable sources are used in applications where the desired frequency is either unknown a priori during the system design or when the desired frequency must be changed or adjustable as a matter of system operation. Tunable sources usually provide continuous or nearly continuous tuning capability across their frequency range of operation. However, tunable sources generally exhibit poor frequency accuracy and frequency stability when compared to high-Q, fixed frequency sources. Therefore, when using unlocked sources for a given system design, a choice must usually be made between high stability, spectral purity, and accuracy on the one hand and frequency tunability on the other hand.
Applications requiring a source with both high-Q frequency stability/accuracy characteristics and frequency tunability generally necessitate the use of a so-called “locked” signal source. In a locked source, a tunable source is locked to or otherwise derived from a fixed or “reference” source. Often the tunable source is locked to a reference source using one or more feedback circuits or feedback loops. In the case of a locked source using a feedback loop, once the feedback loop is closed the tunable source can achieve frequency stability and accuracy that are a function of the reference source frequency stability. Such a configuration of tunable and fixed sources locked together is known in the art as a synthesized source or simply as a frequency synthesizer.
A frequency synthesizer, then, is a signal source that generates an output signal from one or more reference signals. In general, frequency synthesizers produce a signal consisting of a single frequency selected from among a finite set of discrete frequencies available by virtue of the design of the synthesizer. Frequency synthesizers of various forms and designs have been found to be highly useful if not essential in a wide variety of applications including FM car radios, sophisticated radar systems, and test equipment such as spectrum analyzers and signal generators.
In many cases, the synthesized signal produced by a given synthesizer is often at a higher frequency than that of the reference signal(s). The synthesized signal is typically a very stable, spectrally pure, single frequency signal having low or sometimes even very low phase noise. However, unlike other signal sources, such as a free-running voltage controlled oscillator (VCO), a given frequency synthesizer generally is capable of producing only a finite, although often large, number of selectable, discrete frequencies as an output signal. Therefore, frequency synthesizers are most often used where the frequency stability/precision and spectral purity are of paramount importance. Hereinbelow, frequency synthesizers will be referred to as variable frequency sources to distinguish them from the tunable, free-running sources that generally have a continuous tuning range.
A number of different types of frequency synthesizers or methods of frequency synthesis are known in the art including direct frequency generation, direct digital synthesis (DDS), and phase locked loop (PLL) frequency synthesis. The direct frequency generation synthesizer, while not strictly speaking considered a locked source, typically utilizes a combination of frequency multiplication, division, and addition to generate a desired frequency from one or more reference frequencies. Frequency multiplication is accomplished using a non-linear device, such as a step recovery diode or comb generator, to produce a large number frequency harmonics of a reference signal source frequency. Frequency division is typically accomplished using a digital frequency divider. Addition or subtraction of frequencies is implemented using a mixer, which ideally accepts two input signals and produces two output signals, one output signal at the frequency that is the sum of the two input signal frequencies and the other output signal at a frequency that is the difference of the two input signal frequencies. The combination of multiplication, addition, subtraction and division allows the direct frequency generation synthesizer to produce a finite number of output signal frequencies.
The DDS uses a digital to analog converter (DAC) to convert a digital data stream into an analog output signal. The digital data stream is a digital representation of a sampled version of the desired output signal, thus the DDS directly synthesizes the output signal. In a PLL synthesizer, a negative feedback loop is used to compare and “phase lock” the output signal of a tunable frequency source, such as a VCO, to a stable reference signal produced by one or more reference sources. When locked, the PLL output frequency is typically a multiple of the reference signal or linear combination of the reference signal and other signals generated by the synthesizer. There are also hybrid synthesizers that combine one or more of these or other various frequency synthesis approaches.
As is the case with frequency synthesizers in general, there are also many ways to realize a PLL synthesizer. In some applications, a simple single loop approach is acceptable or even preferred. In other instances, more complicated, multiple loop approaches are required. Generally, for high-performance applications, such as a local oscillator LO of a high-performance spectrum analyzer, the microwave synthesizer invariably involves multiple loops to optimize phase noise, spurious performance, sweep rate, and locking speed.
FIG. 1
illustrates a block diagram of a basic, single loop PLL synthesizer (SLS) used to synthesize a signal from a stable reference. The basic SLS comprises a voltage-controlled oscillator (VCO)
10
, a loop frequency divider
16
, a reference oscillator
18
, a phase/frequency comparator or detector (PFD)
20
, and a loop integrator
22
. The VCO
10
produces an output signal, the frequency of which is proportional to an input control voltage. The frequency divider
16
to create a lower frequency signal divides the output signal produced by the VCO
10
. The frequency divider
16
is an apparatus that accepts a signal at a frequency f and produces an output signal at a frequency f/N where N is the division factor of the frequency divider.
The signal produced by the frequency divider
16
is compared by the PFD
20
to a reference frequency signal produced by the reference oscillator
18
. The PFD
20
, in turn, produces an error voltage signal that is proportional to the phase/frequency difference between the frequency of the output signal of the frequency divider
16
and the reference signal frequency fref. The error voltage is integrated by the loop integrator
22
to produce the input control voltage of the VCO
10
.
In some implementations, an output amplifier
12
and a loop amplifier
14
are included in the basic SLS. The output amplifier
12
is used to amplify the output signal produced by the SLS. The loop
Agilent Technologie,s Inc.
Mis David
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