Patent Application
20120139586
The present application claims priority of European patent application 10193257.2 filed on Dec. 1, 2010.
The present invention relates to a frequency synthesizer and a corresponding frequency synthesizing method.
Frequency synthesizers are key building blocks for many microwave systems. They are found in many modern devices, including radio receivers, mobile telephones, satellite receivers, GPS systems, radars, etc. There are three main synthesizer architectures, in particular direct analog, direct digital and indirect (Phase Locked Loop) synthesizers. The requirements of the microwave systems are getting tough, so that the known synthesizers cannot fulfill requirements such as phase noise, switching speed, fine resolution, and frequency sweep. Recently, new hybrid architectures were developed which combines direct digital synthesizer (DDS) and phase locked loops (PLL) which, however, can also not fulfill all these requirements.
Millimeter wave/sub-THz frequency synthesizers which can generate ultra-broadband signals with fine frequency resolution and low phase noise and which can sweep linearly are not available and not known in the art. There are synthesizer architectures known having either limited bandwidth (as e.g. the architecture described in Dengler, R. J., Cooper, K. B., Llombart, N., Chattopadhyay, G., Bryllert, T., Mehdi, I., Siegel, P. H., “Toward real-time penetrating imaging radar at 670 GHz,” Microwave Symposium Digest, 2009 MTT '09, IEEE MTT-S International, pp. 941-944, 7-12 Jun. 2009), or coarse frequency resolution (using e.g. a PLL), or high phase noise (high multiplication factor), or combinations of these features (but not having all these features).
Dengler, R. J., Cooper, K. B., Llombart, N., Chattopadhyay, G., Bryllert, T., Mehdi, I., Siegel, P. H., “Toward real-time penetrating imaging radar at 670 GHz,” Microwave Symposium Digest, 2009 MTT '09, IEEE MTT-S International, pp. 941-944, 7-12 Jun. 2009 describes an imaging THz radar system in which an up-converter is used to mix a first fixed (high) frequency signal from a synthesizer with a second tunable (lower) frequency signal from a tunable source (chirper). The desired band of the output signal of the mixer is selected by use of a filter.
It is an object of the present invention to provide a frequency synthesizer and a corresponding frequency synthesizing method providing as many of the above mentioned features as possible, i.e. providing a large bandwidth, fine frequency resolution and low phase noise. Further, the ability to perform a continuous, preferably linear frequency sweep (required e.g. for radar applications) shall preferably be provided.
According to an aspect of the present invention there is provided a frequency synthesizer comprising:
According to a further aspect of the present invention there is provided a corresponding frequency synthesizing method comprising the steps of:
Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed frequency synthesizing method has similar and/or identical preferred embodiments as the claimed frequency synthesizer and as defined in the dependent claims.
The present invention is based on the idea that the base signals (i.e. the synthesized frequency output signal from the frequency signal generation unit) are generated by good phase noise oscillators (e.g. oscillators having a higher quality factor in the resonator tank of the oscillator). This kind of oscillators has very narrow bandwidth. However, fixed frequencies are required from the frequency signal generation unit. Afterwards, by mixing this frequency (i.e. the synthesized frequency output signal) with the frequency tuning signal from the frequency tuning unit and selecting the desired band of the mixer signal, a very low phase noise signal (the frequency synthesizer output signal), e.g. in the millimeter wave frequency band, can be generated (advantageously having a much larger bandwidth than signals that can be obtained by known frequency synthesizers.
Thus, with the frequency synthesizer and the frequency synthesizing method according to the present invention, ultra-broadband signals (e.g. in the frequency range 480-960 GHz; according to a preferred embodiment of the invention, this frequency is achieved by applying an additional frequency multiplication) with fine frequency resolution, high linearity, high chirp rate and good phase noise can be generated.
Further, as proposed in preferred embodiments of the present invention, different frequency bands (e.g. an upper side band and a lower side band of the mixer signal) can be used continuously with smooth transition between them, and a high linearity during frequency sweep can at the same time be achieved.
Generally, said frequency signal generation unit may simply comprise one (or more) oscillator, e.g. a dielectric resonator oscillator, for generating said synthesized frequency output signal at a predetermined frequency (in case of two or more oscillators at different frequencies). In a preferred embodiment, however, said frequency signal generation unit comprises at least one (preferably two or more) frequency signal generation loop circuit including a phase detector that compares the frequency and/or phase of a feedback signal received from a feedback loop to the phase of said first reference signal to obtain a control signal, an oscillator that generates a synthesized frequency output signal based on said control signal, and a feedback loop including a frequency down-conversion unit that down-converts said synthesized frequency output signal into said feedback signal. One of the advantages of such an embodiment is significantly better performance with regard to phase noise.
According to another aspect the present invention provides a frequency synthesizer comprising:
These and other aspects of the present invention will be apparent from and explained in more detail below with reference to the embodiments described hereinafter. In the following drawings
If needed, as in some embodiments and depending on the particular implementation of the frequency signal generation unit 24 and/or the frequency tuning unit 28, further reference signals may be provided by the reference signal source 22. Such further (low phase noise) reference signals may include (low phase noise) reference signals 50 for phase detectors and/or necessary low phase noise LO (local oscillator) signals for mixers provided in particular implementations of the frequency signal generation unit 24, and/or low phase noise reference signals (clocks) 52 for direct digital synthesizers (DDS) provided in particular implementations of the frequency tuning unit 28.
In addition, a control unit (not shown) may be provided for controlling the elements of the frequency synthesizer 20.
The frequency signal generation unit 24 provides certain fixed frequencies with a frequency difference fD. The frequency tuning unit 28 provides a linear continuous sweep with the bandwidth B. The frequency selection unit 30, which preferably also includes a multiplication unit, selects the desired frequency band from the mixer signal 46, e.g. in an embodiment either the lower sideband or the higher sideband. Then, the signal from the selected frequency band is preferably filtered, amplified and multiplied. Thus, a continuous, linear, ultra-broadband frequency sweep in a desired frequency range, e.g. in the mm-wave/THz frequency range, can be realized.
Preferred embodiments of the frequency signal generation unit 24a, 24b are shown in
By way of taking frequency signal generation loop circuit 60 as an example a preferred embodiment of the frequency signal generation loop circuits 60, 62, 64 shall be explained.
A phase detector 70 (also called phase frequency detector; PFD) compares the frequency and/or phase of a feedback signal 80 received from a feedback loop to the frequency and/or phase of said first reference signal 40 (in particular, detects frequency and/or phase differences) to obtain a control signal 82. A loop filter 71 is coupled to the output of the phase detector 70 for filtering the control signal 82 output from the phase detector 70. A controlled oscillator 72, e.g. a voltage controlled oscillator (VCO), is coupled to the output of the loop filter 71 and generates a synthesized frequency output signal 84 based on said control signal 82. Said synthesized frequency output signal 84 is output by an output unit 73, e.g. a splitter, which also provides the synthesized frequency output signal 84 to a feedback loop including a frequency down-conversion unit 74 that down-converts said synthesized frequency output signal 84 into said feedback signal 80.
The frequency down-conversion unit 74 preferably comprises a frequency divider 75 and a mixer 76, which down-mixes the output signal of the frequency divider 75 with a second reference signal 50a provided by said reference signal source 22 to obtain said feedback signal 80. Further, in some embodiments additional filters 77, 78 (in particular band-pass filters) in front of and/or behind the mixer 76 are provided in the feedback loop.
The VCOs 72 are preferably narrowband (high Q) oscillators to have very good phase noise characteristics. The mixer 76 in each frequency signal generation loop circuit 60, 62, 64 down-converts the synthesized frequency output signal 84 to the phase detector frequency. To cover a broader spectral range, i.e. to enable a frequency sweep over a broader bandwidth, the mixers 76 of the various frequency signal generation loop circuits 60, 62, 64 are provided with different second reference signals 50a, 50b, 50c and the controlled oscillators 72 are accordingly working at different frequencies. The frequencies of the oscillators are chosen in a way that the “chain” of upper and lower sidebands (see
The embodiment of the frequency signal generation unit 24a shown in
A first embodiment of a frequency tuning unit 28a is depicted in
In a simple embodiment as shown in
Other embodiments of the frequency tuning unit 28 are available as, for instance, shown in
Such a hybrid structure provides a very linear high resolution frequency sweeping. In order to have continuous sweep at the output of the frequency synthesizer, two hybrid DDS/PLL loop circuits, i.e. the frequency tuning sub-units 100, 102 are implemented in this embodiment. While one loop circuit is providing the LO input of the mixing unit 26, the other loop circuit is getting ready for the next sweep. By use of the switch 104 the necessary loop circuit is selected. Depending on the selection the upper frequency band or the lower frequency of the RF output of the mixing unit 26, the sweep direction of the hybrid DDS/PLL loop circuit is determined.
Various embodiments of the frequency selection unit 30 are shown in
The embodiment of the frequency selection unit 30a shown in
The embodiment of the frequency selection unit 30b shown in
The embodiment of the frequency selection unit 30c shown in
In particular, as shown in
The first frequency band 151 is the lower side band after up-conversion by the first frequency signal generation loop circuit 60. The second frequency band 151 is the lower side band after up-conversion by the second frequency signal generation loop circuit 62. The third frequency band 153 is the lower side band after up-conversion by the third frequency signal generation loop circuit 64. The fourth frequency band 154 is the upper side band after up-conversion by the first frequency signal generation loop circuit 60. The fifth frequency band 155 is the upper side band after up-conversion by the second frequency signal generation loop circuit 62. The sixth frequency band 156 is the upper side band after up-conversion by the third frequency signal generation loop circuit 64.
Hence, if, for instance, the synthesized frequency output signal 42 from the first frequency signal generation loop circuit 60 is switched onto the mixing circuit 26, the mixing signal 44 covers the first and fourth frequency bands 151, 154 having a considerable gap (in frequency direction) in between. Consequently, the requirements on the subsequent filter in the frequency selection unit 30 for filtering out the desired frequency band are less strict, i.e. the filter curve can be less steep as, for instance, in the known frequency synthesizers where there is no (or only a small) gap between neighboring frequency bands, as shown in
An example for the frequency allocation in a practical implementation could be as follows (referring again to the example explained with reference to
Preferably, the sweep of the frequency tuning unit signal 44 should be in different directions for the lower side bands (i.e. the frequency bands 151, 152, 153) than the upper side bands (i.e. the frequency bands 154, 155, 156). If the multiplication factor is 16, the output frequency of the frequency synthesizer will be from 480 to 960 GHz. If the used DDS in the frequency tuning unit 28 works with 32 bit, the reference clock of the DDS is 1 GHz and the output of the DDS is 100 MHz, the signal at the output of the DDS will have 0.23 Hz resolution, and at the output 48 of the frequency synthesizer has a fine resolution of 368 Hz (in the 480-960 GHz band).
A simple embodiment of a frequency signal generation unit 24c is shown in
It should be noted that particular elements used in the device and method of the present invention are generally known. This holds particularly for elements like loop filters, local oscillators, phase detectors, mixers and frequency dividers. Hence, in embodiments of the proposed frequency synthesizer standard elements can be used for those elements with the required settings and/or dimensions to achieve the desired effects.
The proposed frequency synthesizer can synthesize linear continuous frequency sweeps in microwave and millimeter wave frequencies. Any deterministic ultra-broadband frequency shapes can be generated, in particular in preferred embodiments using a digital signal generation in a DDS. The synthesized frequency has low phase noise in lower and higher in-band offset frequencies. The frequency synthesizer has very fine resolution (Hz), which mainly depends on the DDS performance. It is also capable of synthesizing multitude waveforms such as very linear, quadratic, cubic frequency chirps or deterministic deviations from linear frequency ramps.
The invention has been illustrated and described in detail in the drawings and foregoing description, but such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.
In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
Any reference signs in the claims should not be construed as limiting the scope.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10193257.2 | Dec 2010 | EP | regional |
