Use of stable, adjustable active feedback analog filters in frequency synthesis

Abstract

The present invention provides a method and apparatus for generating RF signals. [Solution] A method and apparatus for generating an RF signal uses digital signal components to generate a composite RF signal having multiple frequency components. An analog filter is used to filter the composite RF signal. The analog filter is an adjustable active feedback circuit having one or more variable resonators and a variable gain block connected to a signal loop defined by a passband. The analog filter is adjusted so that the passband of the analog filter overlaps with one or more desired frequency components of the multiple frequency components of the composite RF signal, and the passband has a relative bandwidth of about 1% or less.

Description

Technical Field

[0001] This application relates to frequency synthesis for radio frequency (RF) applications, and more particularly to high-speed frequency switching applications such as frequency hopping communication.

Background Art

[0002] The need for any frequency in wireless communication has led to the creation of numerous devices for frequency synthesis. Currently, several frequency conversion techniques are available in the prior art, but with the emergence of various frequency hopping communication systems, Direct Digital Sequence (DDS) has become widely used.

[0003] Conventional DDS is a technique for using a digital data processing block as a means for generating both a frequency-adjustable and phase-adjustable output signal that references a fixed-frequency precision reference clock source. In essence, the reference clock frequency is "divided" in the DDS architecture by a scaling factor described in a programmable binary tuning word stored in a frequency control register (FCR). This "division" generally limits the maximum synthesized frequency to the Nyquist frequency limit, which is half of the reference oscillator clock frequency. A block diagram of a conventional DDS 10 is shown in FIG. 1 and includes digital signal components such as a frequency control register (FCR) 12, a numerically controlled oscillator (NCO) 14, a reference oscillator 16, a digital-to-analog converter 18, and a reconstruction low-pass filter 19.

[0004] Conventional numerically controlled oscillators (NCOs) are DSP implementations of general-purpose local oscillators (LOs) required for desired transceiver frequency conversion and signal correlation. NCOs are driven by the same reference oscillator clock and controlled by a frequency transformer (FCR). The tuning word is typically 24-48 bits long and is largely driven by a lookup table (LUT) with high resolution in terms of phase and amplitude increments, resulting in the need for large read-only memory (ROM). Often included within the NCO is a broadband adjustable DSP-based filter, typically used to remove undesirable spurious output signals. Adding a high-speed digital-to-analog converter (DAC) to the NCO yields an analog sinusoidal RF output suitable for driving analog components, where the term RF is not limited to low or high frequencies. Typically, this analog output is further passed through a low-pass filter to remove all undesirable high-frequency spurious components from the resulting frequency synthesis. Conventional DDSs typically rely heavily on digital signal processing. Figure 2 shows an example of a 7MHz signal generated by a 100MHz clock DDS with a large ROM LUT featuring a 14-bit accumulator, 4-bit dither, and a 12-bit DAC.

[0005] A significant advantage of DDS output is that it is phase-coherent regardless of the imposed frequency and phase modulation. Furthermore, similar to continuous frequency sweeps, transitions from one frequency output to another may be time-synchronous during the transition. This is important in modern radar and radio communication implementations. The usefulness of DDS lies in the fact that frequency transitions can be instantaneous over wide frequency intervals without losing phase coherence, as is typical of LO based on phase-locked loop (PLL) frequency synthesis.

[0006] Therefore, in communication systems, DDS-based modulated signals can remain coherent from one transmit data burst to the next, which can be mapped to higher data throughput. In frequency-agile radar applications, the transmit signal remains coherent across multiple pulses. Thus, more signal information about the target can be abstracted from the return echo signal. For example, the distance to the target can be estimated from the phase of the return signal as the frequency changes.

[0007] Some of the drawbacks of the DDS frequency synthesis architecture may include: - Power: DSP-based signal processing and DSP-based NCOs have relatively high power requirements. For example, a device operating at 3.5 GHz can consume up to 4W, which poses a significant challenge for handheld communications. - Output Frequency: In principle, a DDS can generate a sinusoidal output from 0Hz to any harmonic of the clock frequency. However, due to potentially high time-domain response distortion at higher combined frequencies, the DDS is effectively limited to half of the reference clock frequency with respect to higher frequencies. Furthermore, the output frequency is limited by the Nyquist frequency, which is half of the clock frequency. - Large-capacity memory required for the control FCR. - Circuit footprint.

[0008] However, the main drawback of DDS is the number of spurious frequency components present around the intended frequency components of the synthesized sinusoidal signal. These not only interfere with frequency conversion but also interfere with adjacent frequency bands, potentially resulting in intermodulation distortion.

[0009] A pure sinusoidal frequency reference signal with a frequency spectrum undistorted by spurious frequency components near the desired frequency is beneficial to the high-performance signal processing architectures of modern RF communication architectures. DDS LUTs and DACs must support high clock rates so that there is a sufficient number of DAC readout samples per cycle of the RF signal generated by the DDS, but future 5G networking requirements are far above 20 GHz. Creating this pure composite frequency can be challenging using DDS technology. [Overview of the project]

[0010] According to one embodiment, a method for generating a signal is provided, the method comprising: generating a composite signal having a plurality of frequency components using a digital signal component; filtering the composite signal using an analog filter having a passband, wherein the analog filter has an adjustable active feedback circuit having one or more variable resonators and a variable gain block connected to a signal loop; and adjusting the analog filter such that the passband of the analog filter overlaps with one or more desired frequency components of the plurality of frequency components of the composite signal, and the passband has a relative bandwidth of about 1% or less.

[0011] In other embodiments, the method may further include, alone or in combination, one or more of the following elements: the composite signal may be frequency-transformed such that multiple frequency components include multiple transformed frequency components; the composite signal may be filtered before and after the composite signal is frequency-transformed; one or more desired frequency components may be filtered from frequency components that may be close to 0.01% to 0.1% of one or more desired frequency components; the analog filter may include multiple analog filters, and the multiple analog filters may include multiple separate passbands; the method may further include the step of constructing a composite signal and the step of adjusting the analog filters to one or more different desired frequency components.

[0012] According to one embodiment, an apparatus for generating a signal is provided, the apparatus comprising: a synthesizer that generates a composite signal having a plurality of frequency components; an analog filter connected to process the composite signal, the analog filter having a passband for filtering the composite signal, the analog filter having an adjustable active feedback circuit having one or more variable resonators and a variable gain block connected to a signal loop; and a controller connected to the synthesizer and the analog filter. The controller may include commands to adjust the analog filter such that the passband of the analog filter overlaps with one or more desired frequency components of the plurality of frequency components, the analog filter can generate a relative bandwidth of about 1% or less of the passband.

[0013] In other embodiments, the apparatus may, alone or in combination, further include one or more of the following elements: a frequency converter adapted to transform a composite signal such that multiple frequency components include multiple transformed frequency components; an analog filter may be connected between the synthesizer and the frequency converter, and may further have additional analog filters connected downstream of the frequency converter; one or more desired frequency components may be filtered from frequency components within a range of about 0.1% to about 1% of the desired frequency components; the analog filter may include multiple analog filters; the synthesizer may be adapted to adjust multiple frequency components, and the controller may further include instructions to adjust the analog filters to different desired frequency components.

[0014] In other embodiments, the features described above can be combined in any reasonable combination, as will be recognized by those skilled in the art. [Brief explanation of the drawing]

[0015] These and other features will become clearer from the following description with reference to the attached drawings. The drawings are for illustrative purposes only and are not limiting in any way. [Figure 1] This is a block diagram of a conventional direct digital sequencing (DDS) circuit. [Figure 2] This graph shows the time-domain response of a DDS system that generates a 7MHz signal using a 100MHz clock. [Figure 3a] This is a block diagram of ATF-DDS. [Figure 3b] This is a block diagram of an analog filter having one or more variable resonators in an active feedback loop. [Figure 3c] This is a block diagram of an analog filter having one or more variable resonators in an active feedback loop. [Figure 3d] This is a block diagram of an analog filter having one or more variable resonators in an active feedback loop. [Figure 4] This is a plot of the spectrum of a conventional DDS that generates a 7MHz signal using a 100MHz clock, showing one of the high-frequency spria stones at 93MHz. [Figure 5] Figure 4 shows a plot of the smooth normalized frequency response of an analog filter centered on a 7MHz signal and the structured normalized output spectrum of the ATF-DDS. [Figure 6] This graph shows the spectrum of the frequency-converted DDS output with a fundamental frequency of 7 MHz and a clock frequency of 100 MHz, the frequency response of the analog filter, and the normalized output spectrum of the ATF-analog filtered DDS signal. [Figure 7] This plot shows the frequency-converted DDS output, the tuned analog passband, and the filtered frequency response centered at 193 MHz. [Figure 8] This is a block diagram of an ATF filter and a combination of frequency conversion and DDS for generating a desired sinusoidal output carrier signal. [Figure 9] This is a block diagram of a DDS with an ATF filter and an additional power detector as a calibration element. [Modes for carrying out the invention]

[0016] Next, a method and apparatus for generating an RF signal will be described. Digital signal components are used to generate a composite signal having multiple frequency components. The composite signal is filtered by an analog filter. The analog filter may be an adjustable active feedback circuit having one or more variable resonators and a variable gain block connected to the signal loop. The analog filter applies an adjustable passband to the composite signal to select one or more DDS composite frequency components. By narrowing and / or adjusting the passband, the method and apparatus make it possible to generate an analog signal having a desired frequency content. This may be used to generate an approximation of a desired continuous single-frequency analog signal at a desired frequency with the smallest bandwidth. Circuits implemented using such an analog adjustable filter (ATF) are referred to herein as ATF-DDS. The frequency synthesis circuits described herein use the term DDS, but it will be understood that the circuits may use digital signal components other than well-known DDS designs, and the circuits may be based on different circuit designs used to generate a composite signal that can then be appropriately filtered to produce a desired output signal.

[0017] In one example, FIG. 3a shows a block diagram of an ATF-DDS20 that uses a narrowband, analog, high-performance adjustable filter 100 downstream of the DDS10. In this example, the ATF-DDS20 includes digital signal components similar to those shown in FIG. 1, such as a frequency control register (FCR) 12, a numerically controlled oscillator (NCO) 14, a reference oscillator 16, and a digital-to-analog converter 18. These components are known in the art and are connected and operated as known in the art. The outputs of these components are input to the analog filter 100 to generate a desired analog output 22. A controller 24 for controlling the analog filter 100, such as for adjusting the passband as described herein, is preferably included. Also, the controller 24 can be connected to control one or more other circuit components and can operate based on user input, pre-programmed parameters, or a combination thereof. Specific control techniques for the ATF-DDS will not be discussed further. Note that the ATF-DDS20 shown in FIG. 3a does not include a reconstruction low-pass filter 19 as shown in FIG. 1, but instead includes an analog filter 100 for generating an analog output. It will be understood that other filters and other components may be used downstream of the DDS10 if required by a particular intended application.

[0018] The active feedback analog filter 100 can be used to provide a narrow bandpass filter that is adjustable with respect to both the center frequency and the width of the passband. This allows various degrees of selectivity to be applied to the composite signal generated by the DDS10. For example, the passband can be adjusted to have a width of 1% or less of the relative frequency and can be adjusted in frequency to extend over a desired set of frequencies. The passband can also be adjusted to have a wider frequency if desired for some situations. In this way, the user can select one or more frequency components from among the frequency components within the composite signal generated by the DDS10. In some examples, the passband can be adjusted to select one or more desired frequency components that can be close to 0.01% to 0.1% of the adjacent unwanted frequency components. The ATF-DDS20 can be designed with a more complex structure that can generate more than one passband, for example, to select frequency components that are separated by non-adjacent frequency components, unwanted frequency components, or other unwanted features on the spectrum. Additionally, by adjusting the ATF100, the frequency components of the analog output signal are also changed, which can be useful, for example, in frequency hopping applications. Other, more complex circuit designs can also be used for various applications.

[0019] Some examples of adjustable bandpass filters that can be used as the analog filter 100 are described in U.S. Patent No. 10,050,604 (Nielsen et al.) entitled "Variable Filter". These filters can be designed to be stable, adjustable active feedback filters that are used as narrow bandwidth filters to achieve improved performance and frequency range. Examples of the analog ATF filter can be shown in FIGS. 3b to 3d.

[0020] Referring to Figure 3b, the analog adjustable filter 100 may include an input 102, an output 104, a resonator coupling 106, a variable resonator 108, and a feedback loop 110 with a variable scaling block 112. Referring to Figure 3c, the analog adjustable filter 100 includes multiple variable resonators 106 connected in series. Referring to Figure 3d, the analog filter 100 may include variable resonators 106, each of which may have an individual variable feedback loop 110a with a scaling block 112a connected within the overall feedback loop 110. It will be understood that the number of resonators and the overall circuit architecture may vary according to user preference or the requirements of a given application.

[0021] The designs described herein can be implemented, for example, by utilizing smaller components that use less power, and can be implemented at the chip level without off-chip components, compared to other known DDS designs.

[0022] The structured frequency-domain response of a synthesized 7MHz DDS signal using a 100MHz clock, as generated by the conventional techniques described above, is shown by labeled line 42 in Figure 4, showing the line spectral component at 7MHz. These are higher-order frequency terms, such as 93MHz and 107MHz, which generate stepwise distortion in the time-domain response of Figure 2, as seen in the highly structured signal of Figure 7.

[0023] Typically, a low-pass interpolation filter is used with the DDS to suppress these higher-order frequency terms, leaving the fundamental frequency, which is 7 MHz in this example. The phase noise of this desired fundamental tone may include phase noise from the reference clock signal driving the DDS and jitter from the DDS's digital processing.

[0024] Therefore, a DDS with a low-pass filter works well for frequency synthesis when the desired fundamental frequency is a fraction of the clock frequency. To generate different frequencies, the DDS output can be converted using a frequency converter. For example, the frequency can be up-converted as is known in the art, such as by a fixed-frequency LO signal. Unwanted frequency spurious signals are generally removed from the DDS output before up-conversion using a fixed low-pass filter to control the spread of frequency spurious signals. Up-conversion brings the bandwidth of the DDS closer to the bandwidth of the clock frequency, as both positive and negative frequencies can be generated by an orthogonal DDS that generates both in-phase and quadrature-phase components. However, such single-sideband up-conversion typically requires two DDS-DAC outputs for in-phase and quadrature signal synthesis, as well as a pair of matched frequency mixers. Mismatch in the mixer pair can introduce frequency spurious signals into undesirable sidebands. Furthermore, as the DDS frequency approaches the Nyquist limit (half the clock frequency), the low-pass filter transition requirements become excessive. Thus, there is a compromise between frequency spurious performance and the complexity of a given fixed low-pass filter. Other methods of frequency transformation can also be used with appropriate modifications.

[0025] A key utility of DDS is the precise synthesis of quasi-sinusoidal signals that are time-phase coherent over any number of frequency steps. As mentioned above, good performance requires a precise, high-speed DAC and a large LUT. Typically, the frequency range is limited to a portion of the DDS clock frequency for the reasons mentioned above. Therefore, applications such as communication transceivers and radar that rely on DDS synthesis frequency hopping require high-performance, high-speed DDS. This is typically not a problem in large, expensive platforms where power consumption, circuit size, and cost are secondary concerns, but in smaller, lower-cost components, low-cost DDS may not provide sufficient performance. An example of low-cost DDS may be in certain applications where the LUT is part of the firmware for an FPGA.

[0026] Extension of the synthesized output signal frequency beyond the synthesizer clock frequency.

[0027] The architecture described herein enables the achievement of frequencies beyond the synthesizer clock frequency by coupling a digital signal component from a low-complexity DDS with an active-feedback narrowband variable frequency filter, which is referred to herein as an ATF-DDS. For simplicity, the digital signal component is referred to herein as a DDS. In one example, a variable frequency filter may be used to track a desired fundamental frequency component of the DDS output and suppress undesirable spurious components. This results in a clean output spectrum with a single dominant frequency component, as shown in Figure 5 labeled as line 52 and Figure 7 labeled as line 72. The ATF-DDS may be used to provide a low-distortion time-domain sinusoidal signal and signal correlation at the receiver, which is ideal for transmit signal generation. In other examples, the ATF-DDS may be used to select more than one frequency component by filtering adjacent frequency components, by using an analog filter with more than one passband, or by using multiple ATF filters.

[0028] ATF-DDS allows for a significantly lower clock reference frequency because analog filters can be used to select a wide range of frequency components in the DDS output without being affected by the DDS's Nyquist frequency limitation. Therefore, the resulting output frequency range can be extended to several times the reference clock frequency, instead of being limited to a fraction of the clock frequency, as in a DDS without such variable frequency filters. This can allow for a reduction in not only the required clock frequency but also the size of the ROM LUT. Furthermore, variable frequency analog filters can be used to remove much of the wideband quantization noise arising from finite-precision DACs. This reduces the required precision of the DAC and, because fewer sample bits are needed, also reduces the size of the ROM LUT.

[0029] Figure 5 shows an example of a high-performance analog filter applied to the 7 MHz DDS signal described above. The normalized frequency response of the analog variable frequency narrowband analog filter is shown at the top, and the resulting ATF-DDS structured spectrum filtered by the analog filter is shown at the bottom. The analog filter used in this example consists of a cascaded high-performance active analog adjustable filter, as described by Nielsen et al. In particular, the ATF herein has two cascaded variable filter components of the referenced patent, where the number of main poles in the filter section can vary. Additional architectures for ATFs using additional poles and / or additional adjustable filter modules are not excluded.

[0030] As another example, consider the aforementioned DDS (100MHz clock and 7MHz fundamental) upconverted by a 200MHz square wave LO. A portion of the DDS spectrum between 180MHz and 220MHz without the analog filter is shown in Figure 6 as a structured upper trace, labeled line 66. Note that both the 207MHz upper sideband modulation spectral component and the 193MHz lower sideband spectral component are present along with many other spectral components generated by the upconversion process. As previously mentioned, the smooth center trace, labeled line 62, is the frequency response of the analog filter. The lower structured trace, labeled line 64, is the resulting filtered spectrum, showing the desired upper sideband selection at 207MHz.

[0031] As this simulation reveals, a DDS operating at a 100MHz clock can synthesize quasi-sinusoidal waveforms over a tuning bandwidth several times that of the DDS clock.

[0032] Consider the application of a frequency-hopping communication system where the frequency increment of each hop can extend beyond the DDS clock frequency. It requires jumping between different mixer spectral components. However, all of these spectral components are phase-coherent with respect to the original DDS clock. Therefore, phase coherence is maintained across frequency hops. This allows additional modulation information to be packed into the carrier waveform, increasing the overall information throughput.

[0033] In another example shown in Figure 7, the DDS output is not upconverted at LO. Instead, the harmonic components of the DDS output are utilized by appropriately tuning an analog filter. A plot of the frequency transfer function of the same ATF is shown, which is now centered at the frequency of a 193 MHz component with a Q of 167, labeled as line 72, and the resulting structured output filter spectrum below is labeled as line 74.

[0034] Further combinations of DDS configurations, analog filters, and frequency conversions may be considered to achieve the desired results. Figure 8 shows an example of a design modification, where a moderate DDS 10 output filtered by an ATF 100 is converted to a higher frequency using a frequency conversion block 202 connected to a fixed LO 204, and then further filtered at a microwave frequency. Any frequency can be achieved by appropriately configuring the DDS in combination with the fixed LO 204, or by providing an adjustable LO 204. In another example, the analog filter 100 may have a multi-pole bandpass response to enable an analog signal with a desired bandwidth. In yet another example, the analog filter 100 may be fitted with filter responses having multiple different passbands.

[0035] ATF calibration

[0036] To calibrate an ATF-DDS, a power detector may be added, which can be as simple as the built-in diode / DAC. As shown in Figure 9, a calibration method in the context of a DDS with two ATF filters may be as follows:

[0037] DDS can be programmed with specific clock frequencies and fundamental frequencies, and the resulting frequency spectrum can consist of precisely known deterministic frequency components.

[0038] The ATF can then be swept by frequency, recording a pattern of frequency components based on the output of the power detector.

[0039] The amplitude at a specific frequency can then be maximized by frequency control.

[0040] The DDS frequency may then be slightly altered, and a decrease in the power detector output is recorded.

[0041] The power detector output can be used to estimate the filter bandwidth.

[0042] Accordingly, the ATF loop gain can be set.

[0043] Comparison of performance between high-performance ATF-DDS and conventional DDS technology.

[0044] While DSP-DDS datasheets can be very detailed, Table 1 shows some of the key differences between existing DDS examples and the same DDS integrated with an adjustable active feedback analog adjustable filter (ATF-DDS). It should be understood that the results listed in Table 1 are for illustrative purposes only and may vary depending on the specific circuit. [Table 1]

[0045] As can be seen, incorporating an adjustable active feedback analog filter into a DDS architecture can result in a synthesizer with a substantially larger footprint, at significantly lower power consumption, and with virtually no loss of linearity or noise reduction compared to a DDS without such an analog filter, potentially leading to a substantially higher frequency range.

[0046] In some examples, lower-resolution DDSs, such as DDSs with lower clock rates and / or lower bit resolution DACs, can be used with one or more analog adjustable filters (ATFs) to synthesize low-distortion sinusoidal signals by using ATF-enabled selection of multiple output frequency components resulting from the DDS. This may also involve the use of frequency conversion to generate signals at arbitrary frequencies. The ATF automatically adjusts to the desired frequency component and the bandwidth adjusted as needed, isolating this frequency component from other undesirable frequency components. This can be a very flexible approach, as the ATF passband can be dynamically programmed according to specific application requirements. Other advantages may include the following capabilities: • Narrow the bandwidth of the ATF-DDS output for greater suppression of unwanted frequency components; • Increase bandwidth for faster output changes; • For ATF-DDS with phase and amplitude modulation, adjust the ATF to a commensurate bandwidth; • Enables self-calibrated, agile LO-based frequency synthesis, created from LUTs with moderate phase and amplitude resolution, which can result in improved spectral purity and phase coherence regardless of any applicable frequency-hopping modulation.

[0047] In this patent document, the terms “have, include” are used in their non-restrictive sense to mean that the item following the term is included, but items not specifically mentioned are not excluded. A reference to an element with the indefinite article “one (a)” does not rule out the possibility that there are more than one element unless the context explicitly requires that only one of the elements exists.

[0048] The following claims should not be limited by the preferred embodiments described above in the examples and drawings, and should be given the broadest interpretation consistent with the overall description.

[0049] The following note is added. (Note 1) A method for generating a signal, wherein the method is: A step of generating a composite signal having multiple frequency components using digital signal components; A step of filtering the composite signal using an analog filter having a passband, wherein the analog filter has an adjustable active feedback circuit having one or more variable resonators and a variable gain block connected to a signal loop; The process includes the step of adjusting the analog filter such that the passband of the analog filter overlaps with one or more desired frequency components of the multiple frequency components of the composite signal, and the passband has a relative bandwidth of about 1% or less; method. (Note 2) The step of frequency-converting the composite signal such that the plurality of frequency components include a plurality of converted frequency components, The method described in Appendix 1. (Note 3) Further includes the step of filtering the composite signal before the composite signal is frequency converted, after the composite signal is frequency converted, or both before and after the composite signal is frequency converted. The method described in Appendix 2. (Note 4) The one or more desired frequency components are filtered from among the frequency components that are close to 0.01% to 0.1% of the one or more desired frequency components. The method described in Appendix 1. (Note 5) The analog filter includes multiple analog filters, The method described in Appendix 1. (Note 6) The plurality of analog filters have a plurality of separate passbands. The method described in Appendix 5. (Note 7) The steps of constructing the composite signal and adjusting the analog filter to one or more different desired frequency components are further included. The method described in Appendix 1. (Note 8) A device for generating a signal, wherein the device is: A synthesizer that generates a composite signal having multiple frequency components; An analog filter connected to process the composite signal, wherein the analog filter has a passband for filtering the composite signal, and the analog filter has an adjustable active feedback circuit having one or more variable resonators and a variable gain block connected to the signal loop; A controller connected to the synthesizer and the analog filter, wherein the controller is: The command includes adjusting the analog filter such that the passband of the analog filter overlaps with one or more desired frequency components of the plurality of frequency components, and the analog filter can generate a relative bandwidth of the passband of approximately 1% or less. A controller and; Device. (Note 9) The frequency converter further comprises a frequency converter configured to convert the composite signal such that the plurality of frequency components include a plurality of converted frequency components. The apparatus described in Appendix 8. (Note 10) The analog filter is connected between the synthesizer and the frequency converter and further has an additional analog filter connected downstream of the frequency converter. The apparatus described in Appendix 9. (Note 11) The one or more desired frequency components are filtered from among the frequency components that represent approximately 0.1% to approximately 1% of the one or more desired frequency components. The apparatus described in Appendix 8. (Note 12) The analog filter includes a plurality of analog filters, The apparatus described in Appendix 8. (Note 13) The synthesizer is configured to adjust the plurality of frequency components, and the controller further includes instructions for adjusting the analog filter to one or more different desired frequency components. The apparatus described in Appendix 8.

Claims

1. A method for generating a signal, wherein the method is: The steps include: generating a composite signal having a range of frequency components by direct digital sequencing (DDS) using digital signal components; The steps include: filtering the composite signal using an analog filter having a passband, wherein the analog filter has an adjustable active feedback circuit having one or more variable resonators and a variable gain block connected to a signal loop; The analog filter is adjusted such that the passband of the analog filter overlaps with one or more desired frequency components of the frequency components within the range of the composite signal, and the passband has a relative bandwidth of about 1% or less; The one or more desired frequency components include a frequency component that is greater than half the reference oscillator clock frequency of the composite signal and is other than the reference oscillator clock frequency. method.

2. The further step includes frequency-transforming the composite signal such that multiple frequency components include multiple transformed frequency components. The method according to claim 1.

3. The further step includes filtering the combined signal before the combined signal is frequency-converted, after the combined signal is frequency-converted, or both before and after the combined signal is frequency-converted. The method according to claim 2.

4. The one or more desired frequency components are filtered from among frequency components close to 0.01% to 0.1% of the one or more desired frequency components. The method according to claim 1.

5. The analog filter includes a plurality of analog filters, The method according to claim 1.

6. The aforementioned plurality of analog filters have a plurality of separate passbands. The method according to claim 5.

7. The steps of constructing the composite signal and adjusting the analog filter to one or more different desired frequency components are further included. The method according to claim 1.

8. Adjusting the analog filter further includes adjusting the bandwidth of the passband. The method according to claim 1.

9. A device for generating a signal, wherein the device is: Direct digital sequencing (DDS) generates a composite signal with a range of frequency components; An analog filter connected to process the composite signal, wherein the analog filter has a passband for filtering the composite signal, and the analog filter has an adjustable active feedback circuit having one or more variable resonators and a variable gain block connected to a signal loop; A controller connected to the DDS and the analog filter, wherein the controller is: The command includes adjusting the analog filter such that the passband of the analog filter overlaps with one or more desired frequency components of the frequency components in the range, and the analog filter can generate a relative bandwidth of the passband of approximately 1% or less. The one or more desired frequency components include a frequency component that is greater than half the reference oscillator clock frequency of the composite signal and is other than the reference oscillator clock frequency. A controller and; having Device.

10. The system further comprises a frequency converter configured to transform the composite signal such that multiple frequency components include multiple transformed frequency components. The apparatus according to claim 9.

11. The analog filter is connected between the DDS and the frequency converter, and further comprises an additional analog filter connected downstream of the frequency converter. The apparatus according to claim 10.

12. The one or more desired frequency components are filtered from among the frequency components that represent approximately 0.1% to approximately 1% of the one or more desired frequency components. The apparatus according to claim 9.

13. The analog filter includes a plurality of analog filters, The apparatus according to claim 9.

14. The DDS is configured to adjust multiple frequency components, and the controller further includes commands for adjusting the analog filter to one or more different desired frequency components. The apparatus according to claim 9.

15. Adjusting the analog filter further includes adjusting the bandwidth of the passband. The apparatus according to claim 9.

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