Abstract:
A broadband multibit digital radio-frequency (RF) signal is synthesized digitally. to convert the digital signal to a high-power analog signal for RF transmission. Each bit (or cluster of bits) of the digital signal is first separately amplified using a fast switching amplifier with a controlled dc power supply voltage. The DC voltages are weighted to match the significance of the bits, and controlled by a set of calibrated DC reference sources to maintain high precision. The amplified digital signals from the various bits are then combined and passed through an appropriate analog filter to generate the RF signal to be transmitted. Such a signal can exhibit broad bandwidth, high dynamic range, excellent linearity, and low noise. Preferred embodiments of this system can incorporate superconducting electronic elements. For ultimate precision, a set of primary or secondary DC voltage standards can be used to regulate the switching amplifier supply voltages.

Description:
[0001]    This application claims priority from a provisional application titled Multibit Digital Amplifier For Radio-Frequency Transmission bearing Ser. No. 60/931,128 filed May 21, 2007. 
     
    
     BACKGROUND 
       [0002]    This invention relates to radio-frequency (RF) transmission and more particularly to digital RF transmission. 
         [0003]    A conventional prior art digital transmitter system, as shown in  FIG. 1 , includes a digital baseband signal ( 11 ), typically a multibit narrowband signal at the Nyquist sampling frequency in the range of 1 MHz. This is then converted to an analog baseband signal  13  using a narrow band digital-to-analog converter (DAC),  12 . The baseband analog signal  1 . 3  is then fed to an analog mixer  14  to which is fed the output  15  of an analog local oscillator (LO)  16 . The output of mixer  14  is then fed to an analog bandpass filter  17  which is used to eliminate undesired mixer products. The resulting analog RF signal,  18 , is amplified by a conventional linear amplifier  19  and fed to a transmission antenna  110  for broadcasting. 
         [0004]    An alternative prior art system shown in  FIG. 2  has been gaining attention in recent years. In this system the signal may be maintained in digital form until much closer to the antenna by employing a concept which is termed “Software Radio” or “Software-Defined Radio” (SDR). Here the digital baseband signal ( 11 ) is first upsampled using a fast digital interpolation filter  21  to produce an upsampled signals  22 . The up-sampled signals  22  are then multiplied by means of a digital multiplier  23  which is supplied with sampling signals  24  generated by a digital Local Oscillator  25  (operating at a frequency of approximately 1 GHz) to generate a multi-bit digital-RF signal  26 . Thus, a digital local oscillator and a digital up converter are used to generate a multibit digital RF signal,  26 . This Nyquist-rate multi-bit signal  26  may be converted to an even faster oversampled single-bit digital signal with the same in-band dynamic range, using an “Oversampling Code Converter” (OCC),  27 . The OCC  27  may be comprised of a digital delta-sigma modulator, or alternatively a “staggered thermometer code” circuit as described in U.S. Pat. No. 6,781,435. The oversampled bitstream  28  at the output of OCC  27 , identified in  FIG. 2  as a single bit pulse width modulated (PWM) oversampled digital RF signal, can then be passed through an analog bandpass filter  29  to create a broadband analog RF signal  210 , which can then be amplified via a power amplifier such as linear amplifier  19  for transmission to a transmitting antenna such as  110 . 
         [0005]    While the architecture for a digital-RF transmitter shown in  FIG. 2  has been discussed in the prior art, to Applicants&#39; knowledge it has never actually been implemented for a broadband RF signal, because it requires an oversampling code converter (e.g., OCC  27 ) which has to operate faster than can be achieved by existing circuit technology. Thus, the digital-RF approach is difficult or impossible to achieve with conventional technology, given the very fast multi-GHz sampling and digital processing rates required. 
         [0006]    Another problem with processing the multibit digital RF signals  26  (and/or the bit stream out of OCC  27 ) is that these signals, especially if generated using Josephson junction (JJ) based circuits, are of very low amplitude and need to be greatly amplified to increase their voltage/current (power) level before application to an antenna for transmission. One approach, shown in  FIG. 3A , is to take the low power analog RF signal  210  from bandpass filter  29  (see  FIG. 2 ) and feed it to a high-quality linear analog amplifier  39 . But this mode of power amplification is power-inefficient if linearity is to be maintained, and can introduce noise into the signal. 
         [0007]    An alternative approach, as shown in  FIG. 3B , is to maintain the signal in a single-bit digital format. Here the output  28  of the OCC is applied to a single switching amplifier  311  whose output swings quickly between the voltage rails of the power supply, e.g., an amplifier which may be operated as a class S or class D amplifier. If the amplifier switching is fast enough, this will reproduce the input PWM stream with larger amplitude, with good power efficiency and without distortion. However, although the switching scheme of  FIG. 3B  is more efficient than the scheme of  FIG. 3A , it requires circuits that need to switch faster than is available using existing amplifier technology. That is, there is no known single amplifier fast enough to perform this function, at the frequencies of interest. 
         [0008]    Thus, although the concepts of  FIGS. 3A and 3B  have been discussed (see, for example, FIG. 8 in P. Asbeck et al., “Digital Signal Processing up to Microwave Frequencies”, IEEE Trans. MTT vol 50, no 3 pp. 900-909, 2002), to Applicants&#39; knowledge, they have never been implemented for a broadband RF signal, since the required switching speeds are greater than can be obtained using conventional technology. 
         [0009]    Accordingly, a problem exists in processing the signals at the high frequencies (e.g., gigahertz range) of interest. This problem is resolved in circuits and systems embodying the invention. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention is designed to achieve the performance advantages of a broadband, high-dynamic-range all-digital-RF transmitter, without requiring an unreasonably high rate of digital processing and switching. This is achieved through the increased use of bit-parallel processing through the entire digital amplification chain. A parallel array of amplifiers is used, each amplifier having a different operating voltage to reflecting the weighting of a given bit or bit cluster. The precision of the combined RF signal may be maintained by carefully controlling each operating voltage to match an appropriate stable reference standard. A preferred embodiment of the invention combines fast digital superconducting electronic elements with fast semiconductor switching amplifiers. For ultimate precision, a Josephson primary voltage standard may be used to control the operating voltages supplied to the amplifiers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    In the accompanying drawings like reference characters denote like components and; 
           [0012]      FIG. 1  is a block diagram of a prior art digital transmission system; 
           [0013]      FIG. 2  is a block diagram of a prior art software-defined radio (SDR) transmission system; 
           [0014]      FIGS. 3A and 3B  are block diagrams of prior art power amplifiers for digital RF transmission; 
           [0015]      FIG. 4  is a block diagram of a parallel digital amplifier for digital RF transmission in accordance with the invention; 
           [0016]      FIG. 5  is a block diagram of a parallel-serial digital amplifier for digital RF transmission in accordance with the invention; 
           [0017]      FIG. 6  is a block diagram of a voltage reference generator for generating precision operating voltages and distributing the operating voltages to amplifiers in accordance with the invention; 
           [0018]      FIG. 7  is a block diagram of a combination circuit suitable for use in practicing the invention; and 
           [0019]      FIG. 8  is a simplified semi-block semi-schematic diagram of a switching amplifier suitable for use in practicing the invention. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0020]    As already noted, this invention relates to the processing of multibit (parallel) digital signals RF signals (e.g.,  26 ) which may be generated, for example, as shown in  FIG. 2 , by digitally up sampling digital baseband signals to produce upsampled signals  22  which are then multiplied via a multibit digital multiplier (e.g.,  23 ) to produce corresponding multibit Nyquist rate digital RF signals (e.g.,  26 ). 
         [0021]    A technology that has the required speed to accomplish the desired processing and transmission of signals is based on Josephson junctions (JJs), and referred to as “Rapid Single Flux Quantum” logic, or RSFQ. Complex RSFQ circuits have been demonstrated with clock speeds up to 40 GHz, and simple circuits with speeds up to 800 GHz. This is much faster than any other integrated-circuit electronic technology. However, the characteristic voltage of JJ circuits is extremely small, of the order of 200 microvolts for the current fabrication technology. Therefore, if JJs are to be used for a transmitter, a very large amplification factor is needed. This requires the use of semiconductor transistor power amplifiers, which are not quite as fast. 
         [0022]    The invention may be explained by reference to  FIG. 4  which shows a multi-bit digital-RF signal  26  synthesized at (or slightly above) the Nyquist frequency corresponding to the band of interest. In the embodiment shown in  FIG. 4 , there are N bits; starting from bit  1 , which is defined as the least significant bit (LSB), extending to bit N, which is defined as the most significant bit (MSB). The number of bits may be any number deemed appropriate for the application and may vary over a wide range (e.g., from less than 8 to more than 30). Each bit, B(i), is applied to a corresponding switching amplifier  41 ( i ). 
         [0023]    As shown in  FIG. 8 , each one of the switching amplifiers  41 ( i ) has a signal input,  411 ( i ), a signal output,  421 ( i ), and two power terminals,  424 ( i ) and  426 ( i ); the power terminals for the application therebetween of an operating voltage. Each switching amplifier is supplied with an operating (supply) voltage corresponding to the significance (order or weight) of the signal input bit applied to that amplifier. The amplifier to which the least significant bit (LSB) is applied has the smallest supply voltage (i.e., V LSB ) applied to it. For the binary system of  FIG. 4 , the supply voltage of each succeeding amplifier, corresponding to a more significant (higher) bit, has a supply voltage that is a factor of two (for a binary system) larger than the previous most significant bit, etc. For the circuit of  FIG. 4 , it is assumed that each succeeding bit increases by a factor of two (2) and that each higher bit is applied to a corresponding amplifier. The amplitude of the operating voltages applied to the various amplifiers  41 ( i ) may then be expressed as: 
         [0000]        V ( i )=2 (i−1)   V   LSB ; 
         [0024]    where (i) is the order of the bit and (i) varies form 1 to N. 
         [0025]    For example, a 12-bit digital RF signal with a 1 GHz bandwidth could have a 2 GS/s output sampling rate, and a parallel array of 12 switching amplifiers, with supply voltages that might range in factors of two from 8 Volts down to 3.9 milliVolts ( 1/256 V). Thus, in circuits embodying the invention, the order of the bits is converted to a specific voltage. This provides a bit to voltage amplitude conversion. For example, assuming bit  1  (B 1 ) to be the LSB, a value of V LSB  is assigned to the bit, a voltage of 2 V LSB  is assigned to bit  2 , and so on for the next bits. 
         [0026]      FIG. 4  represents a new parallel digital amplifier for a digital-RF transmitter. The N bits of (1 to N) of a Nyquist-rate multibit digital-RF signal ( 26  in  FIGS. 2 and 4 ) are amplified in parallel, with an array of switching amplifiers [ 41 ( 1 ) to  41 (N)]. This replaces and is used instead of the Oversampling Code Converter (OCC) of the prior art, as shown in  FIG. 3B . In the circuit of  FIG. 4 , the operating voltages [(K)V LSB ] for the switching amplifiers [ 41 ( 1 ) to  41 (N)] increase by a factor of two from one bit to the next higher order bit, with a maximum for the most significant bit (MSB). For ease of illustration, in  FIG. 4 , each amplifier is shown biased between ground and (K)V LSB ; where K varies between 1 and 2 N−1 . However, it should be appreciated that the operating voltage may be disposed about ground. That is, each amplifier  41 ( i ) is connected between two voltage rails [e.g.,  424 ( i ) and  426 ( i )] and positive (+V) and negative (−V) voltages may be applied to the rails to generate the desired operating voltage between the two rails to be applied to the corresponding amplifier. 
         [0027]    Referring back to  FIG. 8 , note that a switching amplifier  41 ( i ) may include stages (e.g.,  413 ,  415 ,  417 ) of pre-amplification and level shifting to boost the bit input signal (which may be on the order of 0.2 millivolts) to a level (of several volts) which will enable the output of complementary field effect transistors (FETs) P 11 , N 11  to clamp the output e o (i) to the lower rail,  424 ( i ), or the upper rail,  426 ( i ), depending on the value of the bit input signal. In operation, either: (a) P 11  will be turned on and N 11  will be turned off clamping the output of the amplifier to the positive rail; or (b) P 11  will be turned off and N 11  will be turned on clamping the output of the amplifier to the negative rail. As is known in the art, this type of witching is highly power efficient. Note that in systems embodying the invention the electronic circuitry leading up to the output stage of the switching amplifier may be superconducting devices or a mixture of superconducting and semiconductor devices. 
         [0028]    As noted above, the switching amplifiers,  41 ( i ), are preferably (but not necessarily) comprised of an output stage with two switches in series between the two voltage rails, with an output terminal between the two switches, such that only one of the two switches is closed at any time. The switches may be comprised of transistors such as FETs. Therefore the output voltage switches between the two voltage rails. The switches are controlled by a switching controller (e.g., circuits  413 ,  415 ,  417 ) which rapidly shifts between the two configurations depending on the input voltage level. If the input consists of digital pulses, the output consists equally of digital pulses, but of larger amplitude. Such switching amplifiers are known for their high power efficiency, since in principle virtually all of the power is delivered to the load. 
         [0029]    It should be appreciated that the signal transmission system and the associated circuitry is designed for synchronous operation and production of the signals at the outputs of the amplifiers; i.e., they are intended to occur at essentially the same time. In order to achieve the highest precision of the multi-bit digital amplifier, the amplified voltage outputs representing the various bits must be properly synchronized relative to each other. This may be accomplished by appropriate timing of the delays in the preamplifiers  413 ,  415 ,  417  and/or the use of delay networks to ensure that all bit signals, whether requiring more or less amplification, have essentially equal delays. Alternatively, clocking signals may be used to keep the bit signals aligned. Fore example, synchronization may be achieved by using a set of latches and a common clock signal, derived from the clock signal of the digital inputs. In some cases, amplifiers for the various bits may switch with different speeds, due to the different output slew rates of the different amplifiers. In such a situation, appropriate delays may be inserted in the lines for the various bits, either before or after the amplifiers, to ensure optimum phase synchronization of the various parallel components. 
         [0030]      FIG. 4  illustrates that the signal input to each switching amplifier is of (the same) very low amplitude (e.g., Va). The gain of each switching amplifier is controlled (see  FIG. 8 ) to ensure that the input signal causes the output of each switching amplifier to switch between the positive rail (e.g., V LSB , 2V LSB , 4V LSB , etc. . . . ) and the negative rail (e.g., shown as ground in  FIG. 4 , but which could have another voltage applied). Note that, as shown in  FIG. 4 , the amplitude of the output of the Nth switching amplifier would correspond to the value of the operating voltage applied to the Nth switch amplifier  41 (N). The outputs of the switching amplifiers, corresponding their respective input bits, are combined in RF combiner circuit  43  which is designed to suitably combine the outputs of the switching amplifiers. That is, the amplified digital-RF signals from the various bits present at the outputs of the amplifiers are combined via an RF power combiner  43 , and then passed through an analog bandpass filter  45  to generate the RF signal to be broadcast. 
         [0031]      FIG. 6  illustrates a system for generating operating voltages suitable for distribution to the switching amplifiers used to practice the invention. A multi-output DC voltage reference generator (VRG)  47  may be used to generate a multiplicity of different operating voltages, V (i), as (i) varies from 1 to N. These voltages may be distributed via corresponding individual and separate voltage supplies  48 ( i ) to the switching amplifiers. Thus,  FIG. 6  shows a Voltage Reference Generator (VRG)  47  which can generate N different voltage levels [where V 1  equals to V LSB  up to V(N) equal to 2 (N−1) V LSB ] to provide the operating voltages to the N different switching amplifiers. This corresponds to the operating voltages for the N bits shown in  FIG. 4  where the different and subsequent operating voltage levels are set in ratios of 2 (or powers thereof). The VRG  47  may be any power supply which can provide precise, stable voltage values for the N reference levels from the millivolt to the Volt level. (The VRG need not supply significant currents, but can be used to provide stabilization of the amplifier voltages against noise and drift). It is particularly important that the voltage for the MSBs be stable and precise when N is large. For example, for a 12-bit amplifier, supply fluctuations of the MSB amplifier of 1 part in  4000  are larger than the entire output of the LSB amplifier. 
         [0032]    The VRG  47  may be a Josephson voltage standard (JVS) which can be used to generate the operating voltages supplied to the amplifiers. Such a standard may consist of more than 20,000 Josephson junctions in series, in which a precision microwave frequency of 77 GHz is converted to a series of DC voltages with precision and stability that is much better than 1 part per million, and virtually defines the standard Volt. It can also generate discrete voltage levels with up to 16 bits of resolution. By way of example, a JVS may be used as the VRG in either of two ways. In one application, the JVS is sequentially programmed to cycle through each of the N voltage levels, with a synchronous switch directing the appropriate output to a sample-and-hold circuit [e.g.,  48 ( 1 ) to  48 (N)] and then to the reference input of each amplifier supply. In another application, a special JVS chip may be fabricated with multiple voltage taps along the series array that permit N parallel outputs with binary-scaled reference voltages. Because of the fundamental Josephson frequency-voltage relation V=hf/2e, where h is Planck&#39;s constant and e is the charge on the electron, a JJ can convert a frequency of 100 GHz to a voltage of 207 microvolts, still too small to be very useful. 
         [0033]    A practical approach has been to boost the voltage by using a long series array of JJs, at the expense of speed. In fact, the volt is now defined internationally using an integrated circuit composed of approximately 20,000 JJs in series, to select dc voltages up to 10 V, to a precision of parts per billion. This is the Josephson Primary Voltage Standard (reviewed in “Applications of the Josephson effect to metrology”, by S. Benz and C. Hamilton, Proceedings of the IEEE, 2004), which was developed by the US National Institute for Standards and Technology (NIST), and is now fabricated and marketed commercially by Hypres. 
         [0034]    Alternatively, where extremely high precision voltages are not needed, conventional voltage references (with appropriate taps) may be used to provide the needed operating voltages. 
         [0035]    The circuit of  FIG. 4  may be compared to the previously discussed approach of  FIGS. 3A and 3B , where the multibit signal is converted at low power to a heavily-oversampled single bit stream, which is then amplified. Such a prior art code converter requires up-sampling by a large factor, which would be 2̂n for an n-bit signal and a first-order sigma-delta modulator (or an equivalent first-order code converter). For the 12-bit, 1 GHz example above, this would require sampling at 8192 GS/s, which is clearly impractical for any technology. 
         [0036]      FIG. 5  shows an alternative embodiment to the full parallel processing of the bits shown in  FIG. 4 . In  FIG. 5  a compromise is made between maximum parallelism and the oversampled serial approach. In  FIG. 5 , each cluster of 3 bits ( 26   a ,  26   b ,  26   c ,  26   d ) is converted to a corresponding oversampled single bit stream ( 27   a ,  27   b ,  27   c ,  27   d ) with a sampling rate that is increased by a factor of 2 3 =8. (This conversion may be carried out using a delta-sigma modulator, or a digital encoder such as that shown in U.S. Pat. No. 6,781,435 to Gupta and Kadin.) For the 12-bit, 1 GHz example, there would now be 4 parallel output switching amplifiers ( 52 ( i ), switching up to 16 GS/s, with supply voltages that might range (in factors of 8) from 8 Volts down to ( 1/64) Volts. Referring to  FIG. 5 , a 12-bit digital-RF signal is shown, for example, grouped into 4 clusters of 3 bits each. Evidently, other groupings (clusters) are permissible, as well as the number of bits per grouping/clusters. An oversampling code converter [ 51 ( a )- 51 ( d )] converts each 3-bit cluster to an oversampled single bitstream (here at 8× the sampling rate). Each cluster&#39;s bitstream is amplified with a switching amplifier ( 52   i ) having an operating voltage that increases by a factor of 2 3 =8 from one cluster to the next. This requires that switching amplifiers operate 8× faster than for the fully parallel approach of  FIG. 4 , with a reduction of hardware by about a factor of 3. The circuit/system of  FIG. 5  demonstrates that a system designer can trade off between speed and hardware for a given application. 
         [0037]    In  FIGS. 4 and 5  the operating voltages applied to the switching amplifiers varies between ground and a positive value. But, as already noted the operating voltage could vary between some (+V) and (−V) or even between ground and some negative value. 
         [0038]    The optimum design of a transmission system depends on the balance between the speed of the available technologies and the acceptable level of hardware duplication. For any of the designs of the current invention, the linearity and dynamic range of the fully digital approach is to be maintained. 
         [0039]    Superconducting RSFQ circuits are particularly fast, with 2-4 ps pulses and digital clock speeds of 20-40 GHz standard. The most advanced semiconductor power amplifiers, such as those composed of GaN high-electron-mobility transistors (HEMTs), have characteristic frequencies up to 90 GHz, corresponding to digital frequencies up to ˜10 GHz. So, combining these two technologies should yield a practical approach to a broadband all-digital transmission system. 
         [0040]    Note that in the approach of this invention, there is not a separate digital-to-analog converter (DAC) followed by amplification; the two functions are closely integrated together. The signal is maintained in digital format through the amplification chain, although the different gain factors for the various parallel bits permits simple addition to create a combined signal that may be close to the desired analog signal. The signal is not fully analog until it passes through the analog bandpass filters ( 45  in  FIG. 4 ,  55  in  FIG. 5 ) in front of the antenna (see  FIGS. 4 and 5 ). 
         [0041]    A broadband multi-carrier signal can incorporate many narrowband signals with sharply differing power levels. A digital transmitter system of the current invention must have sufficient dynamic range to include the weakest signals while avoiding saturation or distortion from the largest signals. In a fully digital system, nonlinear distortion (such as intermodulation) is avoided. However, it is also critical that the contributions from the parallel bit amplifiers all be properly matched in gain. For fast switching amplifiers, this reduces to controlling the dc supply voltages. This can be achieved by locking the supply voltage to an appropriate precision reference standard. The best standard, of course, is the Josephson voltage standard that virtually defines the volt, with parts per billion stability and 16 bits of dynamic range. This requires a cryogenic system, but that may already be available for the RSFQ digital synthesizer. 
         [0042]    In summary, the present invention provides a practical way to achieve an all-digital RF transmitter for GHz-bandwidth systems, which can provide a large dynamic range and low noise. 
         [0043]    In  FIGS. 4 and 5 , there is shown a broadband RF power combiner ( 43  in  FIG. 4 ,  53  in  FIG. 5 ) which can suitably combine the outputs of the switching amplifiers. The outputs of the amplifiers [ 41 ( i ) and  52 ( i )] include signal components which extend over the entire frequency range from DC to multiple GHz. This requires a very fast combiner circuit.  FIG. 7  illustrates a combiner circuit ( 43  or  53 ) using a standard operational amplifier (op-amp) adder circuit. In  FIG. 7 , each output [e o (i)] from a switching amplifier is connected to and terminated with a corresponding resistor (R 1 , R 2 , . . . ) connected to the inverting input  711  of an op-amp  710 . Each resistor R(i) should ideally be matched to the transmission line impedance (typically of order 50 ohms) to prevent signal reflections. If the feedback resistor (Rf) is also 50 ohms, the signal gain would be unity, and the output voltage Vo=−(V 1 +V 2 + . . . Vn). The signal inversion would not typically be a problem. If it is, a non-inverting op-amp adder may be used instead. For precision operation of this combiner circuit, the input resistors are preferably precisely matched to each other. 
         [0044]    It may be difficult to find an op-amp with sufficient bandwidth for this operation. However, it may not be necessary to maintain the full bandwidth, particularly if the desired analog RF output lies within a specific RF band. Then DC and other out-of-band components would need to be filtered out before the antenna, and some of this filtering could be obtained in (or before) the combiner circuit. Several types of known resonant, relatively narrow-band, passive RF combiners, which are available commercially, could provide acceptable alternatives to the op-amp circuit of  FIG. 7 . 
         [0045]    A more generalized type of switching amplifier, than the one shown and discussed above, which could be used to practice the invention, is one in which the output is tuned to resonate in a narrow bandwidth, usually by coupling to a passive resonator. A Class E amplifier is of this type. In this case, the output consists not of simple digital pulses, but rather of sinewaves that can be turned on and off. While the present invention focuses on a broadband transmitter using a set of classic digital switching amplifiers, the same system architecture could also be applied to a narrowband transmitter with a multi-bit digital input, here using resonant power amplifiers to maximize in-band power efficiency.