Abstract:
Very low phase noise radio frequency (RF) source having multiple discrete frequency outputs used, for example, to calibrate phase noise measurement systems. The calibrator output frequencies can be tailored for a particular application using a scalable architecture.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 12/687,283 filed Jan. 14, 2010, incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to the field of automatic test systems within the radio frequency (RF) field and more particularly, to automatic test equipment for sourcing very low phase noise RF signals for use in, for example, calibrating phase noise measurement systems. As such, an enhanced phase noise calibrator in accordance with the invention can include an RF signal generating system that is used as an RF source to calibrate the performance of phase noise measurement equipment and systems. 
         [0003]    The present invention also relates to a method for calibrating a phase noise measurement system using a novel RF signal generating system. 
       BACKGROUND OF THE INVENTION 
       [0004]    Phase noise in RF systems is often the result of short term deterministic and random frequency fluctuations about a nominal carrier frequency. These fluctuations typically have a duration of less than a few seconds and are usually represented and viewed in the frequency domain. As the performance of phase noise measurement systems improves towards the thermal noise floor dictated by kTB noise, or −174 dBm/Hz, there is a urgent need for corresponding RF sources with lower phase noise to calibrate these systems. 
         [0005]    Effinger et al., U.S. Pat. No. 4,516,085, relates to a microwave frequency synthesizer using a plurality of switchable low noise oscillators. The frequency synthesizer includes a plurality of oscillators  20 , a plurality of switches  21  one associated with each of the oscillators  20 , and a high speed isolation switch  22  that cooperates with the switches  21  to select one of the oscillators  20 . Another switch  23  is operative to select a multiplication factor via a multiplier  30  and filter  31 , leading to another switch  32 . From switch  32 , an isolator  33  provides impedance matching to a mixer  41 . The other input to the mixer  41  comes from one of a plurality of other oscillators in a master oscillator bank  10  through a switch  11 , filter means  12  and isolator  13 . 
         [0006]    A significant problem with the frequency synthesizer of Effinger et al. is that the large number of electronic components between the oscillators  10 ,  20  contribute phase noise to the signal, in spite of the presence of the filter means  12  and filters  31 . 
         [0007]    Bernhard et al., U.S. Pat. No. 4,206,421, relates to an arrangement for synchronizing a free-swinging oscillator system to a reference signal of a substantially lower frequency. A regulating circuit  6  connects to the oscillator  4  to control the frequency thereof. A quartz oscillator  1  produces a reference signal that is subsequently connected to a frequency multiplier and/or frequency divider  2 , an amplifier  3  and a bandpass filter  7 . An output signal from the filter  7  is fed to a harmonics mixer  4 ′ connected to the free-swinging oscillator  4  and which produces a pattern of harmonics. A selective amplifier  5  feeds an intermediate frequency obtained from the harmonics mixer  4 ′ to the regulator circuit  6 . A filter  8  connected to the quartz oscillator  1  produces harmonics which are also coupled to the regulating circuit  6 . By phase or frequency comparison, the regulating circuit  6  adjusts the running frequency of the free-swinging oscillator. In essence, Bernhard et al. describes a classic phase lock loop (PLL) circuit which synchronizes (locks) one oscillator to a reference oscillator. 
         [0008]    These prior art arrangements do not include a streamlined arrangement of electronic components that enable a signal with very low noise to be generated, i.e., a very pure signal. Rather, for example, the presence of a significant number of switching components in Effinger et al. provide the exact opposite effect in that they introduce noise into the signals being processed. 
       SUMMARY OF THE INVENTION 
       [0009]    An RF signal generating system that can generate RF signals for use in calibrating phase noise measurement equipment and systems in accordance with the invention includes a plurality of medium to high power, very low phase noise crystal oscillators that is configured with various stages of multiplication, amplification and filtering to provide the basis for a multiplexed arrangement of frequencies which can then be heterodyned using a frequency mixer to produce a desired range of frequencies. The upconverted (or downconverted) output may then be amplified to maintain sufficient drive level to avoid thermal noise contributions. The amplified signal may then be filtered to isolate the desired product(s) from the frequency mixer output. The filtered upconverted output may be used directly or processed further to extend the range of frequency outputs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The following drawings are illustrative embodiments of the invention and are not meant to limit the scope of the invention as encompassed by the claims. 
           [0011]      FIG. 1  shows a simplified overall block diagram of a main conversion stage of an RF signal generating system for an enhanced phase noise calibration system in accordance with the invention. 
           [0012]      FIG. 2  shows an optional stage to follow the main conversion stage to multiplex additional filtering or additional stages of multiplication/amplification/filtering to generate additional low phase noise frequency outputs in the band(s) of interest. 
           [0013]      FIG. 3  is a schematic showing the manner in which the control unit controls a switch to select a multiplication factor to thereby select a particular multiplied frequency for output to a frequency mixer. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0014]    Referring to the accompanying drawings wherein like reference numerals refer to the same or similar elements,  FIG. 1  shows an embodiment of an RF signal generating system for use in a system and method for calibrating a phase noise measurement system in accordance with the invention. The RF signal generating system is designated generally as  10  and includes a main crystal oscillator  12  with very low phase noise whose output signal is routed through a multiplier chain  14  consisting of a multiplier with integer multiplication factor M 1 , which is then amplified via an amplifier or comparable structure, filtered via a filter or comparable structure, and routed to a frequency mixer  16 . This signal will serve as a local oscillator (LO) input to the frequency mixer  16  and is utilized in generating all subsequent output frequencies. Thus, in this embodiment, the main crystal oscillator  12  is the only oscillator that provides a signal to the LO input of the frequency mixer  16  and thus the signal output at a single time or signals output at different times from the main crystal oscillator  12  will be the only signals used in generating all output frequencies from the frequency mixer  16 . 
         [0015]    An important feature of the invention that the main crystal oscillator  12  is not a direct digital synthesizer that provides a signal at a desired frequency. Rather, since the invention is designed with the overall objective to reduce phase noise, the oscillator  12  should be an oscillator that does not generate significant phase noise and thereby provides a pure signal, such as a crystal oscillator. 
         [0016]    Since this multiplier chain  14  will be common to all frequencies, some form of mechanical frequency adjustment and/or electrical frequency control may be included to allow for nominal adjustment of the frequencies produced. 
         [0017]    In an alternate embodiment, not shown, a multiplexed plurality of oscillators, either operating at their fundamental frequency or an integer multiple thereof is used to source the LO port of the frequency mixer  16  to allow a greater range of discrete frequencies to be generated. Thus, oscillator  12  represents one of one or more such oscillators while multiplier chain  14  represents one of one or more such multiplier chains. 
         [0018]    The intermediate frequency (IF) port of the frequency mixer  16  may be sourced from a multi-pole switch, i.e., n−1 way switch  18 , that allows a plurality of frequencies to be applied to the frequency mixer  16 . This sourcing is provided such that there is no intervening structure that may impart phase noise to the signal from the n−1 way switch  18  to the IF port of the mixer  16 . Thus, the signal from the n−1 way switch  18  comes directly to the IF port of the mixer  16 , without passing through, for example, a filter, or another switch. This lack of intervening structure serves to maintain the purity of the signal. 
         [0019]    The signals applied at the inputs of the multi-pole switch  18  can each be sourced directly from a very low phase noise crystal oscillator  20   n  or optionally routed through a respective multiplication stage or multiplier chain  22   n , each consisting of a multiplier with integer multiplication factor M n , which is then amplified via an amplifier or comparable structure, and filtered via a filter or comparable structure. This sourcing is provided such that there is no intervening structure that may impart phase noise to the signal from each of the oscillators  20   n  to the n−1 way switch  18 , i.e., the only electronic structure between each oscillator  20  and the respective input to the n−1 way switch  18  is the respective multiplier chain  22 . This lack of intervening structure serves to maintain the purity of the signal. 
         [0020]    The integer multiplication, in the form of a doubler, tripler, quadrupler, etc., may be applied to one or more of the multiplexed very low phase noise crystal oscillators  20   n . 
         [0021]    The range and number of output frequencies produced at the RF output port of the frequency mixer  16  can be tailored to any specific application by selecting, via a control unit  38  coupled to the multi-pole switch  18 , the number of fundamental oscillators  20   n  providing signals to the multi-pole switch  18 , appropriate fundamental oscillator frequencies, and multiplication factors used in the multiplier chains  22   n  in conjunction with each fundamental oscillator  20   n . The multiplication factor used with each fundamental oscillator  20   n  need not be the same as the prior or subsequent stage. Higher multiplication factors may require the use of one or more cascaded stages of multiplication to achieve the desired multiplication factor. The control unit  38  is connected to each of the multiplier chains  22   n  and may, in one embodiment, select the multiplication factor of the multiplier of each multiplier chain  22   n  to be different such that the control unit  38  is able to control the multiplier chains  22   n  and the n−1 way switch  18  to select a particular multiplied frequency for output to the IF port of the frequency mixer  16  (see  FIG. 3 ). The control unit  38  would be programmed with information about the oscillators  20   n  and the available multiplication factors in the multiplier chains  22   n  and then would control the switch  18  interposed between the oscillators  20   n  and the frequency mixer  16  to provide for the desired input frequency signal at the IF port of the frequency mixer  16 . 
         [0022]    In this regard,  FIG. 3  shows control lines from control unit  38  indicating that the control unit  38  can control switches to create a signal path from each multiplier chain  22   n  to the IF port of the frequency mixer  16 . 
         [0023]    The system thus enables implementation of a fundamental main oscillator  12  in conjunction with a mixing/amplifying/filtering stage, via multiplier chain  14 , and a multiplexed bank of fundamental oscillators  20   n  to produce a selectable plurality of discrete low phase noise frequency outputs. Moreover, it can be seen that the mixer  16  receives only signals generated by a fundamental oscillator main oscillator  12  and one of the oscillators  20   n , each of which has been passed through a multiplier chain, multiplier chain  14  in the case of the signal from the main oscillator  12  and one of the multiplier chains  22   n  in the case of the signal from one of the oscillators  20   n . 
         [0024]    Another interesting feature of the invention is that switch  18  is only associated with the oscillators  20   n  and not with the main crystal oscillator  12 . Since the output signal from the multiplier chain  14  is not directed to or through the switch  18 , the frequency mixer  16  receives output signals at the LO input continuously from the main oscillator  12 , which signals have been directed into and through the multiplier chain  14 . Frequency mixer  16  also receives output signals at the IF input from the oscillators  20   n  via the switch  18 . 
         [0025]    The output from the RF port of the frequency mixer  16  may then be directed to an amplifier  24  and then to a bandpass filter  26  to provide upconverted output for use in calibration a phase noise measurement system  40 . Additional and alternative uses of the output from the RF port of the frequency mixer  16  are also envisioned. Depending on the range of frequencies involved for an application, an alternate embodiment, not shown, may utilize a multiplexed bank of amplifiers and/or bandpass filters to handle the range of resulting very low phase noise RF mixer output frequencies. Thus, amplifier  24  and bandpass filter  26  each represent one of one or more such amplifiers and bandpass filters. 
         [0026]    The structure shown in  FIG. 1  provides a very streamlined, i.e., with a minimal amount of electronic components. 
         [0027]    Referring now to  FIG. 2 , the upconverted output from filter  26  may be routed through another multiplexed stage  28  of signal filtering and/or multiplication. In this stage  28 , the range of input frequencies output from the filter  26  are applied to a multi-pole switch, i.e., n+1 way switch  30 . This stage  28  includes a bypass path in which the input to switch  30  is filtered further via a filter  32  and fed into another multi-pole switch, i.e., n+1 way switch  34 , before arriving at the common output, which may be directed to the phase noise measurement system  40 . There is no multiplication or amplification of the signal passing through the bypass path. 
         [0028]    Stage  28  also includes one or more multiplication stages  36   n , each multiplication stage  36   n  consisting of an integer multiplier (doubler, tripler, etc.), amplifier and bandpass filter, so that the multiplication stages  36   n  allow the range of very low phase noise frequencies to be extended to higher frequencies. Additional stages, or multiplier chains, could be added as well, although each stage of conventional integer multiplication adds 20 log (M) dB of phase noise relative to its input so there are practical limitations to how extensible the architecture can be. Control unit  38  selects, via switch  34 , of the multiplication stages  36   n , situated between switches  28  and  34 , based on a frequency of a desired calibration signal for the phase noise measurement system. This selection may be effected in a similar manner as control unit  38  selects a signal to use to provide to the IF input of the frequency mixer  16 , discussed above with reference to  FIG. 3 . 
         [0029]    It is envisioned that stage  28  may be used in lieu of the first stage shown in  FIG. 1  as an alternate means of providing multiple very low phase noise signal sources. That is, the fundamental oscillator  12  may be coupled to the switch  30 , without the interposition of the multiplier chain  14 , frequency mixer  16 , amplifier  24  and bandpass filter  26 , and provide its output signal directly to the switch  30 . 
         [0030]    Control of the switching of switches  18 ,  30 ,  34  can be handled in various ways. In one embodiment, an embedded microprocessor or microcontroller serves as the control unit  38  and would be used to select the appropriate switch position(s) based on the desired output frequency. Other constructions of a control unit for controlling switches readily present themselves to one of ordinary skill in the art to which this invention pertains in view of the disclosure herein and are contemplated to be within the scope and spirit of the invention. 
         [0031]    Power saving measures could be implemented to power down multiplication stages not in use. Control could be further extended to include built-in-test (BIT) units and programs, power monitoring units and programs, etc. 
         [0032]    At the L-Band frequencies involved, the following single sideband (SSB) phase noise ranges can be met (all of the upper and lower limits of the ranges are approximate values): 
         [0033]    100 Hz carrier offset: −105 to −110 dBc/Hz 
         [0034]    1 kHz carrier offset: −135 to −140 dBc/Hz 
         [0035]    10 kHz carrier offset: −150 to −155 dBc/Hz 
         [0036]    100 kHz carrier offset: −155 to −160 dBc/Hz 
         [0037]    500 kHz carrier offset: −159 to −164 dBc/Hz 
         [0038]    ≧1 MHz carrier offset: −159 to −164 dBc/Hz 
         [0039]    These values represent the currently achievable SSB phase noise specifications using this approach and are not meant to limit the scope of the invention as improved crystal oscillators and/or multiplication circuits may allow for performance improvements in the future. 
         [0040]    The RF signal generating system for use in an enhanced phase noise calibrator described above is designed to source multiple output signals in the L-Band and S-Band range of frequencies, but the output frequency should not limit the scope of the invention as it may be adapted to other frequency bands. Control of the RF signal generating system for the enhanced phase noise calibrator may be through an IEEE-488 bus but the method of control should not limit the scope of the invention as it may be adapted to other parallel control means (MXI, etc.), serial (LXI, USB, RS-485, etc.), RF (Bluetooth, Wi-Fi, Zigbee, UWB, etc.) or optical (IR, fiber-optic, etc.) control buses or via discrete control. 
         [0041]    Power for the RF signal generating system for the enhanced phase noise calibrator may be provided by linear analog power supplies, rather than switching power supplies, to limit introduction of other potential sources of phase noise which could be introduced in the form of spurious or harmonic signals relating to power line frequencies, switching frequencies, electromagnetic interference (EMI) or other undesirable contributions. Ideally, operation would be from a direct current (DC) source such as a battery, but for the operating environment in which it would be used, a DC source does not provide the most practical approach. 
         [0042]    In the embodiments described above, the oscillators are described as being crystal oscillators. However, other types of low phase noise oscillators may be used in the invention without deviating from the scope and spirit thereof. However, direct digital synthesizers, and other types of frequency synthesized sources that generate signals with significantly greater phase noise than crystal oscillators, are expressly not used in the invention. 
         [0043]    The foregoing invention provides, in view of the minimal electronic componentry, a signal with very low phase noise. By providing an input signal to the IF port of a frequency mixer  16  from one of a plurality of oscillators  20   n , with this signal passing only through a multiplier chain  22   n  and the switch  18 , there are no excess components that generate unwanted phase noise. There are no extra switches or other electronic structure that can inherently introduce phase noise to the signal. As such, the IF port input system consists solely of the oscillators  20   n , the multiplier chains  22   n  and the switch  18 . There is no other structure that materially affects the signal processing. 
         [0044]    Similarly, by providing an input signal to the LO port of a frequency mixer  16  from oscillator  12 , with this signal passing only through a multiplier chain  14 , there is little room to enable phase noise to be generated. There are no extra switches or other electronic structure that can inherently introduce phase noise to the signal. As such, the LO port input system consists solely of the oscillator  12  and the multiplier chain  14 . There is no other structure that materially affects the signal processing. 
         [0045]    Purity of the signal provides by the frequency mixer  16  is thus very high, resulting from the purity or low phase noise of the two signals input to the frequency mixer  16 . 
         [0046]    An innovative aspect of the invention is that signals from all of the low phase noise generating oscillators  20   n  are continuously powered to insure all fundamental oscillators are stabilized and achieve optimum operating parameters. Unused multiplier/amplifier chains are powered down to provide additional signal isolation such that any potential sources of spurious signals are minimized and do not contaminate the signal purity of the multiplier/amplifier chain of interest. This powering down is effected by the control unit  38  being electrically coupled to and controlling each of the multiplier chains  22   n  (see the connections between the control unit  38  and the multiplier chains  22   n  in  FIGS. 1-3 ). As such, control unit  38  has at its control, any one of the signals and can control the switch  18  to pass through any one of the signals to the IF port of the frequency mixer  16  as well as control of the various multiplier chains  22   n . When a change in the signal output from the switch  18  to the frequency mixer  16  is desired, the control unit  38  also enables the appropriate one of the multiplier chains  22   n . Since all fundamental oscillators are continuously powered and operating, there is minimal delay when switching to another discrete frequency. 
         [0047]    The method in accordance with the invention therefore has continuous generation of a plurality of discrete frequency signals, with all of these signals being readily available to be provided to one port of the frequency mixer  16  solely by control of switch  18  effected by control unit  38 . Moreover, the other port of the frequency mixer is also sourced from a low phase noise generating oscillator  12 . Indeed, in one embodiment, all of the oscillators  12 ,  20   n  are the same, i.e., crystal oscillators. 
         [0048]    In the prior art discussed above, i.e., Effinger et al. and Bernhard et al., there is no consideration of the desire to generate a very low phase noise signal, and arranging for this by providing very low phase noise oscillators as in the present invention and a minimal electronic structure between the oscillators and the mixer. Effinger et al. is said to be a low noise microwave frequency synthesizer but overlooks the fact that the numerous switches and signal isolators actually create phase noise. Thus, the presence of no fewer than four different switches between the oscillators  20  and the frequency mixer  41  is highly disadvantageous as it introduces significant phase noise to the signal being input to the frequency mixer  41 . These four switches are required in accordance with the teachings of Effinger et al. 
         [0049]    Accordingly, with the minimal structure in the invention, i.e., with the crystal oscillator  12  providing a continuous signal to the mixer  16  via multiplier chain  14  and crystal oscillators  20   n  providing continuous signals to the switch  18  via multiplier chains  22   n , the signals provided to the mixer  16  are very low phase noise, i.e., of exemplary purity. Construction of an electronic circuit with these components with a view toward providing a very low phase noise signal, i.e., a very pure signal, is not obvious from Effinger et al. and Bernhard et al., neither of which appreciate the possibility of using low phase noise crystal oscillators to provide input to both ports of the same frequency mixer, with one port being sourced from one of a plurality of different oscillators depending on a desired frequency. The overriding goal of the invention to provide a low phase noise signal, with the limited structure disclosed herein, therefore cannot be derived from Effinger et al. and Bernhard et al., nor Grandfield et al. (U.S. Pat. No. 4,791,377). The invention simply addresses different issues and concerns than those addressed by Bernhard et al. and Grandfield et al. 
         [0050]    While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention. Indeed, it is envisioned that any feature shown in or described in connection with one embodiment may be applied to any of the other embodiments shown or described herein to the extent not inconsistent with a particular feature of that embodiment.