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:
FIELD OF THE INVENTION 
       [0001]    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. 
         [0002]    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 
       [0003]    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. 
       SUMMARY OF THE INVENTION 
       [0004]    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 
         [0005]    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. 
           [0006]      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. 
           [0007]      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. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0008]    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. Since this 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. 
         [0009]    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. 
         [0010]    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 . 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. 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 . 
         [0011]    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. 
         [0012]    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. 
         [0013]    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. 
         [0014]    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. 
         [0015]    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 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. 
         [0016]    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 . 
         [0017]    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. 
         [0018]    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. 
         [0019]    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): 
         [0020]    100 Hz carrier offset: −105 to −110 dBc/Hz
       1 kHz carrier offset: −135 to −140 dBc/Hz   10 kHz carrier offset: −150 to −155 dBc/Hz   100 kHz carrier offset: −155 to −160 dBc/Hz   500 kHz carrier offset: −159 to −164 dBc/Hz   ≧1 MHz carrier offset: −159 to −164 dBc/Hz       
 
         [0026]    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. 
         [0027]    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. 
         [0028]    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. 
         [0029]    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. 
         [0030]    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.