Abstract:
A spectrum analyzer is provided with frequency-scalable circuit architectures that extend the bandwidth of the spectrum analyzer using an array of couplers. The array of couplers is distributed along the RF signal path at one end, and interfaced to one or more frequency-translation devices such as mixers or samplers at the other. In a first architecture, a single mixer is employed with an LO signal applied to one input and coupler outputs providing RF signals to another input, with switching controlled to select one coupler&#39;s RF output to provide to the mixer. In a second architecture, a separate mixer is used, one for each coupler RF signal, with switches selecting one of the mixer IF outputs to select a desired output frequency. Both the first and second embodiments eliminate switching and its associated loss and frequency limitations from the main RF signal path to enable wideband high-dynamic-range spectrum analysis.

Description:
BACKGROUND 
   1. Technical Field 
   The present invention relates to components used in broadband spectrum analyzers. More particularly the present invention relates to circuitry for extending the frequency range of a spectrum analyzer into the millimeter-wave spectrum. 
   2. Related Art 
   Currently available broadband spectrum analyzer architectures make use of switched filters along the RF signal path as a means of implementing frequency-band pre-selection.  FIG. 1  provides circuitry illustrating the use of switched filters to allow frequency-band pre-selection in a broadband spectrum analyzer. The circuitry includes a low-frequency spectrum analyzer (LFSA)  2 . The LFSA  2  receives RF and IF signal inputs. In low-frequency mode, the RF signal input is connected directly to the LFSA  2  by means of switch  7 . In high-frequency mode, switches  6  and  7  are connected to one of bandpass filters  4  to provide an RF signal to mixer  8 . The IF output of mixer  8  is then provided as a second RF input to LFSA  2 . 
   As the frequency range of the architectures using switched filters illustrated in  FIG. 1  extends into the millimeter-wave spectrum, frequency-dependent RF-switch limitations occur. Switching limitations such as increased loss and reduced bandwidth begin to take their toll on spectrum analyzer performance. The direct results of these limitations are a decrease in the dynamic range of the instrument, and severe boundaries on its operating frequency bandwidth. 
   In order to overcome the later limitation, techniques using external mixers have been developed to extend the frequency range of the spectrum analyzer.  FIG. 2  illustrates such an external mixer configuration, also known as an external frequency-range extension. The circuitry includes a LFSA  2  that receives an RF signal directly in a low-frequency mode, and receives the RF signal through an external mixer  12  in a high-frequency mode. In low-frequency mode, the RF signal input is connected directly or by means of switch  9  to a low frequency RF input of the LFSA  2 . In high-frequency mode, the switch  9  directs the RF signal through a frequency range extender  10  that includes mixer  12 . The mixer  12  output provides a second RF input to the LFSA  2 . The LO input signal to the mixer  12  is generated inside the LFSA  2 . 
   One drawback to using the technique illustrated in  FIG. 2  is that measurement must typically be performed manually within separate high-frequency bands each requiring an external frequency range extender. Manual measurements are necessary due to the fact that typically the extension mixer  12  covers only a portion of the high-frequency band and must be replaced with different extension mixers for covering other bands. In addition, the lack of filtering on the IF port of the external mixers adds to the tediousness of the measurements. 
   It would be desirable to extend the operating frequency range of a LFSA without the drawbacks and the limitations of the frequency dependent RF-switch of  FIG. 1 , or the need to perform measurements manually within separate frequency bands as when an external mixer of  FIG. 2  is used. 
   SUMMARY 
   Embodiments of the present invention provide frequency-scalable architectures that extend the bandwidth of spectrum analyzers while removing the limitations imposed by the presence of switches, or external mixers along the RF signal path. This is made possible by frequency multiplexing the various frequency bands using a chain of couplers in the RF path with lengths chosen to yield maximum coupling at the band centers, along with one or more mixers. 
   Two circuit embodiments provide frequency-scalable architectures for extending the operating bandwidth of low-frequency spectrum analyzers. In both embodiments, frequency extension is achieved by means of an array of couplers distributed along the RF signal path at one end, and interfaced to one or more frequency-translation devices such as mixers or samplers at the other. In the first architecture embodiment, a single mixer is employed with an LO signal applied to one input and the output of one of the couplers providing RF signals to the other input by means of controlled switching. In a second architecture embodiment, multiple mixers are used, one for each RF coupler signal, with switches selecting one of the mixer IF outputs to select a desired RF frequency range for down conversion. 
   Unlike existing spectrum analyzers in which frequency-band pre-selection is achieved by means of switches along the main RF signal path, the architectures of embodiments of the present invention employ coupler-based multiplexing. This approach results in the elimination of switching and its associated loss and frequency limitations from the main RF signal path, and enables wideband high-dynamic-range spectrum analysis. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details of the present invention are explained with the help of the attached drawings in which: 
       FIG. 1  illustrates the use of switched filters to allow frequency band pre-selection in a broadband spectrum analyzer; 
       FIG. 2  illustrates an external mixer configuration used to extend the frequency range of a spectrum analyzer; 
       FIG. 3  shows circuitry for extending the bandwidth of a low frequency spectrum analyzer (LFSA) according to an embodiment of the present invention using an array of couplers tapped onto the RF signal path, along with a single frequency translation device for translating the coupler outputs to provide to a LFSA; and 
       FIG. 4  shows circuitry for extending the bandwidth of a LFSA according to another embodiment of the present invention using an array of couplers tapped onto the RF signal path along with multiple mixers, and IF switches to select one mixer IF output to provide to the LFSA. 
   

   DETAILED DESCRIPTION 
     FIG. 3  shows circuitry for extending the bandwidth of a low-frequency spectrum analyzer according to one embodiment of the present invention. The circuit of  FIG. 3  uses an array of couplers  102   1 . . . N  arranged along the RF signal path. The frequency spectrum of the RF signal is split into N+1 bands corresponding to the N couplers  102   1 . . . N  and the low-frequency band of the LFSA  116 . The frequency range of each coupler  102   1 . . . N  corresponds to a different one of the N+1 bands with some overlap, as illustrated by the respective symbol R 1 . . . N . A 0 th  band corresponds to the frequency range of the LFSA  116 , the 0 th  band being uncommon to all the couplers  102   1 . . . N  while the i th  (i=1 . . . N) corresponds to the band of each coupler that is directed through one of the filters  120   1 . . . N . 
   The RF input signals appearing at the input port through variable attenuator  101  with frequencies falling in the 0 th  band are diverted to the LFSA  116  by means of a diplexer (D)  110 . In contrast, signals having frequencies in one of the other bands (i=1 . . . N) couple strongly through a corresponding one of the cascade of couplers  102   1 . . . N  to filters  120   1 . . . N . Power remaining in the RF signals as they reach the diplexer  110  is diverted toward a broadband termination where it is absorbed. While in general the frequency bands (i=1 . . . N) of the couplers  102   1 . . . N  need not be contiguous, continuity of the frequency spectrum renders this property desirable. 
   The filters  120   1 . . . N , switches  106   1 . . . N , amplifiers  122   1 . . . N , and additional couplers  104   1 . . . N  provide a selected one of the signals from couplers  102   1 . . . N  to a first input (RF input) of a single frequency translation device, such as a mixer  100 . The frequency translation device can also be a sub-harmonic mixer. Selection of one of the couplers  102   1 . . . N  is provided by controlling connection of one of the switches  106   1 . . . N . The off state of the switches  106   1 . . . N  is made absorptive by proper choice of switch type and off-state termination as illustrated. The filters  120   1 . . . N  are band-pass filters with a range that substantially eliminates the frequency band of a subsequent filter. The different filter ranges are illustrated by the symbols F 1 . . . N . The amplifiers  122   1 . . . N  are optional, and do have a frequency range like the filters, illustrated by symbols G 1 . . . N , substantially covering only the range of their respective connected filter  120   1 . . . N . Each of the couplers  104   1 . . . N  has a range corresponding to one of the couplers  102   1 . . . N  to which it is connected, as illustrated by the respective symbols L 1 . . . N . The through path of the couplers  104   1 . . . N  is connected by an amplifier  124  to the first RF input of mixer  100 . 
   The mixer  100  is a single device in the embodiment of  FIG. 3  that provides for down conversion of a selected RF signal to an IF signal for providing to the LFSA  116 . A local oscillator (LO) that is synchronized with the LFSA is provided to the second input (LO) of the mixer  100 . The mixer  100 , thus, converts the RF signal received at its first input to an IF output signal using the LO signal from its second input. The IF output of the mixer  100  is provided through a bandpass filter  126  to remove undesired harmonics, and is then provided through amplifier  128  to a second input of the LFSA  116 . 
     FIG. 4  shows circuitry for extending the bandwidth of a LFSA according to another embodiment of the present invention. The circuitry of  FIG. 4  uses an array of couplers  202   1 . . . N  provided in the RF signal path, similar to  FIG. 3 . Instead of a single mixer as in  FIG. 3 , however,  FIG. 4  uses multiple mixers  200   1 . . . N  and switches  206   1 . . . N  to select the output of one of mixers  200   1 . . . N  to provide the IF output to LFSA  216 . 
   Similar to  FIG. 3 , in  FIG. 4  the frequency spectrum of the RF signal is split into N+1 bands corresponding to the couplers  202   1 . . . N  and the low-frequency band of the LFSA  216 . The range of each coupler  202   1 . . . N  corresponds to a different one of the N+1 bands with some overlap. A 0 th  band corresponds to the frequency range of the LFSA  216 , the 0 th  band being uncommon to all the couplers  102   1 . . . N  while the i th  (i=1 . . . N) corresponds to the band of each coupler that is directed through to one of the filters  220   1 . . . N . 
   The RF signals appearing at the input port through variable attenuator  201  with frequencies falling in the 0 th  band are diverted to the LFSA  216  by means of a diplexer (D)  210 . In contrast, signals having frequencies in one of the other bands (i=1 . . . N) couple strongly through a corresponding one of the cascade of couplers  202   1 . . . N  to filters  220   1 . . . N . Power remaining in the RF signals as they reach the diplexer  210  is diverted toward a termination where it is absorbed. While in general the frequency bands (i=1 . . . N) of the couplers  202   1 . . . N  need not be contiguous, continuity of the frequency spectrum renders this property desirable. 
   Signals in the frequency bands (i=1 . . . N) appearing in couplers  202   1 . . . N  through the filters  220   1 . . . N  are translated to the IF frequency range of the LFSA  216  using a corresponding one of the mixers  200   1 . . . N . As in the previous architecture, one frequency bands is active at a time while others are disabled. That is, only one of the LO signals LO 1 . . . N  is connected to a corresponding mixer  200   1 . . . N  while others are terminated by means of switches  234   1 . . . N  and adjacent resistive terminations. Therefore, the active frequency band dictates the state of LO switches  234   1 . . . N  and the state of IF switches  206   1 . . . N . 
   The filters  220   1 . . . N  and amplifiers  222   1 . . . N  act on signals from couplers  202   1 . . . N  to provide RF signals RF 1 . . . N  to individual ones of mixers  200   1 . . . N . The filters  220   1 . . . N  are band-pass filters with a range that substantially eliminates the frequency band of a subsequent filter. The different filter ranges are illustrated by the symbols F 1 . . . N . The amplifiers  222   1 . . . N  provide amplification over a similar frequency range as indicated by the symbols G 1 . . . N . 
   LO signals LO 1 . . . N  are provided to mixers  200   1 . . . N  from the output of the second chain of couplers  204   1 . . . N . The second coupler chain  204   1 . . . N  connects to mixers  200   1 . . . N  through bandpass filters  230   1 . . . N  and amplifiers  232   1 . . . N . The bandpass filters  230   1 . . . N  have frequency ranges that substantially eliminate the frequency band of a subsequent filter, as noted by the symbols B 1 . . . N , and amplifiers  230   1 . . . N  provide a similar range as noted by symbols A 1 . . . N . Additional frequency multipliers  236   1 . . . N  can optionally be included to selectively multiply the frequency output of couplers  204   1 . . . N  if desired. The LO signal is provided through the second coupler chain  204   1 . . . N  that is referenced to the LFSA  216 . LO signals whose frequencies fall outside the bands L 1 . . . N  are terminated in resistive load  205 . 
   Selection of one of the outputs of mixers  200   1 . . . N  as an IF signal to provide to LFSA  216  is controlled by connection of one of the switches  234   1 . . . N , as well as one of switches  206   1 . . . N . The switches  234   1 . . . N  are controlled so that only one output is connected to one of mixers  200   1 . . . N . Switches  206   1 . . . N  are controlled so that one of the mixer outputs is then provided through bandpass filter  226  and amplifier  228  to LFSA  216 . 
   The fact that the architectures shown in  FIGS. 3-4  are frequency scalable is evident by noting the varying lengths of the coupler arms. Evident also is the fact that the DC path leading from the RF input port to the LFSA shown in the circuit of both  FIGS. 3 and 4  is preserved for signals falling in the 0 th  band as a result of frequency diplexing with diplexers  110  and  210 . 
   The chains of couplers in the circuits of  FIGS. 3 and 4  can be arranged to reduce frequency dependent loss, to maximize dynamic range or to provide for a desired combination of these goals. To accomplish this, the coupler lengths can be either commensurate or non-commensurate. Further, the spacing between the chained couplers can be either commensurate or non-commensurate. Further, the coupling coefficient of the couplers may or may not differ. By locating high-frequency couplers close to the RF input port, high-frequency ohmic losses can be minimized so as to preserve the high dynamic range of the measurement instrument. 
   To increase dynamic range and to reduce frequency dependent losses, other components besides the couplers of  FIGS. 3 and 4  can be selected appropriately. For example, in one embodiment the bandpass filters can be made tunable so that a user can fine tune dynamic range. The amplifiers can be chosen so that the third-order intercept of the amplifier does not limit the frequency-translation devices. Further, the order of the amplifiers, filters, switches and couplers can be re-arranged to improve performance. Further, the frequency translation devices can be made as identical as possible. Also, the frequency multipliers can be made reactive to keep phase noise as close as possible to 20 log 10 (N), where N is the frequency multiplication coefficient. To reduce frequency-dependent electromagnetic coupling, waveguide channels can be employed to isolate electromagnetically frequency-translation stages, switch stages, or other portions of the circuitry of  FIGS. 3 and 4 . Finally, attenuators can be used to reduce multiple electromagnetic wave reflections between stages. 
   Although the present invention has been described above with particularity, this was merely to teach one of ordinary skill in the art how to make and use the invention. Many additional modifications will fall within the scope of the invention, as that scope is defined by the following claims.