Abstract:
Selectable attenuators are used in the reference and test paths of a VNA, with attenuation automatically inserted or deleted when the Device Under Test (DUT) attenuation reaches predetermined thresholds. Attenuation in the reference channel is removed when the signal in the test channel is sufficient to overcome the leakage of the reference channel. Additionally, attenuation is removed from the test channel when the reference channel has a high attenuation inserted to further increase the difference between the leakage reference signal and the reduced test channel signal thus allowing lower isolation requirements on the two (or more) receiving channels.

Description:
BACKGROUND 
   1. Technical Field 
   The present invention relates to providing attenuation in a Vector Network Analyzer (VNA) receiver to prevent non-linear receiver signal output when measuring insertion loss or gain of a device under test (DUT). 
   2. Related Art 
   Currently available two port VNAs attenuate the reference channel as much as practical to overcome the leakage signal from the reference channel to the test channel. The resulting constructive and destructive addition of the desired measurement signal through the DUT causes a ripple signal to appear. A limitation is reached when the reference signal is decreased enough to cause noticeable noise when measuring low insertion loss devices due to the excess noise in the reference channel itself. This forces the two (or more) measurement channels of the VNA to be designed with inherently high isolation at a considerable cost. 
   Due to these high noise isolation requirements, the design of a combined source and detector, such as a VNA, used to measure the insertion loss or gain of a DUT will typically be bounded by a number of constraints discussed to follow. 
   As a first constraint, the maximum input to the receiver is limited by the point at which the receiver starts to exhibit nonlinearity. For a microwave sampler this level is about −21 dBm. For mixers, depending on the LO drive level, nonlinearity can be exhibited at 0 dBm. 
   As another constraint, the minimum input to the receiver is first limited by KTB Noise. The KTB noise is fixed at −174 dBm in a 1 Hz Bandwidth receiver at 25 Degrees Celsius. This noise increases by a factor of 10 Log(Bandwidth) in Hz. The minimum input to the receiver is also limited by the Noise Figure (NF) of the receive chain. Sampler based systems have a NF of around 30 dB. Mixer based systems have a NF of around 10 dB. The relative merits of mixer vs. sampler based systems is primarily due to complexity of the mixer, although LO frequency generation is also more complex in mixers. Wide bandwidth mixers are constrained by the internal Balun structures, which limit the mixer operation to 100s of KHz to 100s of MHz, or 100s of MHz to 20 to 40 GHz. 
   Isolation is another constraint on receivers. Isolation between mixers is dependent on their inherent RF to LO isolation plus the isolation between the two (or more) LO drive circuits. Isolation between the two LO drive circuits is usually determined by a splitter. The splitter is also constrained by its internal Balun structure. The splitter is limited to the same frequency ranges as the mixers. Samplers on the other hand are simple devices in comparison. The LO (Sampler turn on drive) is a narrow pulse on the order of 10s of picoseconds. The isolation between samplers is accomplished with simple orthogonal microstrip to coplanar waveguide transitions. The LO pulse frequency only has to cover one octave, starting at the lowest frequency of operation. Higher frequencies are converted using under sampling. For convenience, further description will be limited to sampler based systems, although techniques according to embodiments of the present invention are understood to work equally well with mixer based systems. A typical planar dual sampler driven with a common pulse will exhibit about 50 dB of isolation between sampler inputs. Laboratory grade samplers can reach 85 dB. These devices rely on very complex splitters. They are physically large due to the 3D construction of the pulse coupling through the orthogonal plane. Further examples herein are limited to splitters with a limit of 50 dB of isolation. These constraints define the upper and lower limits of a receiver. 
   To measure loss and gain using a source/receiver of a conventional VNA, attenuation is typically provided to the reference channel enough to keep the leakage signal &gt;10 dB below the KTB Noise Floor of the Test Channel. The drawback to this technique is that the KTB noise of the reference channel dominates in low loss measurements. 
     FIG. 1  shows components of a VNA connected to measure a DUT to provide a reference to describe how noise is limited. In  FIG. 1 , the gain or attenuation of each component is labeled. The labels are used to analyze the Reference (R) signal component and the Transmitted (T) signal component as measured by the VNA. 
   The components illustrated in  FIG. 1  for a VNA are conventional and include the signal source  2  for providing a test signal to coupler  4 . The test signal is provided on a through path of the coupler  4  and experiences an incident signal loss (IL) before being provided through a device under test (DUT)  8 . The test signal from the DUT  8  is then received through attenuator (A 2 )  10  and provided for down conversion in downconverter  14 . The downconverter  14  generates harmonics, creating the NFT, or noise figure component for the test signal. The bandpass filter  18  passes the desired harmonic of the test signal (T) to the VNA receiver for evaluation. The receiver noise of the test system  22  is illustrated to be added in at summer  24 . 
   The signal from source  2  is also provided through the coupling path of coupler  4  as a reference signal, and experiences a coupling loss (CPL) before reaching the attenuator (A 1 )  6 . The reference signal from the attenuator is provided to downconverter  12 . The downconverter  12  generates harmonics, creating the NFR, or noise figure component for the reference signal. The bandpass filter  16  passes the desired harmonic of the reference signal (R) to the VNA receiver for evaluation. The receiver noise of the test system  20  is illustrated to be added in at summer  26 . 
   A synopsis of each component and its attenuation contribution is labeled as follows: 
   PIN: VNA Signal Source ( 2 ) 
   IL: Insertion Loss of Through Path of Coupler ( 4 ) 
   CPL: Loss through Coupling Path of Coupler ( 4 ) 
   A 1 : Attenuation through attenuator A 1  ( 6 ) 
   DUT: Attenuation through DUT ( 8 ) 
   A 2 : Attenuation through attenuator A 2  ( 10 ) 
   NFR: Noise Figure of reference downconverter ( 12 ) 
   NFT: Noise Figure of transmitted downconverter ( 14 ) 
   ISO: Isolation Attenuation ( 15 ) between downconverters ( 12 ) and ( 14 ) 
   BWR: Bandwidth of the Reference Filter ( 16 ) in KHz 
   BWT: Bandwidth of Transmitted Filter ( 18 ) in KHz 
   NSR: Noise source contribution of reference receiver ( 20 ) 
   NST: Noise source contribution of transmitted receiver ( 22 ) 
   A number of equations identified below are used in conventional systems to determine the values of A 1  and A 2 , as well as noise in the reference (R) and test (T) signals. The values for A 1  and A 2  in the equations are determined assuming NFR=29 dB, and BWR=23 KHz. The equations to determine A 1  and A 2  are as follows:
 
R=PIN−CPL−A1
 
NSR=10*Log BWR+NFR−174
 
NSR=101.4 dBm (For NSR=29 dB and BWR=23 KHz)
 
R/NSR=R−NSR
 
T=PIN−IL−DUT−A2
 
NST=10*Log BWT+NFT−174
 
NST=101.4 dBm (For NFT=29 dB and BWT=23 KHz)
 
T/NST=T−NST
 
R=R−ISO
 
T/RI=T−RI=T−R+ISO
 
Let T/R1=T/NST+Delta (T/R1&gt;T/NST by Delta dB)
 
T=R+ISO=T−NST+Delta
 
T−(PIN−CPL−A1)+ISO=T−NST+Delta
 
NST−Delta=PIN−CPL−A1−ISO
 
A1=PIN−CPL−NST−ISO+Delta  (Equation 1 for A1)
 
T=PIN−IL−DUT−A2
 
A2=PIN−IL−DUT−T  (Equation 1 for A2)
 
   Next, to provide values for A 1  and A 2  in a typical system, as an example it is assumed that T=−21 dbM at DUT=0 dB. Also, PIN is +9 dB, IL is −4 dB, CPL is −14 dB, ISO is −50 dB and Delta is 15 dB. Applying these values, the following values for A 1  and A 2  are determined from Equation 1 for A 1  and Equation 1 for A 2  as follows:
 
A2=PIN−IL+21  (From Equation 1 for A2)
 
A2=9−4+21
 
A2=26 dB
 
A1=PIN−CPL−NST−ISO+Delta  (From Equation 1 for A1)
 
A1=9−14−(−101.4)−50+15
 
A1+61.4 dB
 
   Next formulas for the signal to noise ratio are determined for both the reference signal (R) relative to NSR and the test signal (T) relative to NST, and the test signal (T) relative to R 1 . The formulas are derived as follows:
 
T/NST=T−NST
 
T/NST=PIN−IL−DUT−A2−NST
 
T/NST=+9−4−DUT−26−(101.4)
 
T/NST=80.4−DUT
 
T/R1=T−R+ISO
 
T/R1=PIN−IL−DUT−A2−R−ISO
 
T/R1=PIN−IL−DUT−A2−(PIN−CPL−A1)+ISO
 
T/R1=PIN−IL−DUT−A2−PIN+CPL+A1+ISO
 
T/R1=CPL+ISO+A1−IL−A2−DUT
 
T/R1=95.4−DUT
 
R/NSR=R−NSR
 
R/NSR=PIN−CPL−A1−NSR
 
R/NSR=+9−14−61.4−(101.4)
 
R/NSR=35.5 dB
 
   For a given signal to noise ratio in dB (SN), the converted Noise Signal (NS) in dB is:
 
NSdB=20*((Log(1+10 −(SN/20) −Log(1−10 −(SN/20) )
 
   For a given signal to interfering signal ratio (SIS) in dB, the converted Ripple (RIP) in dB is:
 
RIPdB=20*(Log(1+10 −(SIS/20) )−Log(1−10 −(SIS/20) )
 
   For two uncorrelated S/N Noise Sources VN 1  and VN 2  in dB, the total Noise VNT in dB is:
 
VN1=10 (SN1/20)  
 
VN2=10 (SN2/20)  
 
VNT=SQR((VN 2 )+(VN2) 2 ))
 
NSTdB=20*((Log(1+VNT)−Log(1−VNT))
 
     FIG. 2  shows a plot of S/N ratio in dB for three different measured signals versus DUT loss in dB. First, a plot  200  of the reference channel signal to noise ratio R/NSR is provided vs. DUT loss. Second, a plot  202  of the test channel signal to noise ratio T/NST is provided vs. DUT loss. Third a plot  204  of the test channel signal to interfering signal T/R 1  is provided vs. DUT loss. 
     FIG. 3  shows a plot of converted noise signals and ripple versus DUT loss in dB. First, a plot  302  of the converted reference channel S/N Rns is provided vs. DUT loss. Second, a plot  300  of the converted test channel S/N ratio Tns is provided vs. DUT loss. Third, a plot  304  of the total converted noise Tns+Rns is provided. Fourth, a plot of the ripple  306  is provided. 
   As seen from  FIG. 3 , the ripple stays 15 dB (Delta) below the test channel noise, thus making it transparent. It is also apparent that the total noise due to Rns is dominant for DUT=&lt;45.4 dB. This is the unfortunate consequence of A 1  being dependant on isolation to get the ripple below the noise. 
   SUMMARY 
   According to embodiments of the present invention, selectable attenuators are used in the reference and test paths of a combined source and detector device, such as a VNA, with attenuation automatically inserted or deleted when the DUT attenuation reaches predetermined thresholds. 
   Attenuation in the reference channel is removed or deleted when the signal in the test channel is sufficient to overcome the leakage of the reference channel. Additionally, attenuation is removed from the test channel when the reference channel has the high attenuation inserted to further increase the difference between the leakage reference signal and the reduced test channel signal thus allowing lower isolation requirements on the two (or more) receiving channels. The embodiments of the present invention can be either mixer or sampler based in construction. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details of the present invention are explained with the help of the attached drawings in which: 
       FIG. 1  shows typical components of a VNA connected to measure a DUT; 
       FIG. 2  shows plots of noise signal ratios vs. DUT attenuation, the noise signals ratios including: (a) R/NST—the receive channel signal to noise ratio, (b) T/NST—the test channel signal to noise ratio, and (c) T/RI the test channel signal to interfering signal test signal ratio; 
       FIG. 3  shows plots of converted noise vs. DUT attenuation, the converted noise including: (a) Rns—the receive channel noise, (b) Tns—the test channel noise, (c) Rns+Tns—the total receive channel and test channel noise, and (d) the ripple due to the finite isolation of 50 dB between the R and T samplers; 
       FIG. 4  shows components of a VNA according to embodiments of the present invention using variable attenuation values for A 1  and A 2 ; 
       FIG. 5  shows plots of noise signal ratios vs. DUT attenuation, with a variable receive channel attenuation A 1 , wherein A 1 =DUT/2+38.7, the noise signals ratios including: (a) R/NST, (b) T/NST, and (c) T/RI; 
       FIG. 6  shows plots of converted noise vs. DUT attenuation, with the variable attenuation A 1 =DUT/2+38.7, the converted noise including: (a) Rns, (b) Tns, (c) Rns+Tns, and (d) the ripple; 
       FIG. 7  shows plots of noise signal ratios vs. DUT attenuation as in  FIG. 5 , with a variable receive channel attenuation A 1  as in  FIG. 5  only up to DUT=45 dB, and A 1  remaining constant at 61.4 dB as in  FIG. 2  with DUT&gt;45 dB; 
       FIG. 8  shows plots of converted noise vs. DUT attenuation with the variable attenuation A 1  only up to DUT=45 dB, and A 1  remaining constant at 61.4 dB as in with DUT&gt;45 dB as in  FIG. 7 ; 
       FIG. 9  shows plots of noise signal ratios vs. DUT attenuation as in  FIG. 5 , with a variable receive channel attenuation A 1  as in  FIG. 5 , as well as a variable test channel attenuation A 2  that goes from 26 to 0 when DUT goes from 0 to 26 dB to satisfy a maximum of −21 dBm on the test channel input; 
       FIG. 10  shows plots of converted noise vs. DUT with the variable attenuation A 1  as well as the variable test channel attenuation A 2  that goes from 26 to 0 when DUT goes from 0 to 26 dB as in  FIG. 9 ; 
       FIG. 11  shows plots of noise signal ratios vs. DUT attenuation as in  FIG. 9 , with a slight reduction in noise at the crossover where Tns=Rns by reverting A 1  back to the constant 61.4 dB with DUT=&gt;71.4 dB; 
       FIG. 12  shows plots of converted noise vs. DUT as in  FIG. 11  with the attenuation A 1  reverting back to the constant 61.4 dB with DUT=&gt;71.4 dB; 
       FIG. 13  shows plots of noise signal ratios vs. DUT attenuation with the attenuation of A 1  and A 2  varied through four sets of values, with A 2 =26 for a DUT of 0-13 dB, A 2 =13 for a DUT of 13-26 dB, A 2 =0 for a DUT&gt;26 dB, and A 1 =45.2 for a DUT of 0 to 39 dB and A=61.4 for DUT&gt;39 dB; 
       FIG. 14  shows plots of converted noise vs. DUT for the values of A 1  and A 2  of  FIG. 13 ; 
       FIG. 15  shows plots of noise signal ratios vs. DUT attenuation with the attenuation of A 1  and A 2  varied through two sets of values, with A 2 =26 for a DUT of 0-26 dB, A 2 =0 for a DUT&gt;26 dB, and A 1 =51.7 for a DUT of 0 to 26 dB and A=61.4 for DUT&gt;26 dB; 
       FIG. 16  shows plots of converted noise vs. DUT for the values of A 1  and A 2  of  FIG. 16 ; and 
       FIG. 17  shows a comparison of the plots for total noise Tns+Rns for each of the  FIGS. 3 ,  6 ,  8 ,  10 ,  12 ,  14  and  16 . 
   

   DETAILED DESCRIPTION 
     FIG. 4  illustrates embodiments of the present invention, wherein variable attenuators  6 A and  10 A are provided to make the attenuation values A 1  and A 2  variable. Variation of the attenuation values A 1  and A 2  enables isolation noise between the reference and transmit receivers to be reduced so that less complex equipment is required in a VNA. 
   1. First Embodiment 
   In a first embodiment of the present invention, A 1  is changed to allow T/R 1 =R/NSR+Delta instead of T/R 1 +T/NST+Delta. One advantage that this provides is to allow the noise due to Rns to be at a minimum for DUT=&lt;45.4 dB and dominated by Tns&gt;45.4 dB. The equations for A 1  and A 2  are rederived as follows:
 
T/R1=R/NSR+Delta
 
T−R1=R−NSR+Delta
 
R1=R−ISO
 
T−R+ISO=R−NSR+Delta
 
T+ISO=2*R−NSR+Delta
 
2*R=T+ISO−NSR−Delta
 
R=(T+ISO+NSR−Delta)/2
 
R=PIN−CPL−A1 (Determined From FIG.  4 )
 
PIN−CPL−A1=(T+ISO+NSR−Delta)/2 (By setting R=R in last 2 equations)
 
A1=PIN−CPL−(T+ISO+NSR−Delta)/2
 
A1=PIN−CPL−PIN/2+(IL+DUT+A2+Delta−ISO−NSR)/2
 
A1=PIN/2−CPL+(IL+DUT+A2+Delta−ISO−NSR)/2  (Equation 2 for A1)
 
   Applying the values from the previous, non-limiting example of NFR=NFT=29 dB, BWR=BWT=23 KHZ, NSR=101.4 dBm, PIN=9 dBm, IL=4 dB, CPL=14 dB, ISO=50 dB and Delta=+15 dB we determine A 1  as follows:
 
A1=PIN/2−CPL+(IL+DUT+A2+Delta−ISO−NSR)/2  (From Equation 2 for A1)
 
A1=+9/2−1 4+(+4+DUT+26+15+50−(−101.4)/2
 
A1=DUT/2+38.7
 
   This equation shows that A 1  is now dependant on DUT. A 1  can now be realized by a variable attenuator that can track the value of DUT by the above relationship of DUT/2+38.7. The value for A 2  remains fixed at 26 dB, as determined previously using Equation 1 for A 2 . 
     FIG. 5  shows a plot of S/N ratio in dB for three different signals for three different measured signals versus DUT loss in dB with A 2  being a variable attenuator that tracks the DUT value. First, a plot  500  of the reference channel signal to noise ratio R/NSR is provided vs. DUT loss. Second, a plot  502  of the test channel signal to noise ratio T/NST is provided vs. DUT loss. Third a plot  504  of the test channel signal to interfering signal T/R 1  is provided vs. DUT loss. 
     FIG. 6  shows a plot of converted noise signals and ripple versus DUT loss in dB with the variable A 1  attenuator that tracks the DUT loss. First, a plot  600  of the converted reference channel S/N Rns is provided vs. DUT loss. Second, a plot  602  of the converted test channel S/N ratio Tns is provided vs. DUT loss. Third, a plot  604  of the total converted noise Tns+Rns is provided. Fourth, a plot of the ripple  606  is provided due to the finite isolation of 50 dB between the R and T samplers. 
   The conditions of  FIG. 6  are substantially the same as the example of  FIG. 4 , with the exception of A 1  changing from a fixed 61.4 dB to a value based on DUT/2+38.7. The results of  FIG. 6  show that the ripple now stays a constant value (Delta) below the reference channel noise, thus, making it transparent. The noise due to Rns is still dominant for DUT=&lt;45.4 dB. This is the minimum noise that can be achieved for Rns while still remaining Delta dB. A further reduction of Rns will cause the decrease of Delta, which in turn will increase ripple. 
   2. Second Embodiment 
   In a second embodiment of the present invention, a slight reduction in total noise can be realized if conditions are reverted back to T/R 1 =T/NST+Delta for DUT&gt;45.4 dB. The noise reduction occurs where Tns=Rns at DUT=45.4 dB. 
     FIG. 7  shows a plot of S/N ratio in dB for three different signals for three different measured signals versus DUT loss in dB with A 1  being a variable attenuator that tracks the DUT value only up until DUT=45.4 dB. First, a plot  700  of the reference channel signal to noise ratio R/NSR is provided vs. DUT loss. Second, a plot  702  of the test channel signal to noise ratio T/NST is provided vs. DUT loss. Third a plot  704  of the test channel signal to interfering signal T/RI is provided vs. DUT loss. 
     FIG. 8  shows a plot of converted noise signals and ripple versus DUT loss in dB with the variable A 1  attenuator that tracks the DUT loss. First, a plot  800  of the converted reference channel S/N Rns is provided vs. DUT loss. Second, a plot  802  of the converted test channel S/N ratio Tns is provided vs. DUT loss. Third, a plot  804  of the total converted noise Tns+Rns is provided. Fourth, a plot of the ripple  806  is provided due to the finite isolation of 50 dB between the R and T samplers. 
   In  FIG. 8 , the conditions are changed only slightly from the conditions of  FIG. 6 . The exception is A 1  being a fixed value of 61.4 dB with DUT&gt;45.4 dB, and then A 1  taking a value of DUT/2+38.7 for DUT&gt;45.4 dB. There are no discontinuities at the crossover point because DUT/2+38.7=61.4 dB at DUT=45.4. With the configuration used for  FIG. 8 , ripple now stays a constant value (Delta) below the reference channel noise for DUT=&lt;45.4 dB and then stays a constant value (Delta) below the test channel noise for DUT&gt;45.4 dB. 
   3. Third Embodiment 
   For a further embodiment of the present invention, the value of A 2  is altered to create a greater dynamic range for the VNA. The value of A 2  in previous embodiments described has been fixed due to the fact that a given maximum input to the test channel down converter of −21 dBm and given that T=PIN−IL−DUT−A 2  forces A 2  to be PIN−IL−DUT+21. For the minimum value of DUT of 0, this forces A 2  to be +9−4−0+21=26 dB. If A 2  is allowed to go from 26 dB to 0 dB when the DUT goes from 0 dB to 26 dB, then the maximum input of the test channel would satisfy the maximum input of −21 dBm. This provides the benefit of an increase of 26 dB of dynamic range. With A 2  modified as described, the equations for A 1  and A 2  are changed as follows:
 
A2=PIN−IL−DUT−T  (From Equation 1 for A2)
 
A2=PIN−IL−DUT+21 (With T=−21 dBm under the new conditions)
 
A1=PIN/2−CPL+(IL+DUT+A2+Delta−ISO−NSR)/2  (From Equation 2 for A1)
 
A1=PIN/2−CPL+(IL+DUT+(PIN−IL−DUT+21)+Delta−ISO−NSR)/2 (by substituting A2)
 
A1=PIN/2−CPL+(PIN+21+Delta−ISO−NSR)/2  (Equation 3 for A1)
 
A1=PIN−CPL+(21+Delta−ISO−NSR)/2  (Equation 3 for A1 simplified)
 
   Applying the values from the previous, non-limiting example of NFR=NFT=29 dB, BWR=BWT=23 KHZ, NSR=101.4 dBm, PIN=9 dBm, IL=4 dB, CPL=14 dB, ISO=50 dB and Delta=+15 dB we determine A 1  and A 2  as follows:
 
A1=PIN−CPL+(21+Delta−ISO−NSR)/2  (from Equation 3 for A1)
 
A1=+9−14+(21+15+50−(−101.4)/2 (With values for DUT=&lt;26 for this embodiment)
 
A1=38.7 dB (for DUT=&lt;26)
 
A2=PIN−IL−DUT+21  (From Equation 1 for A2 with T=21 dB)
 
A2=94−DUT+21 (With values for DUT=&lt;26 for this embodiment)
 
A2=26−DUT (for DUT=&lt;26)
 
A2=PIN−IL−DUT+21  (From Equation 1 for A2 with T=21 dB)
 
A2=+9−4−26+21 (With values for DUT&gt;26 for this embodiment)
 
A2=0 (for DUT&gt;26)
 
A1=PIN/2−CPL+(IL+DUT+A2+Delta−ISO−NSR)/2  (From Equation 3 for A1)
 
A1=+9/2−14+(4+DUT+0+15−50−(−101.4))/2 (With values for DUT&gt;26)
 
A1=9.5+DUT/2+70.4/2
 
A1=DUT/2+25.7 (for DUT&gt;26 for this third embodiment)
 
     FIG. 9  shows a plot of S/N ratio in dB for three different signals for three different measured signals versus DUT loss in dB with A 1  being a variable attenuator that tracks the DUT value only up until DUT=45.4 dB, and A 2  changing values above DUT=26 dB. The three signals include a plot  800  of the reference channel signal to noise ratio R/NSR, a plot  802  of the test channel signal to noise ratio T/NST, and a plot  804  of the test channel signal to interfering signal T/R 1  all versus DUT loss. 
     FIG. 10  shows a plot of converted noise signals and ripple versus DUT loss in dB from the conditions of  FIG. 9 . A plot  1000  shows the converted reference channel S/N Rns vs. DUT loss, a plot  1002  shows the converted test channel S/N ratio Tns vs. DUT loss, a plot  1004  shows the converted noise Tns+Rns, and a plot  1006  shows the ripple due to the finite isolation of 50 dB between the R and T samplers. 
   In  FIG. 10 , the conditions are changed only slightly from the conditions of  FIG. 8 . The exception is A 2  changing from 26 dB to 0 dB for DUT&gt;45.4 dB. The ripple now stays a constant value (Delta) below the reference channel for all values of DUT.  FIG. 10  also shows the extended dynamic range of 26 dB. 
   4. Fourth Embodiment 
   In a fourth embodiment, a further reduction in total noise around the crossover where Tns+Rns at DUT=71.4 db can further be realized. The noise reduction is achieved by reverting back to the condition of T/R 1 +T/NST+Delta for DUT&gt;71.4 dB. This reverts A 1  back to a value of 61.4 dB instead of a value based on the DUT, and reverts A 2  back from 0 dB to 26 dB with DUT&gt;71.4 dB. 
     FIG. 11  shows the effect of this modification with a plot of S/N ratio in dB for three different signals. The three signals include a plot  1100  of the reference channel signal to noise ratio R/NSR, a plot  1102  of the test channel signal to noise ratio T/NST, and a plot  1104  of the test channel signal to interfering signal T/R 1  all versus DUT loss. 
     FIG. 12  shows a plot of converted noise signals and ripple versus DUT loss in dB from the conditions of  FIG. 11 . A plot  1200  shows the converted reference channel S/N Rns vs. DUT loss, a plot  1202  shows the converted test channel S/N ratio Tns vs. DUT loss, a plot  1204  shows the converted noise Tns+Rns, and a plot  1206  shows the ripple due to the finite isolation of 50 dB between the R and T samplers. In  FIG. 12 , the conditions are changed only slightly from the conditions of  FIG. 10 . The exception is A 2  changing back to 0 dB to 26 dB with DUT&gt;71.4 dB, and A 1  changing from a variable based on the DUT to a fixed 61.4 dB. 
   5. Fifth Embodiment 
   With embodiments of the present invention described thus far, A 1  and A 2  have both become variable. Such a variation of A 1  and A 2  is theoretically viable, but realistically fixed attenuators are desirable. Variable attenuations make the calibration tedious if all combinations of A 1  and A 2  were used in calibration. 
   In this fourth embodiment, a limited number of changes for A 1  and A 2 , such as four are used, so only four calibrations will be necessary. The selection of these four values will optimize the total noise while retaining 15 dB (Delta) between Rns for low values of DUT and 15 dB (Delta) between Tns for high values of DUT. The first selection will set the initial DUT value switch point where A 2 =PIN−IL+21 that satisfies the maximum input of −21 dBm to the T channel down converter. This can be satisfied for DUT=&lt;13. A 2  is, thus, determined as follows:
 
A2=PIN−IL+21  (from Equation 1 for A2 with DUT=0)
 
A2=+9−4+21
 
A2=26 for DUT=&lt;13 dB
 
   A 1  for this case will use the Equation based on T/R 1 =R/NSR+Delta stated in the first embodiment.
 
A1=PIN/2−CPL+(IL+DUT+A2+Delta−ISO−NSR)/2  (From Equation 2 for A1)
 
   The value of DUT will be at A 2 /2 so that the next selection point can be at DUT−A 2  for A 2 =0.
 
A1=9/2−14+(4+13+26+15−50−(−101.4)/2
 
A1=45.2 For DUT=&lt;13
 
   Then next break point will occur at DUT=13 where A 2  can be reduced to 13 and still not violate the −21 dBm input to the T Channel Down Converter.
 
A2=PIN−IL−DUT+21
 
A2=+9−4−13+21
 
A2=13 for DUT&gt;13 to =26
 
   The next break point will occur at DUT=26 where A 2  can reduce to 0 and still not violate the −21 dBm input to the T Channel Down Converter.
 
A2=PIN−IL−DUT+21  (From Equation 1 for A2)
 
A2=9/2−14+(4+39+0+15−50−(−101.4)/2
 
A1=45.2 For DUT&gt;26 to =&lt;39
 
   Next, the Equations for A 1  and A 2  revert back to being based on T/RI=T/NST+Delta described in the background of this application.
 
A2=PIN−IL−DUT+21  (From Equation 1 for A2)
 
A2=+94−26+21
 
A2=0 For DUT&gt;39
 
A1=PIN−CPL−NST−ISO+Delta  (From Equation 1 for A1)
 
A1=9−14−(−101.4)−50+15
 
A1=61.4 For DUT&gt;39
 
     FIG. 13  shows the effect of varying A 1  and A 2  over four steps for three different signals for this fifth embodiment. The three signals include a plot  1300  of the reference channel signal to noise ratio R/NSR, a plot  1302  of the test channel signal to noise ratio T/NST, and a plot  1304  of the test channel signal to interfering signal T/R 1  all versus DUT loss. 
     FIG. 14  shows a plot of converted noise signals and ripple versus DUT loss in dB from the conditions of  FIG. 13 . A plot  1200  shows the converted reference channel S/N Rns vs. DUT loss, a plot  1402  shows the converted test channel S/N ratio Tns vs. DUT loss, a plot  1404  shows the converted noise Tns+Rns, and a plot  1406  shows the ripple due to the finite isolation of 50 dB between the R and T samplers. In  FIG. 14 , the conditions are changed only slightly from the conditions shown in  FIGS. 2-3 . The exception is A 1  and A 2  changing from a fixed 61.4 dB and 26 dB respectively for this fifth embodiment. 
   6. Sixth Embodiment 
   In a sixth embodiment, a compromise is made if only 2 sets of values are used for A 1  and A 2 . The first set of values will be used from DUT&lt;26 or the value needed to keep the T channel Down Converter from having an input &gt;−21 dBm for DUT=0. This is determined by the Equation A 2 =PIN−IL−DUT−T from Equation 1 for A 2  where DUT=0 and T=21 dBm. The equation A 1 =PIN/2−CPL+(IL+DUT+A 2 +Delta−ISO−NSR)/2 from Equation 2 for A 1  for DUT&lt;26. The second set of values for A 2  will again be determined by the equation A 2 =PIN−IL−DUT−T where DUT=26 and T=21 dBm. The second set of values for A 1  will be determined from A 1 =PIN−CPL−NST−ISO+Delta from Equation 1 for A 1 .
 
A2=PIN−IL−DUT−T  (From Equation 1 for A2)
 
A2=+9−4−0(−21)
 
A2=26 For DUT&lt;26
 
A1=PIN/2−CPL+(IL+DUT+A2+Delta−ISO−NSR)/2  (From Equation 2 for A2)
 
A1=9/2−14+(4+26+26+15+50−(−101.4))/2
 
A1=51.7 For DUT=&lt;26
 
A2=PIN−IL−DUT−T  (From Equation 1 for A2)
 
A2=+9−4−26−(−21)
 
A2=0 For DUT&gt;26
 
A1=PIN−CPL−NST−ISO+Delta  (From Equation 1 for A1)
 
A1=9−14−(−101.4)−50+15
 
A1=61.4 For DUT&gt;26
 
     FIG. 15  shows the effect of varying A 1  and A 2  over two steps in this embodiment. The three signals include a plot  1500  of the reference channel signal to noise ratio R/NSR, a plot  1302  of the test channel signal to noise ratio T/NST, and a plot  1304  of the test channel signal to interfering signal T/RI all versus DUT loss. 
     FIG. 16  shows a plot of converted noise signals and ripple versus DUT loss in dB from the conditions of  FIG. 15 . A plot  1600  shows the converted reference channel S/N Rns vs. DUT loss, a plot  1602  shows the converted test channel S/N ratio Tns vs. DUT loss, a plot  1204  shows the converted noise Tns+Rns, and a plot  1606  shows the ripple due to the finite isolation of 50 dB between the R and T samplers. In  FIG. 16 , the conditions are changed only slightly from the conditions shown in  FIGS. 2-3 . The exception is A 1  and A 2  changing from a fixed 61.4 dB and 26 dB respectively to the values for this sixth embodiment. 
     FIG. 17  shows a comparison of the plots for total noise Tns+Rns for each of the  FIGS. 3 ,  6 ,  8 ,  10 ,  12 ,  14  and  16 . The labels for the plots for Tns+Rns in  FIG. 17  remain the same from their previous respective figures. 
   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.