Abstract:
The invention relates to a method for measuring objects for measurement, by means of a network analyzer with several measurement ports, at least one signal generator, for stimulating the object for measurement and at least one local oscillator, for measurement of the signal transmitted or reflected from the object for measurement by the superposition principle. According to the invention, on a frequency change, only the frequency of the local oscillator or the frequency of the signal generator is changed but not the frequency of the local oscillator and the signal generator simultaneously.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method, which allows a phase-stable frequency change of the signal generator and of the local oscillator, for example, of a network analyzer. Within the context of the present application, this method is referred to as the Secum-Trahenz method. 
     2. Related Technology 
     A vectorial network analyzer with controllable signal generators and controllable oscillators is known from DE 102 46 700 A1. Only a single local oscillator is often provided in the network analyzers used in practice. Several signal generators are generally provided in more-recent network analyzers with more than two test ports. For example, with a network analyzer comprising three test ports, two signal generators are present, one of which can be switched between two of the three test ports. A mixer, which is connected to a directional coupler, by means of which the wave travelling outwards via the test port or respectively the wave travelling inwards via the test port is de-coupled and fed into the mixer, is provided for each test port. The other input of the mixer is connected to the local oscillator. 
     With network analyzers and also other measuring instruments, for example, with connected signal generators, it is problematic that a frequency change of the signal generators has to be implemented in a phase-stable manner, that is to say, no sudden phase changes should occur during the frequency change. Signal generators generally comprise synthesisers with several phase-locked-loop (PLL) stages with fractional dividers. A division by a fractional-rational division factor is implemented in the dividers. If the division factor is varied not exclusively with regard to its integer component, this generally leads to a sudden phase change. Even if only the integer division factor is varied, a sudden phase change by π/n, where n is a whole number, can occur. The necessity for implementing a phase-stable frequency change is relevant particularly when measuring frequency-converting devices under test, such as mixers. 
     SUMMARY OF THE INVENTION 
     The invention therefore provides a method for the measurement of devices under test, in which no sudden phase change occurs during the frequency change or in which the sudden phase change is at least registered. 
     Accordingly, the invention provides a method for the measurement of a device under test, in particular, using a network analyzer, with several test ports, at least one signal generator for the excitation of the device under test and at least one local oscillator for measuring the signal reflected or transmitted from the device under test according to the principle of superposition: characterised in that, in the event of a frequency change, only the frequency of the local oscillator or the frequency of the signal generator is varied, but the frequency of the local oscillator and of the signal generator are not varied simultaneously. 
     According to the invention, in the event of a frequency change, only the frequency of the local oscillator or the frequency of the signal generator is changed, but the frequency of the local oscillator and the signal generator are not changed simultaneously. This means that any phase changes occurring during the frequency change of the oscillator and during the frequency change of the signal generator can be registered separately from one another and either compensated by adjusting the phase position of the oscillator or respectively the signal generator or can be included in the subsequent evaluation of the measurement. 
     The local oscillator and the signal generators are generally fitted as synthesisers with one or more PLL stages. A frequency change is then implemented by varying the division factor of the divider provided in the PLL stages. In this context, it is advantageous if initially, only the integer component of the division factor is varied, because no phase displacement is caused as a result. Accordingly, a rough frequency change in the proximity of the new target frequency can be achieved by this means. The fine frequency tuning then required can be implemented by varying the component after the decimal point of the division factor, wherein, according to the invention, only either the frequency of the oscillator or of the signal generator is adjusted, but both frequencies are not adjusted simultaneously, so that the phase change of the oscillator and of the signal generator can be registered separately from one another. 
     The method according to the invention can be used in a particularly advantageous manner with frequency-converting devices under test, for example, for testing mixers. In this context, two inputs of the frequency-converting device under test must be excited with different frequencies. With a mixer, one input of the mixer should be provided, for example, with a frequency in the high-frequency input bandwidth, while a signal, which serves as a local-oscillator signal for the mixer under test, must be supplied to the other input. The sum or respectively difference frequency from the two input signals, which is disposed in a completely different frequency range, appears at the output of the mixer. 
     In fact, measurements of mixers with vectorial network analyzers, which are based on reflection measurements including a reference mixer, are already known. In order to implement advantageous transmission measurements, the phase position of the two excitation signals, which are supplied to the mixer, must be accurately known. In the measurement of the output signal, the time position of the phase of the output signal relative to the time position of the phase of the excitation signals is also relevant. The two signal generators required for the excitation of the mixer must therefore be adjusted to a defined phase position relative to one another, and/or the phase position must be accurately known. Accordingly, in order to measure the output signal of the mixer, the relationship of the local oscillator signal, which is used internally within the network analyzer, relative to the generator signals must be accurately known. The method according to the invention for a phase-stable frequency change in adjusting the signal generators or respectively the local oscillator can be used in a particularly advantageous manner in this context. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the invention is described in greater detail below with reference to the drawings. The drawings are as follows: 
         FIG. 1  shows a block circuit diagram of a network analyzer, which can be used within the framework of the invention for testing a mixer; and 
         FIG. 2  shows an exemplary embodiment of the internal structure of the local oscillator illustrated in  FIG. 1  and the signal generators illustrated in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Before the method according to the invention is described, an exemplary test structure, in which the method according to the invention can be used advantageously, will be described with reference to  FIG. 1 . In the exemplary embodiment illustrated in  FIG. 1 , a frequency-converting device under test, for example, a mixer, is tested using a vectorial network analyzer. 
     In the exemplary embodiment presented, the first input port E 1  of the mixer DUT is a high-frequency input, at which the high-frequency signal RF_DUT is received. The second input port E 2  is an input port, at which the local oscillator signal LO_DUT is received. With the test configuration illustrated in  FIG. 1 , the first input port E 1  of the frequency-converting DUT designed as a mixer is connected to the first test port P 1  of the network analyzer NA. The second input port E 2  of the device under test DUT is connected to the third test port P 3  of the network analyzer NA. The mixed-down, intermediate-frequency signal ZF_DUT is connected to the output port A of the device under test DUT. The output port of the device under test DUT is connected to the second test port P 2  of the network analyzer NA. The S-parameters S 11 , that is to say, the reflection of the device under test DUT at the test port P 1 , S 33 , that is to say, the reflection of the device under test DUT at the test port P 3 , S 21 , that is to say, the transmission through the device under test DUT from the test port P 1  to the test port P 2 , and S 23 , that is to say, the transmission through the device under test DUT from the test port P 3  to the test port P 2 , are particularly relevant in this context. 
     The network analyzer NA illustrated in  FIG. 1  is designed as a conventional multi-port, vectorial network analyzer.  FIG. 1  shows only three test ports P 1  to P 3 . Of course, the network analyzer NA can also provide more than three test ports. Several two-port network analyzers can also be connected together in cascade, as illustrated in the priority document DE 10 2006 001 284. 
     In the exemplary embodiment illustrated in  FIG. 1 , each test port P 1 , P 2  and P 3  provides its own signal generator GN 1 , GN 2  and GN 3 . However, this need not necessarily be the case. It is also possible, for example, for only two signal generators to be provided one signal generator being switchable between two test ports. Furthermore, in the exemplary embodiment presented, a common local oscillator LO is provided for all test ports P 1  to P 3 . This need not necessarily be the case. With a more complex network analyzer, an individual local oscillator may be provided for each test port P 1  to P 3 , or a local oscillator can supply two test ports respectively in pairs. 
     The frequency of the signal generators GN 1 , GN 2 , GN 3  and of the local oscillator LO can be varied respectively via dividers T G1 , T G2 , T G3  and T LO , which form a component of a phase-locked loop according to the PLL principle, and which are indicated only schematically in  FIG. 1 , being described in greater detail with reference to  FIG. 2 . The divided-down signal of the signal generators GN 1 , GN 2  and GN 3  is supplied respectively to the associated test port P 1 , P 2  and respectively P 3 . A directional coupler R 1 , R 2  and respectively R 3 , which de-couples the forward wave a 1 , a 2  and respectively a 3  generated by the signal generators GN 1 , GN 2  and GN 3  travelling to the test ports P 1 , P 2  and P 3  respectively and supplies it respectively to an associated mixer M 1 , M 2  and M 3 , is disposed between the signal generators GN 1  to GN 3  and the associated test ports P 1  to P 3 . The return wave b 1 , b 2  and b 3  received via the test ports P 1 , P 2  and respectively P 3  is also de-coupled via the directional couplers R 1 , R 2  and R 3  and supplied to the associated mixer M 1 , M 2  and M 3 . 
     The mixers M 1 , M 2  and M 3  also each receive the signal of the local oscillator LO, divided down as required in the divider T LO . The signal of the forward and return waves mixed down into the intermediate-frequency ranges ZF 1 , ZF 2  and respectively ZF 3  is supplied in each case to an analog/digital converter A 1 , A 2 , A 3 , and the digitised signal is registered in a detector D 1 , D 2  or respectively D 3  with regard to amplitude and phase. A control device or controller C receives the signals received from the detectors D 1  to D 3  and is used at the same time to control the signal generators GN 1  to GN 3 , the local oscillator LO and the associated dividers T G1 , T G2 , T G3  and T LO . The S-parameters are calculated from the forward and return waves, for example, in the control device C, and presented on a display DS dependent upon the measured frequency. 
     An exemplary embodiment of the exact structure of the signal generators GN 1  to GN 3  and the local oscillator LO is presented in  FIG. 2 , wherein it is evident that the signal generators GN 1  to GN 3  and the local oscillator LO are built up with several PLL stages with several dividers  64 ,  67  and  77 . 
     The reference signal REF is transmitted to the local oscillator LO and/or the signal generators GN 1  to GN 3  via the connecting line  31 . In the exemplary embodiment, the frequency of the reference signal REF is initially doubled within the local oscillator LO or respectively in the signal generators GN 1  to GN 3  in a frequency doubler  60  and supplied within the local oscillator LO or respectively signal generator to a first comparison input of a first phase detector  61 . The output of the first phase detector  61  is connected to the control input  63  of a first oscillator  62 . The output of the first oscillator  62  is connected via a first fractional divider  64  to the second comparison input of the first phase detector  61 . Consequently, the first oscillator  62  with the divider  64  and the first phase detector  61  forms a first phase-locked loop PLL, which is synchronized with the reference signal REF. This first phase-locked loop in stage  65  is also referred to as Child_PLL. The divider  64  divides the frequency by the fractional-rational division factor (N.F) CH  with the integer component N and the non-integer component F after the decimal point. 
     The stage  66  adjoining the above is referred to as the Sweep_PLL. A second divider  67 , which is connected to the output of the first oscillator  62 , is provided here. A synchronization component  68  ensures the selection of the fractional-rational division factor (N.F) SY  of the divider  67 . The output of the second divider  67  is connected to a first comparison input of a second phase detector  69 , of which the output is once again connected to the control input  70  of a second oscillator  71 . The output of the second oscillator  71  is connected to a first input of a mixer  72 . A second input of the mixer  72  receives the reference signal REF doubled by the frequency doubler  60 . The output of the mixer  72  is connected to the second comparison input of the second phase detector  69 . In this manner, a second phase-locked loop PLL, which is also synchronized via the reference signal REF, is formed by the second oscillator  71 , the mixer  72  and the phase detector  69 . 
     A third oscillator  74 , of which the control input  75  is connected to a third phase detector  76 , is disposed in a third stage  73 , which is referred to as the Main_PLL. A first comparison input of the third phase detector  76  is connected to the output of the second oscillator  71 , while a second comparison input of the third phase detector  76  is connected via a third divider  77  to the output of the third oscillator  74 . The local oscillator signal or respectively generator signal with the frequency f LO  or f GN1 , f GN2  or f GN3  is available at the output of the third oscillator  74 , which is also referred to as the main oscillator. The frequency f LO  in this context can be tuned over several octaves. The divider  77  also divides the frequency by a fractional-rational division factor (N.F) MA . 
     In particular, but not exclusively in the case of frequency-converting devices under test DUT, the problem arises that the frequency of the signal generators GN 1  to GN 3  and of the local oscillator LO must be changed in a phase-stable manner. The testing of the mixer DUT of the exemplary embodiment shown in  FIG. 1  is preferably implemented according to the invention as follows: 
     In the event of an adjustment of the network analyzer NA, the complex parameters of the signal generators GN 1  and GN 3  at the test ports P 1  and P 3 , namely, the forward waves a 1  and a 3  and the reflected waves b 1  and b 3  at these test ports, can only be measured if the frequency in the signal generators GN 1  and GN 3  is the same within the bandwidth of the intermediate-frequency range ZF 1  and respectively ZF 3 , because the local oscillator LO in the exemplary embodiment is provided only as a single local oscillator at the same measurable, intermediate reception frequency. Similarly, the intermediate frequency of the mixer DUT, or in general, of the frequency-converting device under test, can only be analysed at the test port P 2 , if the local oscillator LO is adjusted for this reception frequency. Accordingly, it is never possible to measure at the three test ports P 1  to P 3  with the same adjustment of the signal generators and the local oscillator, that is to say, for each test point, both the signal generator GN 3  at the test port P 3  and also the local oscillator LO must be adjusted respectively for reception at the test port P 2 . In this context, the phase relationships must not be lost, because otherwise no information can be provided regarding the phase of the mixer product ZF-DUT generated by the mixer DUT with reference to the phase of the input signals RF_DUT and LO_DUT. 
     The frequency of the two signal generators GN 1  and GN 3  at the test ports P 1  and P 3  is initially adjusted to the same frequency, for example, 1 GHz. Any phase differences in the generators GN 1  and GN 3  are known from the calibration with calibration standards and can be taken into consideration accordingly. If the mid-frequency of the intermediate-frequency ranges ZF 1 , ZF 2  and respectively ZF 3  of the network analyzer NA is, for example, 20 MHz, the frequency of the local oscillator LO is initially disposed, in this example, at 1.020 GHz. The phase difference between the test ports P 1  and P 3  can now be determined via the wave values a 1  and a 3  of the forward waves as follows:
 
φ a1 =φ LO −φ port1   (1)
 
φ a3 =φ LO −φ port3   (2)
 
φ a1 −φ a3 =φ port3 −φ port1   (3)
 
     The frequency of the signal generator GN 3  at the test port P 3  must now be brought, through the phase-stable frequency change according to the invention, to the target frequency of the measured signal LO_DUT, which the mixer DUT anticipates at its second input E 2 . If the intermediate frequency ZF_DUT generated by the mixer DUT is 30 MHz, for example, the frequency error between the signal RF_DUT and the signal LO_DUT must be 30 MHz, and accordingly, the frequency of the signal generator GN 3  must be increased from 1 GHz to 1.030 GHz. According to the invention, for this frequency change, only the frequency of the local oscillator LO is varied without a frequency change of the signal generator GN 3 , and then following this in an alternating manner, only the frequency of the signal generator GN 3  but not the frequency of the local oscillator LO is varied. In this context, the step width should be selected to be so small that it is not necessary to leave the bandwidth of the intermediate frequency range ZF 3 . This procedure must be repeated as often as required until the target frequency, in the example, 1.030 GHz, is finally reached. 
     By way of example, the frequency of the local oscillator LO is initially increased by 5 MHz from the original 1.020 GHz to the new value of 1.025 GHz. The intermediate frequency of the intermediate-frequency stage ZF 3  is therefore now 25 MHz instead of the original 20 MHz. The change in the phase Δφ a3,1  of the forward wave a 3  at the test port P 3  in this first stage is now measured, stored in a memory and taken into consideration in the subsequent evaluation. Alternatively, this can also be compensated directly by changing the phase of the local oscillator LO by the same phase-change value Δφ LO =Δφ a3,1 . 
     The frequency of the signal generator GN 3  at the test port P 3  is now increased, for example, by 10 MHz from the original 1.000 GHz to the new value of 1.010 GHz, so that a new intermediate frequency within the intermediate-frequency range ZF 3  from 1025 MHz−1010 MHz=15 MHz is adjusted. The phase change Δφ a3,2  of the forward wave a 3  at the test port P 3  obtained as a result is once again measured and stored. 
     The frequency of the local oscillator LO is now adjusted upwards by a further 10 MHz to 1.035 GHz. The frequency of the intermediate-frequency signal in the intermediate-frequency range ZF 3  is now once again 25 MHz. The associated phase change Δφ a3,3  is once again registered and stored. After this, the frequency of the signal generator GN 3  at the test port P 3  is again adjusted by a further 10 MHz to the new value of 1.020 GHz, so that, once again, an intermediate frequency of 15 MHz is obtained. The phase change Δφ a3,4  associated with this step is also registered and stored. It must be emphasized, that the bandwidth of the intermediate-frequency range ZF 3  and also all other intermediate-frequency ranges ZF 1  and ZF 2  is significantly broader than 5 MHz, that the resulting intermediate frequencies of 15 MHz and also of 25 MHz are disposed within the bandwidth extending around the mid-frequency of 20 MHz. 
     This procedure is repeated until the frequency of the signal generator GN 3  is disposed at 1.030 GHz, and the associated frequency of the local oscillator is disposed at 1.050 GHz. In this context, a step width of 5 MHz is selected instead of 10 MHz, so that the frequency of the intermediate-frequency signal in the intermediate-frequency range ZF 3  resulting after the last step is once again 20 MHz. 
     The decisive advantage of the procedure described above is that the phase difference Δφ between the excitation signals at the test port P 3  and the test port P 1  is now known. It is now: 
     
       
         
           
             
               
                 
                   
                     Δφ 
                     31 
                   
                   = 
                   
                     
                       φ 
                       
                         port 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         3 
                       
                     
                     - 
                     
                       φ 
                       
                         port 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                     + 
                     
                       
                         ∑ 
                         i 
                       
                       ⁢ 
                       
                         Δφ 
                         
                           
                             port 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                           , 
                           i 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     wherein φ port3 −φ port1  was the phase difference originally determined according to equation (3) between the test ports P 3  and P 1  before increasing the frequency of the signal generator GN 3 . The overall change Δφ LO  of the phase position of the local oscillator LO by comparison with the original phase position of the local oscillator φ LO  is known to be: 
     
       
         
           
             
               
                 
                   
                     Δφ 
                     LO 
                   
                   = 
                   
                     
                       ∑ 
                       i 
                     
                     ⁢ 
                     
                       
                         Δφ 
                         
                           LO 
                           , 
                           i 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     The adjustment of the local oscillator LO and the signal generator GN 3  could, of course, also be implemented in the reverse sequence, that is to say, it is possible to begin with the signal generator GN 3 , wherein the concluding stage would be increasing the frequency of the local oscillator LO. The properties of the device under test have no influence on the phase position, because only the forward waves a 1  and a 3 , but not the return waves b 1  and b 3  reflected from the device under test are used. 
     The method described above is referred to within the framework of the present application as the Secum-Trahenz method. 
     The frequency of the local oscillator LO must now be adjusted in such a manner that the intermediate frequency adjusted at the test port P 2  at the output of the mixer M 2  falls within the bandwidth of the intermediate-frequency range ZF 2 . 
     At its output port A, the mixer device under test DUT generates a signal ZF_DUT, of which the frequency corresponds to the difference between the frequencies of the signals RF_DUT and LO_DUT. In the example described above, with a frequency of the signal LO_DUT of 1.030 GHz and a frequency of the signal RF_DUT of 1.000 GHz, a frequency difference of 30 MHz is obtained, which should be analysed with regard to amplitude and phase at the test port P 2 . 
     In order to provide information about the phase of the signal ZF_DUT, the frequency of the local oscillator LO must now be adjusted from 1.050 GHz in a phase-stable manner to 50 MHz, so that the anticipated frequency of 30 MHz of the signal ZF_DUT falls in the middle of the bandwidth of the intermediate-frequency range ZF 2  of 20 MHz. In principle, this can be implemented with the Secum-Trahenz method described above. The adjustment from 1.050 GHz to 50 MHz necessitates an adjustment of 1 GHz, which, with a step width of 10 MHz would require 202 individual steps in order to adjust the local oscillator LO and the signal generator GN 2 . Without further measures, the adjustment with the Secum-Trahenz method would therefore be relatively time-consuming. 
     Accordingly, it is advantageous initially to adjust only the integer components with the dividers of the synthesiser in the local oscillator LO shown in  FIG. 2 , because this has no influence on the phase positions. Accordingly, in a first stage, only the integer component N of the fractional-rational division factor (N.F) CH  of the divider  64  of the Child-PLL in stage  65 , of the division factor (N.F) SY  of the divider  67  of the Sweep-PLL in stage  66  and of the division factor (N.F) MA  of the divider  77  of the Main-PLL stage  73  should preferably be changed. After this rough tuning, the frequency f LO  at the output of the local oscillator LO will already be disposed in the proximity of the target frequency of 50 MHz. 
     In a subsequent fine-tuning stage, the F components after the decimal point of the division factors (N.F) CH , (N.F) SY  and (N.F) MA  should then be changed stepwise in such a manner that the exact target frequency of 50 MHz is reached. This fine tuning is then implemented using the Secum-Trahenz method described above with small step widths, for example, once again of 10 MHz. 
     The fine tuning according to the Secum-Trahenz method can also be omitted as required, if a sufficiently-fine tuning raster is achieved merely by changing the integer components of the division factors ensuring that the signal ZF_DUT received at the port P 2  after mixing with the signal of the local oscillator LO in the mixer M 2  falls within the bandwidth of the intermediate-frequency range ZF 2 . In this case, a network analyzer can be used, which provides only two signal generators GN 1  and GN 3  instead of the three signal generators required for the signals at the test ports P 1  and P 3 . Accordingly, the Secum-Trahenz method cannot be used at test port P 2  because of the absence of a signal generator GN 2 . By way of example, switching the integer component N of the division factor (N.F) MA  at the Main-PLL stage  73  from 4 to 128 would change the original frequency f LO  of the local oscillator LO from 1050 MHz to 1050 MHz·4/128=32.8125 MHz. Accordingly, a mixing in the mixer M 2  with the frequency 30 MHz of the received signal ZF_DUT would lead to an intermediate frequency of 32.8125 MHz−30 MHz=2.8125 MHz, which falls within the bandwidth of the intermediate-frequency range ZF 2 , which is substantially not limited in the downward direction. 
     A third possibility for overcoming the large, sudden frequency change, in the example, from 1.050 GHz to approximately 50 MHz, is provided by a measurement of the harmonic or sub-harmonic of the fundamental frequency f LO  of the local oscillator LO. In this context, the frequency of the local oscillator LO is left at the setting, at which the phase relation was determined according to the Secum-Trahenz method; however, the measurement is made at a receiver frequency, for example, of f LO /9, with the deduction of the device intermediate frequency, in the above example, 20 MHz. 
     The invention is not restricted to the exemplary embodiment described above. In particular, the method according to the invention can also be used with network analyzers comprising more than three test ports and with less than one signal generator per test port. Furthermore, the method is, in principle, not restricted to network analyzers and can also be used with other devices, for example, with signal generators, wherein the use is not restricted to the testing of frequency-converting devices.