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IM SET IMD

DOUBLE DDS

In this section we will focus on measuring OIP3 of designed measuring kit. Own OIP3 we need to know so that we can assess the suitability of the kit for measuring IMD of external passive circuits (BPF, LPF, HPF for example) with higher IP3.

Output of the DDS+AMP+LPF set was connected thru my power reflection bridge (see USEFUL TOOLS) and via attenuator to the Signal Hound Spectrum Analyzer USB-SA44B, which was connected to the PC via USB  (SET - BRIDGE - ATT - SA - PC).

Usually I am using M0WWA ladder attenuator 0-71dB, max. 1W input. We are palying with more than 1W power, so I used another 20dB/5W BIRD fixed attenuator in front of M0WWA one.

At the input of the SA I allways use attenuation, if input signal level is 0 dBm or higher  as a protection against parasitic IM. Head of every picture contains needed informations:

                                                                                       SA INPUT ATT =   Spectrum Analyzer Input attenuator value
                                                                                       DDS+AMP+LPF = Complete IM set
                                                                                       PRB = my Power Reflection Bridge with 43006301 ferrite cores

OIP3 was measured at different levels of the output signal (from 0 to +27.5dBm). The following pictures shown rersults at different outputs signal level.


Signal level at the output of the combiner was 0 dBm (SA input attenuator = 40dB, marker = -40dBm). Noise level of the Spectrum analyzer is approx. at -145dBm. NO INTERMODULATION PRODUCTS ARE VISIBLE!  We CAN NOT calculate OIP3 until intermodulation elements are not visible.  Any calculation would not be accurate in such a case.

Notice: Input attenuator does not affect the noise floor of the Spectrum Analyzer. It was during whole measurement approximatelly at the same  level. It  depends only on SA settings. I tried to go as low as possible with SA noise floor even if the sweep time was longer.


Signal level at the output of the combiner is +5 dBm (SA attenuator = 40dB, marker = -35 dBm). Noise level of the Spectrum analyzer is approx. at -145dBm. NO INTERMODULATION PRODUCTS ARE VISIBLE!  

Similar results I got for +10 dBm, +15 dBm and +20 dBm of output signal level, too. Next picture shows +25 dBm result.


Signal level at the output of the combiner is +25dBm (SA attenuator = 70dB, marker = -45 dBm). Noise level of the Spectrum analyzer is approx. at -145dBm. NO INTERMODULATION PRODUCTS ARE VISIBLE!  


Signal level at the output of the combiner is +25.5dBm (SA attenuator = 70dB, marker = -44.5 dBm). Noise level of the Spectrum analyzer is approx. at -155dBm. IM elements are visible ( at -145 dBm), so we can calculate OIP3:

                                                             IMD3 = 145 - 44.5 = 100.5 dB
                                                             IMD3/2 = 100.5/2 = 50.25 dB
                                                             Po = +25.5 dBm
                                                             OIP3 = IMD3/2 + Po = 50.25 + 25.5 = +75.75 dBm


Signal level at the output of the combiner is +26dBm (SA attenuator = 70dB, marker = -44dBm). Noise level of the Spectrum analyzer is approx. at -155dBm. IM elements are visible ( at -140 dBm), so we can calculate OIP3:

                                                             IMD3 = 140 - 44 = 96 dB
                                                             IMD3/2 = 96/2 = 48 dB
                                                             Po = +26 dBm
                                                             OIP3 = IMD3/2 + Po = 48 + 26 = +74 dBm



Signal level at the output of the combiner is +27dBm (SA attenuator = 70dB, marker = -43dBm). Noise level of the Spectrum analyzer is approx. at -155dBm. IM elements are visible (at -130 dBm), so we can calculate OIP3:

                                                             IMD3 = 130 - 43 = 87 dB
                                                             IMD3/2 = 87/2 = 43.5 dB
                                                             Po = +27 dBm
                                                             OIP3 = IMD3/2 + Po = 43.5 + 27 = +70.5 dBm


Signal level at the output of the combiner is +27.5dBm (SA attenuator = 70dB, marker = -42.5dBm). Noise level of the Spectrum analyzer is approx. at -155dBm. IM elements are visible (at -125 dBm), so we can calculate OIP3:

                                                             IMD3 = 125 - 42.5 = 82.5 dB
                                                             IMD3/2 = 82.5/2 = 41.25 dB
                                                             Po = +27.5 dBm
                                                             OIP3 = IMD3/2 + Po = 41.25 + 27.5 = +68.75 dBm


Analysis of results

1.  For 0 dBm of output signal level we have two results. One, when we measured DDS own IM (see the relevant chapter). Second here, where we measured own IM of whole set (DDS+AMP+LPF+PRB). Results are different. Why? 

In the first case DDS was connected direct via PRB to the SA input attenuator. Have a look on level chain:
    
                                DDS         -           PRB            -           ATT          -           SA                                Result
     carrier              +6 dBm                -6 dB                   -30 dB                 -30 dBm                          0 dBm of carrier
        IM                 -99 dBm               +6 dB                  +30 dB              -135 dBm                         -105 dBm of IM

We count in the first line from left to right, in the second line vice versa. DDS alone has +6 dBm max. output level of the carrier and -99 dBm of IM product level.

Now how it looks during whole set testing (DDS level at minimum, ATT1 = 22 dB). We count similar as in the previous case:

                    DDS     -       ATT1     -      AMP        -       LPF       -       PRB       -       ATT2          -       SA                         Result
carrier        -6 dBm        - 22 dB        +35 dB             -1 dB              -6 dB              -40 dB            -40 dBm             0 dBm of carrier
  IM       <-111 dBm       +22 dB        -35 dB              +1 dB             +6 dB             +40 dB       < -145 dBm           <-105 dBm of IM

In this case IM product is not visible, it means that its level is -145 dBm or less.  DDS alone has -6 dBm min. outupt level of the carrier and less then -111 dBm of IM product level. Notice: We apply a presumption that the amplifier in the range from 0 to +20 dBm works in a linear mode! 
So why IM results for 0 dBm level were different (111 - 99 = 12dB)?  Because DDS loading was different in both cases....(RB in the first case, amplifier in the second case. It was mainly the matter of  impedance matching, plus different conditions for the formation of IM..). 

2.  How it looks for output level +20 dBm?  We count similar way as in the previous case (DDS level at minimum, AAT1 = 2 dB:

                    DDS     -     ATT1     -      AMP      -      LPF     -      PRB      -       ATT2      -        SA                               Result
carrier         -6 dBm       -2 dB         +35 dB         -1 dB            -6 dB           -70 dB             -50 dBm               + 20 dBm of carrier
  IM         <-101 dBm       +2 dB        -35 dB          +1 dB           +6 db           +70 db          <-145 dBm             <-75 dBm of IM

Value -101 dBm can be not valid. SA IM level must be <-155 dBm to have 20 dB difference against 0 dBm of output level (AMP in linear mode). IM product is still not visible.

  IM        <-111 dBm       +2 dB        -35 dB         +1 dB             +6 dB        +70 dB           < -155 dB                < -85 dBm of IM

3.  We have just couple of OIP3 values from the measuring results:

                                 OIP3 of alone DDS as a signal source for IM set                   =     + 52.05 dBm
                                 OIP3 of the set for output level +25.5 dBm                           =     + 75.75 dBm
                                 OIP3 of the set for output level +26 dBm                              =     + 74 dBm
                                 OIP3 of the set for output level +27 dBm                              =     + 70.5 dBm
                                 OIP3 of the set for output level +27.5 dBm                           =     + 68.75 dBm

We will approximate the measured results to a chart similar way as in the first IM set.  In the math formula

                                                                             y = kx + q  

k = 1  and  q = 52.05.  See the following chart for DDS+AMP+LPF IM approximation together with curves taken from the first set measuring.


Conclusion.  Even taking into account possible measurement errors, the Double DDS IM measuring set reached excellent IM parameters. The set is suitable for IM measuring till the value +65 dBm of OIP3 or little more. These results have been achieved thanks to the application of a number of important criteria:

  • Independent signal sources with sinusoidal outputs
  • As high as possible reverse isolation of both RF signal sources
  • Maximum possible linearity of RF amplifiers
  • Maximum suppresion of the second harmonics of both signals at the combiner input
  • As high as possible directivity of the combiner
  • Best impedance matching of all components of the kit  
 
 
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