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Measurements and Results

The two-tone generator was measured in combination with the spectrum analysis modul of the FA-NWT (network tester of the Funkamateur-shop/Berlin) and a spectrum analyzer HP8566B.

The two-tone IMD source was connected with a digital step attenuator and some extra attenuators to each SA-meter. Then the input level was reduced to the extent that the IMP3 could be measured consistently and reliably above the noise floor.


Measuring arrangement for IMD3

 Messanordnung für IP3-Bestimmung


FA-NWT with SA-Modul

Intermodulation strength of the SA-modul input is significantly lower than of the HP spectrum analyzer. With input levels of less than -20 dBm and a noise floor of about -77 dBm, the highest IMD3 were measured.

At 5 kHz signal spacing the intermodulation-free interval was determined with 50.0 dB and at 20 kHz with 49.7 dB.

HP8556B

 The intermodulation-free distance of the measuring system with the HP8566B was higher by more than 30dB.

                

To simplify calculation of IP3 a small calculator was created in Excel. Only the measured output power and the size of the 3rd order intermodulation product must be entered with negative dBm-values preceded by a minus sign. When an attenuator has been introduced between the amplifier output and the input of the spectrum analyzer the value of attenuation is also entered as a negative value. The IP3-calculator can be downloaded in the download area.

       

Filter at the generator output?
To reduce harmonics and higher-frequency mixing products (IMP2 as the strongest spurious is 40dB below the level of the two-tone signal) a 7-pole Chebyshev low-pass filter was inserted
at the output. Although this reduced the spurious as expected, it worsened the IMD3 by 3 to 5 dB, the measured IP3 values greater than 10 dBm. This was due to an insufficient matching of the filter. After revision of the filter it should be investigated to what extent this affects the intermodulation-free range of the measurement system and the IP3 measurements on amplifiers and mixers. Corresponding results are then added here.

            

 

       




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