Single Tone Measurements of Nonlinear Distortions in Wideband RF Amplifiers
It is possible to use single tone method to measure the non-linear distortions if the device under test (DUT) is wideband. The idea is to measure 2nd and 3d harmonic levels generated by DUT. The advantages of the single tone measurements of IP2 and IP3 are that only single signal source is needed. The main problem is that the signal source harmonics must have lower level than expected harmonic levels generated by DUT. To reduce the specific input harmonic, a reject filter can be used. The measurement setup is shown on Fig.1. This setup is used to measure the IP parameters of AAA-1 devices ( www.active-antenna.eu ) in production stage. The signal source is and old but very good signal generator (model PS6 from Wandel&Goltermanm) whose 2nd and 3d output harmonics are below -70 dBc. Perseus ( www.microtelecom.it) DDC receiver is used as a spectrum analyzer to measure the level of the generated by DUT harmonics. This RX is very well calibrated and is an excellent high resolution spectrum analyzer.
The simple serial resonance input filter rejects between 42 to 46 dB the unwanted harmonic. The output fundamental frequency reject filters are needed to avoid the overload of the spectrum analyzer and have similar rejection level. High Q ferrite cores are used with relatively large cross section to avoid any possibility for non-linear distortions. For the same reason ceramic capacitors are not used (only mica or polyester technology). The filters must be mounted in screened boxes to avoid signal leakage (Fig.2, 3) . The total attenuation of the source 2nd or 3d harmonics is above 110 dB. With this simple setup it is possible to measure OIP2 values up to 105 dBm and OIP3 values up to 45 dBm. To measure reliably higher IP values, two stage input reject filter, high quality connectors and cables and careful screening is needed.
Fig. 1 Single tone measurement setup. Load as a high resolution pdf file 1tone_IMD_Measure_v10.pdf
Fig.2 Filters to reject fundamental frequency. Fig. 3 Harmonic reject filter