## 4 Results  
### Measurements of the Input-Referred Intermodulation Effect in a Solid-State Amplifier  
In order to experimentally examine the possibility of IMD generation at the input of an amplifier and measure the power of adequate frequency components, the following test-bed was prepared (Figures 5 and 6). The setup consists of two signal generators, two 3 dB power dividers--which work as combiners--and a spectrum analyzer. Figure 5: Diagram of measurement test-bed. The measurements were conducted using two generators. The output power of the generators was changed simultaneously (two-tone excitation). The power spectrum of the signals returning from the input port of the LNA was observed at one of the ports of a power divider. The power losses introduced by power dividers during power dividing and summing were measured and taken into account in the final processing of the results. The test-bed was calibrated, i.e., there were two configurations (Figure 7) measured in order to find out the attenuation in the signal path, power leakage between ports of the power dividers and spectrum analyzer spurious and nonlinear responses. It was found that the signals with a power equal or less than -80 dBm were covered and influenced by spectrum analyzer-induced spurious signals. The analyzer RSA306B had its spurious response set to a level of -80 dBm, according to its datasheet. In general, the intermodulation products (frequency components) may have very low power levels (i.e., lower than -80 dBm) depending on the excitation power, which in a real dedicated receiver design may be detected with the use of a narrow bandpass filter. The parts of characteristics of IMD power versus input power above about -70 dBm is taken to make an analytical modeling. This model was used to predict IMD power below a level of -70 dBm for purposes of further calculations of the C-PCL system performance. The prediction is based on an extrapolation with the use of the existing slope (theoretically 3:1)  
Figure 6: Picture of measurement test-bed with Avantek SC82-2342M amplifier. Figure 7: Calibration measurements of the test-bed. of the device under test, which results from the fact that the IMD is a third-order product and its power is with a cubic relation to the exciting signal power. For the following analysis, the IMD product at \(2\mathrm{f_{1}}-\mathrm{f_{2}}\) was chosen. The measurements were performed with the use of the following devices under test (DUT):  
* LNA amplifier Avantek SC82-2342M;
* Amplifier Crystek CRBAMP-100-6000;
* Amplifier Mini-Circuits ERA-3;
* DVB-T receiver and tuner in the form of a USB dongle, model R820T. The set of chosen devices corresponds to typical semiconductor amplifiers used in the microwave area. Additionally, a ready-to-use miniaturized full DVB-T receiver was used as a representative of an integrated device. The amplifiers that we tested were chosen in order to have a cross-section of typical devices that are used in the military or dedicated designs, i.e.,:  
* Packaged amplifiers from the market used in military designs;
* Mini-Circuits SMD amplifier commonly used in the microwave engineering world;
* A case of commonly available integrated receiver as an example of a ready-to-use device that may be used in low-cost solutions. The measurements were conducted for frequencies of sounding signals equal to \(\mathrm{f_{1}}=749\) MHz and \(\mathrm{f_{2}}=754\) MHz. These values fit the DVB-T band and may serve as an example for illustration purposes and were chosen as the trade-off between the available bandwidth of the power dividers and amplifiers used for the experiment. The selected results of measurements are shown in Figure 8 and Tables 1-4. These values of power are related to the DUT's input, i.e., the losses of power splitters are taken into account.