Nonlinear components cause mixture of two signals which produce numerous mixing products below and above their frequency with different frequencies and amplitudes (intermodulation products).

The intermodulation products of third order (IMP3) are unfortunately in the very near vicinity of the two signals - only in a distance corresponding to their frequency difference (F1_IP3 = F1 (F2-F1) and F2_IP3 = F2 + (F2-F1) for F2> F1).

Due to this close proximity, the intermodulation products IMP3 for signals with narrow frequency spacing can not be removed by filters and add up to the desired signal as noise.

Since the amplitudes of IMP3 with increasing output power does not increase linearly with the desired signals, but in the 3. Power, they would reach at a specific power output the amplitude of the desired signals. This fictional output power is called IP3 (OIP3 as - O: Output) .

The graph on a logarithmic coordinate system between input and output power (power data in dBm on a linear scale corresponding to a logarithmic representation), the IP3 is represented by the intersection of the straight lines of the main signal and intermodulation product IMP3. The slope of the IMP3 with 3. Power relative to the output  signal in a logarithmic coordinate system corresponds with rise of the lines in a 1:3 ratio (see chart).

The imaginary intersection corresponds to the IP3 (interception point 3rd order). In practice, however, an amplifier droves above a certain power output into saturation, so that not only the gain curve, but also the curve of the intermodulation products increasingly flattened. Consequently, the two curves intersect never - according to this, the IP3 is also never reached and is therefore not measurable. It must be calculated from measured values on two lines. However, because the slope of the lines is known as a ratio of 1:3, the IP3 can be calculated with a single value pair.

Under real conditions many signals with varying size and frequency are mixing. To ever make compareable statements about the intermodulation behavior of electronic modules, a standardized test method with only two signals is required. The two-tone generator provides this and produce signals with equal amplitude and a defined frequency spacing. From the measured output power of the amplifier and the IMP3, the IP3 can be calculated (IP3: imaginary output power, when the 3rd order intermodulation products are of equal size). The IP3 is therefore a fictitious value to characterize the intermodulation behavior under test conditions - no more, no less.

MDS: minimal detektierbares Signal                                       IIP3: Eingangs-IP3

DRim: intermodulationsfreier Dynamikbereich                        OIP3: Ausgangs-IP3

DRlin: linearer Dynamikbereich

Zur Vereinfachung der Berechnung des IP3 wurde ein kleiner IP3-Rechner in Excel erstellt. Lediglich die gemessene Ausgangsleistung und die Größe des Intermodulationsproduktes 3.Ordnung sind einzugeben, wobei negative dBm-Werte mit einem Minuszeichen zu versehen sind. Wenn ein Dämpfungsglied zwischen Verstärkerausgang und Eingang des Spektrumanalysers eingefügt wurde, wird dessen Dämpfung ebenfalls als negativer Wert eingegeben. Zur Bestimmung des IIP3 ist der Vestärkungsfaktor des Messobjekts erforderlich.

IP3-Rechner.xls

Platinenlayout Oszillator

Tips for winding the RF transformer
A 1 m long wire of 0.20 mm CuL was folded in half and twisted slowly with a accu-drill under uniform tension. For this the closed end was hung in a hook bolt and the two free ends are clamped firmly. The twist was finished at about 3 turns/ cm, both ends with uneven twist were cut off. One end soldered to a BNC connector at the other end to a small 100 ohm potentiometer. With the NWA (FA-NWT or VNWA) the impedance of the two-wire line was determined and adjusted by further twisting or untwisting to 50 ohms. It was found that the finished wound transformer at the input and output had a characteristic impedance of 52 ohms. Only with a little bit more twisted wires of 48 ohm impedance the transformer matched exactly to 50 ohm. Presumably, during the winding of the transformer it comes to a low untwisting . The isolation of the combiner inputs increased thereby from -43 dB to over -60 dB.