The applet examines the effect of the compression/limiting characteristics on the intermodulation and dynamic range performance
of a single stage amplifier. Various example and user-input amplifier transfer characteristics are compared using selections from the drop-down
The display on the upper left shows the compression characteristic on a normalised linear scale.
The slope factor scrollbar to the upper left allows the sharpness/slope of the first selection, compression characteristic to be modified. The grey curve shows the response of an ideal hard-limiter, whilst the grey vertical line indicates the input power corresponding to the output 1dB compression point.
The display on the lower left shows the two-tone frequency spectrum and intermodulation products as a function of input drive level, controlled by the lower left signal level scroll bar and the compression slope factor scrollbar where appropriate.
In the applet, the two-tone equal signal level, frequencies chosen are 110MHz and 130MHz. Intermodulation components up to the eleventh order are calculated within the band 10MHz to 490MHz. The frequencies are arbitrary and are scalable to any required band.
The order of the intermodulation components |m ± n|are indicated at the head of the spectral lines. The levels are read by hovering the mouse near the wanted component.
The main display to the right shows the amplifier input/output transfer characteristic on a logarithmic (decibel) scale.
The drop-down menu includes some examples and a user test facility. One example is the simplest characteristic comprising only linear and cubic polynomial terms. The amplifier gain of the given examples are normalised to unity so it is convenient to refer all measurements to the input. User input data is not normalised, but the display is re-scaled and the data is, by convention, referred to the output.
The display indicates the signal levels of fundamental signal, third-, fifth-, and seventh-order components as a function of signal level and/or compression sharpness. The polynomial matching accuracy limits the upper input power for display to 0dBm.
Corresponding -1dB compression and third-order intercept points are indicated, from which single-tone and two-tone dynamic ranges can be calculated. The minimum signal Pmin is set by system factors such as noise level or detection/processing thresholds.
The applet accepts linear or logarithmic user data files of the form, dB Test File. Linear Test File. Specification conforming data can be copied from a text file (Select All followed by Control+C) and input to the data console (after clearing) using keyboard keys Control+V and loaded by mouse-clicking the load button. This data can be viewed by selecting test in the drop-down menu.
Note how sensitive the intermodulation component levels are to the amplifier compression characteristic shape. The nulling of these components with signal amplitude is due to the intermodulation signal contribution of the higher polynomial orders, some of which are not in phase with those produced by the lower orders. This effect can be observed by either, changing the limiter compression curve slope, or adjusting the signal strength.