Full version in "Microwave System Design Tools and EW Applications" published by Artech House, Inc.

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**Amplifier Dynamic Range****
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Introduction This Applet examines the effect of amplifier compression/limiting characteristics on the intermodulation and dynamic range performance. The display on the upper left depicts the linear compression characteristic. The scroll bar to the upper left allows the sharpness/slope of the compression to be modified. The corresponding 1dB compression point referred to the input is indicated. 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 scroll bar and compression slope. As an example, the two tone frequencies chosen are 110MHz and 130MHz. The order of the intermodulation components |m ± n| are indicated
The main display to the right shows the amplifier input/output transfer characteristic on a
logarithmic (dB) scale. The amplifier gain is normalised to unity so it is convenient to refer all
measurements to the input. Actual system gains and limiting levels can be adjusted accordingly.
The display indicates fundamental signal gain and signal levels of 3
Corresponding -1dB compression and 3
The Save Button saves relevant data to the Java Console. Data may be selected on the Console using the mouse (Shift-click) and copied to the clipboard ( Ctrl C ) and saved in a text file for a permanent record or transferring to Excel for example. If visual plots are required, Alt-Printscreen, copies the current window to MS Clipboard.
Background
Broadband RF amplifiers suffer from intermodulation distortion at high input signal levels due to linearity deviations and saturation effects. At very high levels the amplifier enters a compression or limiting region before burnout becomes a problem. High gain limiting amplifiers are designed to be over driven and the intermodulation performance is easier to calculate. Harmonics of the input signals are only important for octave and greater bandwidth amplifiers. Intermodulation components can cause problems in narrower bandwidth amplifiers.
Amplifiers are usually characterised by the
1dB compression level or the 3
2) Amplifier Transfer Characteristic It is usual to describe the transfer characteristic of an amplifier by a power series of the form, where, v is the input and Vo the output signal. In principle, any real characteristic can be approximated by suitable choice of the coefficients of such a series, but if the approximation is to be accurate into saturation then many terms are required. The first term is a DC component. The second is the linear gain. The third and all even power terms are normally zero for symmetrically saturating amplifiers, but would produce even harmonics and intermodulations of the input signals if unbalanced. The higher power odd terms describe the curvature amplifier characteristic in more and more detail.
3) 3 For two tone input signals,
Usually the dominant intermodulations are the third order terms ( )
and ( ). With sharp compression amplifiers, the higher odd order
series terms become more significant but all produce third order intermodulation components which combine or interfere
with the dominant 3 Arising from the cubic power term, the magnitude of the 3 4) Amplifier Model The amplifier compression characteristic used in this model is,
where, v and V A power series approximation is developed in the applet, based on Lagrange Interpolation, to simplify extraction of the intermodulation components using the FFT algorithm and extending it's validity over very wide differential signal dynamic ranges.
In the case where, the cubic term in the amplifier power series predominates,we can use just the first two terms of a limiting amplifier voltage transfer characterisic, where, v let then each fundamental signal output component is of the form, At the 1dB compression point, v The third order intermod component amplitude (at cos(2 omega
At the 3 The power ratio between the 3 This value is close to that observed for slope/sharpness values in the mid-range. For most practical amplifiers, it may be possible to use this basic approximation.
Taking the first two terms of an amplifier saturating characteristic, we get, where, vi , vo are the input and output voltages, and G1 the voltage gain, and V1 is the 1dB compression point. By cascading two stages, the final voltage at the second stage output is,
Extracting the first two terms again, this simplifies to, Comparing equations 1 and 2 the cascaded compression point V12 is found from, Once again, for most practical amplifiers, it may be possible to use this basic approximation. 7) Summary The applet shows the effect of amplifier saturating characteristics on the useful single and two-tone dynamic ranges. pwe |