Nov 30, 2015

AM-AM/AM-PM Diagram

Linearity of amplification is one of the most important characteristics of a broadcasting system. Amplitude (AM) and phase (PM) deviations cause analog signal degradation and lead to increased BER (Bit Error Rate).

In an ideal system, the power level of the input signal will not effect the phase and will be in linear correlation with the amplitude of the output signal. In real life, internal and external processes, such as power supply ripples, thermal drifts, multipath fading etc., would interfere with the signal and, thus, deviate its characteristics. Furthermore, the non-linearity products of an "on-air" transmitters are often filtered out by the transmitter system band-pass filters and cannot be easily detected by analyzing the signal spectrum or SNR. In such situations AVQ1020 becomes a really handy tool displaying the system linearity in its AM-AM/AM-PM diagram. The plot shows amplitude and phase deviations in relation to the amplitude of the input signal.

Transmitter manufacturers employ various techniques to preserve signal physical integrity and to increase transmitter efficiency. Changes in amplitude and phase deviations over time may indicate components' aging and preventive actions can be called to avoid transmission outages.

Samples of AM-AM/AM-PM diagram

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AM-AM/AM-PM Diagram of a system
with good linearity
(red lines are for reference)
AM-AM/AM-PM Diagram of a system
with non-linear interference
(red lines are for reference)

 


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