HF Radio Systems & Circuits
Written for the engineer working in radio communications, regardless of frequency, this comprehensive reference book explains the design of high frequency communications systems and equipment.
TABLE OF CONTENTS HF Radio Systems & Circuits
Chapter 13: Ultralow-Distortion Power Amplifiers
Edward G. Silagi
In applications where a high degree of linearity and low noise are required from the transmitter, external error correction may be added to improve performance. This chapter describes feedforward techniques for low transmitter noise and distortion.
13.1 Introduction
Linear PA output
Linear power amplifier (LPA) performance may be characterized by the contents of the amplifier output spectrum. The output of the LPA generally includes the desired signal plus a number of undesired signals. The undesired signals take the form of noise and distortion. Noise transmitted by the LPA includes amplified exciter noise and internally generated noise. It may be described by the equivalent amount of power existing in a 1-Hz bandwidth (dBW/Hz).
The distortion existing at the LPA output is usually described in terms of harmonic and IMD content and also spurious responses. Harmonic frequencies exist at integer multiples of the desired transmit frequency. Harmonic amplitudes may be measured by injecting a CW signal at the input of the LPA and observing the output at the harmonic frequencies. Intermodulation distortion is created by the mixing of two or more different frequency tones in the LPA. The IMD products exist at sum and difference frequencies of the mixing tones and their harmonics. They may be measured by injecting two equal amplitude tones of different frequency at the input of the LPA while observing the output with a spectrum analyzer. Spurious responses, or oscillations, occur at random frequencies and can exist while the LPA is being driven or undriven.
Copyright Noble Publishing Corporation 1998 under license agreement with Books24x7
