Recognizing that the radio spectrum is finite, the Federal Communications Commission (FCC) encourages the development and use of technology that makes efficient use of the radio spectrum. To this end, the FCC has recently allocated a region of the radio spectrum from 220-222 MHz for the purpose of spurring the development and acceptance of new narrowband technologies. The 220-222 MHz range allocated by the FCC is divided up into 400 5 kHz-wide frequency ranges paired to provide 200 channels.
The present invention relates to a spectrum efficient narrowband technology designed to operate efficiently in this 220-222 MHz range. Specifically, the present invention will be discussed in detail below in the context of amplitude modulated RF signals such as those transmitted by single sideband (SSB) two-way radios operating in the 220-222 Mhz frequency range; however, it should be clear that the principles of the present invention may be adapted to operate efficiently in other systems requiring amplification of an RF signal and in frequency ranges other than 220-222 Mhz. Accordingly, the scope of the present invention should be determined with reference to the claims appended hereto and not the following detailed discussion.
Non-linearities in radio frequency (RF) power amplifiers used to amplify SSB signals introduce distortion into these signals. More specifically, RF power amplifiers create undesired intermodulation distortion (IMD) products that result in the amplified RF output signal not being a faithful reproduction of the RF input signal. Odd-order IMD is particularly bothersome because these signals fall close in frequency on either side of the desired RF output signal and thus cannot readily be removed by RF filtering.
Furthermore, to increase efficient use of the frequency spectrum, SSB signals are often broadcast as part of a channelized system in which a first SSB signals may be broadcast in a channel adjacent to a second SSB signal. The odd-order IMD mentioned above can fall inside an adjacent channel, thus interfering with the SSB signal in the adjacent channel. The level of IMD that falls into adjacent channels is strictly regulated throughout the world to increase the reliability of channelized radio systems.
Accordingly, in order to develop an SSB system that both works properly and meets government regulations, RF power amplifiers must be as linear as possible across their operating frequency range.
Generally speaking, attempts to linearize power amplifiers may be categorized as feedback control, feedforward control, or predistortion. The most common method of amplifier linearization is feedback control. While feedback control works well at relatively lower frequencies, feedback control does not work well at higher frequencies. At higher frequencies, feedforward and predistortion are generally used. Of the foregoing linearization methods, feedback systems are preferable for the type of RF power amplifier discussed herein because they are closed loop systems that are less susceptible to changes caused by aging and environmental factors.
The present invention thus relates to feedback type control circuits for RF power amplifiers. Negative feedback control systems operate basically as follows. A differencing or error stage is provided prior to the RF amplifier. The output of the amplifier is sampled and fed back to the differencing stage. The differencing stage takes the difference between the fed back signal and the original input signal. A feedback system as just described reduces the amount of net operating gain but also reduces IMD relative to the desired signal.
Additionally, in the present application, it is desirable to place a bandpass filter prior to the amplifier to be linearized. This allows the use of a low power bandpass filter that is compact in size, which is important for use in mobile radio systems.
However, in order to operate correctly, negative feedback systems as described above require that the feedback signal be at or near 360.degree. (or integer multiples thereof) out of phase with the input signal. Providing a bandpass filter in the feedback loop before the amplifier introduces relatively large phase delay in the signal to be amplified that renders the feedback loop unstable.