Baseband signal processing circuit for a radio communication apparatus

Abstract: A baseband signal processing circuit for a radio communication apparatus has an amplifier for amplifying the amplitude of an input baseband signal, a limiter for limiting the amplitude of the amplified baseband signal, an amplitude detector for detecting the amplitude of the output signal of the amplifier, and a control unit for controlling the gain of the amplifier in response to the output of the amplitude detector. When the amplitude detector detects an amplitude greater than predetermined one, the control unit controls the gain of the amplifier in such a manner as to maintain the output of the amplifier at a constant level. As a result, the output level of the amplifier is prevented from exceeding the limiter level of the limiter. This allows the maximum frequency transition to be set up with no regard to frequency and suppresses distortions regarding signals exceeding the standard frequency transition.

Description: The present invention relates to a radio communication apparatus and, more particularly, to an improved baseband signal processing circuit including an amplifier and a limiter. A conventional baseband signal processing circuit known to the applicants, who are not aware of a publication describing the circuit, has an amplifier for level adjustment, a limiter, a splatter filter, and a modulator. The amplifier amplifies an input baseband signal and feeds the resulting signal to the limiter. The limiter limits the amplitude of the amplified baseband signal to prevent the frequency band from broadening in the event of transmission or, in the case of FM, limits the deviation. On receiving the output of the limiter, the splatter filter removes the frequency band for broadband from the limiter output. The output of the splatter filter is applied to the modulator. In the conventional circuit described above, when the input signal to the limiter has an extremely great amplitude (infinite), the output of the limiter has a rectangular waveform whose amplitude has been limited at a predetermined level. The splatter filter removes, among the frequency components of the rectangular wave, high frequency components. However, when only the fundamental frequency component of the frequency of the rectangular wave is passed through the splatter filter, the output of the filter has a greater amplitude than the rectangular wave. Specifically, the amplitude is 4/π times greater than the amplitude of the rectangular wave. More specifically, at the stage following the splatter filter, the amplitude is 4/π times greater in the frequency range in which only the fundamental frequency component passes than in the frequency range in which all the frequency components pass, despite that the limiter limits the amplitude. Regarding an FM (Frequency Modulation) transmitter, the limiter limits the maximum frequency transition. However, the maximum frequency transition has to be further reduced in the frequency range in which the output amplitude of the splutter filter is 1, since it should be limited with 4/π used as a reference. This reduces the difference between the standard and maximum frequency transitions and, therefore, apt to cause distortion to occur at signal levels exceeding the standard frequency transition. In the specification of United States patent number 3,991,370 which was issued on November 9 1976, there was proposed a system for limiting the amplitude of audio input signals including a first feed back loop for reducing the gain of an amplifier to which the signals are applied and a second feed back loop for providing a frequency dependent signal for returning the amplifier to its original gain. In the specification of European patent application number 90304264.6, which was published under number 394063 on October 10 1990, the transmitting portion of a radio communication apparatus is described. This apparatus selectively transmits input baseband signals in response to a selection signal, and a level controller having a controllable gain controls the selected baseband signal so that the selected signal has a level which is determined by the controllable gain. A gain controller determines the gain according to the input baseband signal. In a quarterly journal entitled UKW Berichte, J21956 F, dated March 1974 there is, at pages 155-167, an article by Joachim Kestler entitled "SSB-Exciter Mit HF-Clipper", in which a circuit diagram of a pre-amplifier for a crystal microphone is shown on page 162. This circuit includes amplifiers, an amplitude limiter and an amplitude detector following one of the amplifiers so that the output of the amplifier is at a constant level. A feature of an arrangement to be described below is the provision of a baseband signal processing circuit for a radio communication apparatus which allows the maximum frequency transition to be set up with no regard to frequency and reduces distortion even when the signal exceeds the standard frequency distortion. In the UKW Berichte article referred to above, it is recognized that voltage spikes should not be allowed to over-drive the filter or modulator. Accordingly this reference proposes that the limiter threshold only be reached during overshoots or transients from the controlled amplifier at the commencement of speech. The arrangement to be described below proposes, in contrast, that an amplifier be controlled to a lower level than the threshold of a following limiter, such that the amplitude level at the output of a filter does not exceed the amplitude level at the input of the filter. The feature employs the phenomenon, illustrated by the Fourier analysis of a square topped wave, which is provided by an amplitude limiter, that the amplitude of the fundamental is higher than that of the original wave. In the arrangement to be described as an example, a baseband processing circuit for a radio communication apparatus includes an amplifier for amplifying the amplitude of an input baseband signal to thereby produce an amplified baseband signal, a limiter for limiting the amplitude of the amplified baseband signal, an amplitude detector for detecting the amplitude of the amplified baseband signal to thereby output a detection signal, and a control unit responsive to the detection signal for controlling, when the amplitude detector detects an amplitude greater than a predetermined amplitude, the gain of the amplifier such that the output of the amplifier remains at a constant level. The arrangement to be described also illustrates a baseband signal processing method for a radio communication apparatus including the steps of, amplifying the amplitude of an input baseband signal to produce an amplified baseband signal, limiting the amplitude of the amplified baseband signal, detecting the amplitude of the amplified baseband signal to produce a detection signal, and controlling, when an amplitude greater than a predetermined amplitude is detected, the gain of the amplified baseband signal such that the amplified baseband signal remains at a predetermined level. The following description and drawings disclose a previously proposed arrangement, and by means of an example the invention which is characterised in the appended claims, whose terms determine the extent of the protection conferred hereby. In the drawings:- FIG. 1 is a block diagram schematically showing a conventional baseband signal processing circuit for a radio communication apparatus; FIGS. 2A-2C show signal waveforms representative of the operation of the conventional processing circuit; FIG. 3 is a block diagram schematically showing a baseband signal processing circuit embodying the present invention; and FIG. 4 is a graph useful for understanding the operation of the embodiment. To better understand the present invention, a brief reference will be made to a prior art baseband signal processing circuit for transmission, shown in FIG. 1. As shown, the processing circuit has an amplifier 1 for level adjustment, a limiter 2, a splatter filter 3, and a modulator 4. The amplifier 1 amplifies an input baseband signal and feeds the resulting signal to the limiter 2. The limiter 2 limits the amplitude of the amplified baseband signal to prevent the frequency band from broadening in the event of transmission or, in the case of FM, limits the deviation. On receiving the output of the limiter 2, the splatter filter 3 removes the frequency band for broadband from the limiter output. The output of the splatter filter 3 is applied to the modulator 4. In the above construction, when the input signal to the limiter 2 has an extremely great amplitude (infinite), the output of the limiter 2 has a rectangular waveform. Specifically, when the limiter 2 limits an input signal having an extremely great amplitude as shown in FIG. 2A, it produces an output having a rectangular waveform whose amplitude has been limited at a predetermined level, as shown in FIG. 2B. The splatter filter 3 removes, among the frequency components of the rectangular wave, high frequency components. However, when only the fundamental frequency component of the frequency f of the rectangular wave is passed through the splatter filter 3, the output of the filter 3 has a greater amplitude than the rectangular wave. Specifically, when the rectangular wave shown in FIG. 2B is subjected to Fourier transform, the result is expressed as: f(x) = 4 π [sin x + sin 3x 3 + sin 5x 5 + ..... ] Hence, after the rectangular wave is passed through a filter which passes only the fundamental frequency component, the following equation holds: f(x) = 4 π · sin x As a result, the amplitude is 4/π times greater than the amplitude of the rectangular wave. Such a waveform is shown in FIG. 2C. The above-described conventional processing circuit is disadvantageous in that the amplitude of the output of the limiter 2 changes with the signal frequency, and in that distortion is apt to occur at signal levels exceeding the standard frequency transition. Referring to FIG. 3, a baseband signal processing circuit embodying the present invention and free from the above-discussed problems will be described. In the figures, the same or similar components are designated by the same reference numeals, and redundant description will be avoided for simplicity. As shown, the processing circuit has an amplifier 1 for amplifying a baseband signal to be transmitted. A limiter 3 limits the amplitude of the output of the amplifier 1. The output of the limiter 3 is routed through a splatter filter 3 to a modulator 4. This part of the construction is identical with the conventional arrangement. The processing circuit further includes an amplitude detector 5 for detecting the amplitude of the output of the amplifier 1, and a control unit 6 for controlling the gain of the amplifier 1 in response to the output of the amplitude detector 5. In operation, when the output level of the amplifier 1, i. e., the level of the input signal to the limiter 2 is lower than the limiter level, the control unit 6 does not effect any control over the amplifier 1. On the other hand, when the input signal to the limiter 2 has a higher level than the limiter level, the control unit 6 controls the amplifier 1 such that the output level of the amplifier 1, i. e., the limiter input level remains constant. More specifically, the control unit 6 controls the gain of the amplifier 1 in response to the output of the amplitude detector 5 such that the input and output of the amplifier 1 have a relation shown in FIG. 4. As a result, the output of the amplifier 1 remains constant even when a signal having a great amplitude is inputted to the amplifier 1, substantially preventing signals having higher levels than the limiter level from reaching the limiter 2. This prevents the limiter 2 from producing a rectangular wave and, therefore, prevents the amplitude level of the output of the splatter filter 3 from exceeding that of the input to the filter 3. It follows that controlling the maximum frequency transition of an FM transmitter to below the input level of a splatter filter as discussed earlier is not necessary. This is successful in eliminating distortions at levels above the standard frequency transition. It is to be noted that the limiter 2 is also essential when a level exceeding the range wherein the output level of the amplifier 1 can be maintained constant, so that the amplitude level may be limited. While the embodiment lowers the output level of the amplifier 1 to the limiter level, the limiter level may be replaced with a level slightly higher than the limiter level so long as the high frequency components of the limiter output are not great. In summary, it will be seen that the present invention provides a baseband signal processing circuit which suppresses the level of higher harmonics included in the output of a limiter and thereby prevents the amplitude from increasing at a stage subsequent to a splatter filter. This unprecedented advantage is derived from the fact that the processing circuit detects the amplitude of the output of an amplifier and controls the amplifier such that the output level of the amplifier does not exceed the limiter level. The present invention, therefore, allows the maximum frequency transition to be set up with no regard to frequency and insures a sufficient distortion factor for signals exceeding the standard frequency transition. It will be understood that, although a particular arrangement has been described to illustrate the invention, variations and modifications thereof, as well as other embodiments, may be made within the scope of the appended claims.

Claims: A baseband processing circuit for a radio communication apparatus, including an amplifier (1) for amplifying an input baseband signal thereby to produce an amplified baseband signal, a limiter (2) tending to limit the amplitude of the amplified baseband signal to an effective maximum level determined by the limiter (2), a filter (3) for removing high frequency components of the limited baseband signal, an amplitude detector (5) for detecting the amplitude of the amplified signal thereby to provide a detection signal, and a control unit (6) responsive to the detection signal for controlling the gain of the amplifier (1) characterized in that the control unit (6) controls the gain of the amplifier (1) and that the output of the amplifier is at a level which is lower than the effective level that would be determined by the limiter (2) in the absence of the control unit (6), the output amplitude of the filtered signal from the filter (3) not exceeding the amplitude of the limited signal that is input to the filter (3). A baseband processing circuit as claimed in claim 1 characterized in that the gain of the amplifier (1) is controlled such that its output level does not exceed a level π/4 times the effective maximum level that would be determined by the limiter (2) in the absence of the control unit (6). A method of processing a baseband signal for a radio communication apparatus, including the steps of: (a) amplifying an input baseband signal to produce an amplified baseband signal; (b) limiting the amplitude of the amplified baseband signal to an effective maximum level determined by a limiter (2); (c) filtering high frequency components from the limited baseband signal; (d) detecting the amplitude of the amplified signal thereby to provide a detection signal, and (e) controlling the amplification of the baseband signal in response to the detection signal, characterized by the further steps of: (f) so controlling the amplification that the amplified baseband signal does not exceed a level which is lower than the effective level that would be determined by the limiter (2) in the absence of the controlling step (e), such that the filtered signal does not exceed in amplitude the actual limited signal that is subjected to filtering. A method of processing a baseband signal as claimed in claim 3 characterized in that step (e) is performed such that the amplitude of the amplified baseband signal does not exceed a level only π/4 times the effective maximum level that would be determined by the limiter (2) in the absence of the controlling step (e).

Inventors: IWAHASHI KOJI, IWAHASHI, KOJI, Iwahashi, Koji, c/o NEC Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1991
Main classification: H04B1
IPC codes: H04B1, H03G3, H03G11