One type of AM-RF transmitter includes a pulse width modulator responsive to an information signal source for driving a power pulse amplifier connected with a low pass filter in a power supply circuit for an RF amplifier. Examples of specific transmitters of this type are found in Swanson U.S. Pat. Nos. 3,506,920 and 3,588,744, as well as Bruene et al, U.S. Pat. No. 3,413,570.
In the Swanson patents and in several embodiments of the Bruene et al patent the pulse width modulator operates at a fixed frequency. The width of the pulses is varied in response to the instantaneous amplitude of the information signal supplied to the modulator. In response to the instantaneous amplitude of the information signal having a zero value, the two levels of the pulse width modulated signal during a particular period of the signal are equal in duration. During peak positive portions of the information signal the pulse width modulated signal typically has a high level for a much greater time than a low level. In contrast, during peak negative portions of the information signal, the length of the high level portion of the pulse width modulated signal is considerably less than the length of the low level portion of the signal. As the frequency of the information signal increases and decreases the rate of change between sequential pulses of transitions between the high and low levels respectively increases and decreases.
The fixed frequency of the pulse width modulated signal is selected to satisfy frequency performance requirements of the transmitter in replicating the information signal source. Typically, the information signal source is an audio source, such as voice or music. In general, the highest audio frequency which must be reproduced at the output of the low pass filter determines how high the fixed frequency of the pulse width modulated source is to be. For example, a transmitter designed to provide a maximum response for an audio signal having a maximum frequency of 10 KHz requires a much higher operating frequency for the pulse width modulator than a transmitter designed to provide a maximum response capable of handling a 5 KHz signal.
Another important consideration in determining the fixed pulse width modulation frequency is the amount of tolerable distortion. In general, the lowest distortion for the highest frequency information signals is achieved by having a high ratio of fixed pulse width modulation frequency to the highest frequency of the information source.
An additional consideration in selecting the fixed frequency of the pulse width modulator is modulator conversion efficiency. In a pulse type transmitter including a high power pulse amplifier, as included in transmitters of the type with which the present invention is utilized, switching losses increase as pulse frequency increases. In other words, as the ratio of pulse duration time to switching time becomes greater, switching losses decrease.
The stated considerations regarding distortion and efficiency are contrary to each other. In consequence, the designer of this type of RF-AM transmitter must make a compromise in selecting the fixed frequency of the pulse width modulator. Usually the compromise involves selecting the lowest frequency which will satisfy distortion limits for the highest information frequency the transmitter is designed to handle, i.e., the highest information frequency for which the transmitter specification requires a distortion measurement. If no distortion measurement is required above 5000 Hz, i.e., the modulation handled by the transmitter has a maximum frequency of 5000 Hz, a pulse frequency in the 50-60 KHz range is usually selected. If distortion measurements must be made at 7500 Hz, a pulse frequency of 70 KHz or higher is required, resulting in a lower efficiency than occurs with a transmitter having a 50 KHz fixed pulse width modulation frequency.
In FIG. 6 of the Bruene et al patent is disclosed a transmitter of the type including a variable frequency pulse width modulator which drives a power pulse amplifier for supplying a DC voltage to an anode-cathode path of an RF amplifier by way of a low pass, integrating filter. The variable frequency pulse width modulator of the Bruene et al patent, however, does not satisfy the distortion and efficiency problems of the prior art fixed frequency pulse width modulated transmitters. In the Bruene et al transmitter, the instantaneous amplitude of an audio signal source controls the frequency of an assymetrical multivibrator. The multivibrator derives a bilevel signal having a constant duration high level and a variable duration low level. The variable duration low level has the shortest duration when the modulating information signal source has a positive peak value; the variable duration low level is longest in response to the information having a negative peak value.
The switching times of the assymetrical multivibrator, hence the transitions from the high to the low level, are determined by the instantaneous value of the information source at the time a transition is to take place. In essence, the assymetric multivibrator samples the amplitude of the information signal source at the time of a transition, and the sampled amplitude determines the length of the variable, low level duration. There is a considerable likelihood that variations in the information signal source which occur during the fixed duration high level output of the assymetric multivibrator are not reflected and are not replicated in the output of the low pass filter driving the RF amplifier. Hence, distortion with the variable frequency device disclosed in FIG. 6 of Bruene et al is very likely. The problem of distortion as occurs in the fixed frequency pulse width modulator is not considered or inherently solved by the FIG. 6 configuration of Bruene et al.
It is, accordingly, an object of the present invention to provide a new and improved AM-RF transmitter including a pulse width modulator which causes the transmitter to operate with higher efficiency and lower distortion.
Another object of the present invention is to provide a new and improved RF-AM transmitter of the type including a pulse width modulator, wherein the frequency of the pulse width modulated signal is varied in such a manner as to enable high frequency variations of an information signal to be accurately reproduced, and to enable the transmitter to operate in a high efficiency mode.
An additional object of the present invention is to provide a new and improved AM-RF transmitter of the type including a pulse width modulator wherein a modulating signal applied to a power supply terminal of an RF amplifier has reduced distortion, particularly at higher frequencies of an information signal.