Method of and apparatus for controlling modulation of digital signals in frequency-modulated transmissions

A method of adjusting a signal amplitude of timed digital signals to a reference voltage, comprising:

• during a period of non-transmission of such timed digital signals, sensing a bias voltage representative of an unmodulated frequency at a signal terminal of a radio transmitter of frequency modulated signals;
• upon receipt of a transmit-enable signal, storing the sensed bias voltage as a reference voltage;
• generating signal voltages of a predetermined negative and positive offset amplitude with respect to said stored reference voltage;
• and
• selectively applying the generated signal voltages in sequence corresponding to a predetermined sequence of high and low data pulses of such timed digital signals to the signal terminal.

Apparatus for generating a frequency modulated signal at a stable center frequency in accordance with digital data, said apparatus comprising:

• a) a voltage controlled oscillator having an input and an output, and being responsive to an input modulation signal for generating the frequency modulation signal at the stable center frequency at said output of said voltage controlled oscillator;
• b) a phase lock loop interconnected between said input and said output of said voltage controlled oscillator and comprising a reference oscillator for generating at its output a reference frequency signal, a phase detector connected to said reference oscillator output and to said voltage controlled oscillator output for receiving the reference frequency signal and the frequency modulated signal and for providing at its output a signal indicative of the phase difference therebetween, and a filter coupled to said phase detector for filtering the phase difference signal and applying a filtered signal to said input of said voltage controlled oscillator, whereby the center frequency is stabilized;
• and
• c) means for providing the input modulation signal as a sequence of high and low modulation level signals in accordance with the digital data and for inserting the input modulation signal into said phase lock loop, said inserting means comprises a switch inserted into said phase lock loop and operable between a closed state for completing said phase lock loop and an open state for interrupting said phase lock loop.

Apparatus for generating a frequency modulated signal at a stable center frequency in accordance with digital data, said apparatus comprising:

• a) a voltage controlled oscillator having an input and an output, and being responsive to an input modulation signal applied to its input for generating at its output the frequency modulation signal at the stable center frequency;
• b) a phase lock loop interconnected between said input and said output of said voltage controlled oscillator and comprising a reference oscillator for generating at its output a reference frequency signal, a phase detector connected to said reference oscillator output and to said voltage controlled oscillator output for receiving the reference frequency signal and the frequency modulated signal and for providing at its output a signal indicative of the phase difference therebetween, and a filter coupled to said phase detector for filtering the phase difference signal and applying a filtered signal to said input of said voltage controlled oscillator, whereby the center frequency is stabilized;
• c) means for providing the input modulation signal as a sequence of high and low modulation level signals in accordance with the digital data and for inserting the input modulation signal into said phase lock loop;
• and
• d) a track and hold circuit coupled within said phase lock loop and operable in a track mode wherein said phase lock loop is maintained closed and a control voltage applied to said first input of said voltage controlled oscillator is sampled and stored, and a hold state wherein said phase lock loop is opened and the stored sample of the control voltage is applied to said input of said voltage controlled oscillator to maintain the center frequency of the frequency modulation signal at its center frequency.

Apparatus for generating a frequency modulated signal at a stable center frequency in accordance with digital data, said apparatus comprising:

• a) a voltage controlled oscillator having a first input serving as a reference modulation port, a second input serving as a voltage controlled oscillator port and an output, and being responsive to an input modulation signal applied for generating at its output the frequency modulation signal at the stable center frequency;
• b) a phase lock loop interconnected between said input and said output of said voltage controlled oscillator and comprising a reference oscillator for generating at its output a reference frequency signal, a phase detector connected to said reference oscillator output and to said output of said voltage controlled oscillator for receiving the reference frequency signal and the frequency modulated signal and for providing at its output a signal indicative of the phase difference therebetween, and a filter coupled to said phase detector for filtering the phase difference signal and applying at its output a filtered signal to said input of said voltage controlled oscillator, whereby the center frequency is stabilized;
• c) means for providing the input modulation signal as a sequence of high and low modulation level signals in accordance with the digital data and for inserting the input modulation signal into said phase lock loop;
• and
• d) balancing means coupled to said providing means for receiving and apportioning the input modulation signal into a first portion applied to said reference modulation port and a second portion applied to said voltage controlled oscillator modulation port, said balancing means apportioning said first and second portions independently of each other based upon a data transmission parameter, whereby the frequency modulation signal may be adjusted to account for variation in the modulation sensitivity over a range of the date transmission parameter.

Apparatus for generating a frequency modulated signal at a stable center frequency in accordance with digital data, the digital data comprises at least first and second data levels, said apparatus comprising:

• a) a voltage controlled oscillator having an input forming a modulation port and an output, and being responsive to an input modulation signal for generating the frequency modulation signal at the stable center frequency at said output of said voltage controlled oscillator;
• b) a phase lock loop interconnected between said input and said output of said voltage controlled oscillator and comprising a reference oscillator having an input forming a reference modulation port and an output for generating a reference frequency signal at said output of said reference oscillator, a phase detector connected to said output of said reference oscillator and to said output of said voltage controlled oscillator for receiving the reference frequency signal and the frequency modulated signal and for providing at its output a signal indicative of the phase difference therebetween, and a filter coupled to said phase detector for filtering the phase difference signal and applying a filtered signal to said input of said voltage controlled oscillator, whereby the center frequency is stabilized;
• c) means for providing the input modulation signal as a sequence of high and low modulation level signals in accordance with the digital data and for inserting the input modulation signal into said phase lock loop;
• and
• d) balancing means comprising a first programmable reference modulation shaping circuit and a second programmable voltage controlled oscillator shaping circuit, each of said first and second shaping circuits having an input for receiving the digital data and for generating respectively a first reference modulation signal and a second voltage controlled oscillator modulation signal, each of said first and second modulation signals comprising a sequence of the high and low modulation voltages in accordance respectively with the first and second data levels of the digital data;
• and
• e) means for programming distinctly the amplitudes of the high and low modulation voltages of the first and second modulation signals, whereby the first and second modulation signals are apportioned between said reference modulation port and said voltage controlled oscillator modulation port.