Method and apparatus for minimizing the effects of frequency selective fading on a PCM/FM data signal

A demodulator for minimizing the effects of frequency selective fading in multipath transmission of pulse code modulated/frequency modulated telemetry data, said demodulator comprising:

• a variable frequency oscillator for generating a local oscillator signal;
• a first mixer connected to said variable frequency oscillator to receive said local oscillator signal, said first mixer receiving a first radio frequency signal;
• said first mixer combining said first radio frequency signal and said local oscillator signal to generate a second radio frequency signal;
• a fixed frequency oscillator for generating a fixed frequency signal;
• a second mixer connected to said fixed frequency oscillator to receive said fixed frequency signal and said first mixer to receive said second radio frequency signal, said second mixer combining said fixed frequency signal and said second radio frequency signal to generate a third radio frequency signal;
• each of said first, second and third radio frequency signals being a carrier for said pulse code modulated/frequency modulated telemetry data;
• a bandpass filter connected to said second mixer to receive said third radio frequency signal and then provide an analog signal, said bandpass filter attenuating said analog signal in a first state resulting in a detectable difference between the amplitude of said analog signal in said first state and the amplitude of said analog signal in a second state;
• said bandpass filter having an intermediate frequency bandwidth, said local oscillator signal being adjusted to provide a frequency offset to said second radio frequency signal which is about one half of the intermediate frequency bandwidth of said bandpass filter;
• and
• an amplitude modulation detector connected to said bandpass filter to receive said analog signal, said amplitude modulation detector detecting the amplitude of said analog signal in said first state and the amplitude of said analog signal in said second state to provide an analog pulse code modulated telemetry data signal having voltage spikes removed from said analog pulse code modulated telemetry data signal.

A method for minimizing the effects of frequency selective fading in multipath transmission of pulse code modulated/frequency modulated telemetry data, comprising the steps of:

• at a receiving location, receiving a first radio frequency signal;
• generating a local oscillator signal, said local oscillator signal having a frequency which is adjustable;
• subtracting said first radio frequency signal from said local oscillator signal to provide a second radio frequency signal;
• generating a fixed frequency signal;
• subtracting said second radio frequency signal from said fixed frequency signal to provide a third radio frequency signal, each of said first, second and third radio frequency signals being a carrier for said pulse code modulated/frequency modulated telemetry data;
• filtering said third radio signal in a bandpass filter having an intermediate frequency bandwidth to provide an analog signal;
• attenuating said analog signal in a first state resulting in a detectable difference between the amplitude of said analog signal in said first state and the amplitude of said analog signal in a second state;
• adjusting said local oscillator signal to provide a frequency offset to said second radio frequency signal which is about one half of the intermediate frequency bandwidth of said bandpass filter;
• and then
• detecting the amplitude of said analog signal in said first state and the amplitude of said analog signal in said second state to form an analog pulse code modulated telemetry data signal having voltage spikes removed from said analog pulse code modulated telemetry data signal.