Carrier supply for frequency division multiplexed systems

Abstract

The terminals in the frequency division multiplexed systems of the prior art employ many steps of modulation whose frequency variations, due to changes in the frequencies generated by the carrier oscillators, add to produce a frequency offset which is unacceptable in modern wideband systems. The terminals of the present invention use additional modulators connected to the carrier oscillators to reduce the frequency offset of the transmitted signal to the frequency variations of only a single oscillator.

What is claimed is: .[.1. A transmitter for a frequency division multiplexed system having n steps of modulation to produce an output signal which includes a modulated input signal comprising a first plurality of n modulators having their respective first input and output connected in a series string between the input and output of said transmitter, a plurality of n oscillators each having individual output frequencies, a second plurality of n-1 modulators having their respective first and second inputs connected to individual pairs of adjacent oscillators of said plurality of n oscillators to form a series string of alternate oscillators and modulators beginning with a first oscillator and ending with a last oscillator of said plurality of n oscillators, means connecting said first oscillator of said plurality of n oscillators to the second input of said first modulator of said first plurality of modulators connected to the input terminal of said transmitter, means for combining the output of said first oscillator of said plurality of oscillators with the output of the modulator of said first plurality of modulators connected to the input of said transmitter, and means connecting the output of each of said second plurality of modulators with the second input of each of said first plurality of modulators except the one connected to the input of said transmitter, the output frequency of each of said second plurality of modulators being a combination of the frequencies of the oscillators connected thereto, whereby the frequency offset in the transmitted signal is due only to the frequency variation of the said last oscillator of said plurality of n oscillators..]. .[.2. A transmitter for a frequency division multiplexed system in accordance with claim 1 wherein each of said first plurality of modulators is lower single sideband and the output signal of each of said second plurality of n-1 modulators is at a frequency which is the sum of the frequencies of the oscillators connected thereto..]. .[.3. A transmitter for a frequency division multiplexed system in accordance with claim 1 wherein each of said first plurality of modulators is upper single sideband and the output signal of each of said second plurality of n-1 modulators is at a frequency which is the difference of the frequencies of the oscillators 
     
        connected thereto..]. 4. A receiver in a frequency division multiplexed transmission system for demodulating an input signal modulated n times .[.comprising.]..Iadd., the input signal including a carrier pilot signal, said receiver comprising: a phase detector coupled to the input of said receiver, .Iaddend.a plurality of n demodulators having their respective first input and output coupled in a series string between the input and output of said receiver, a plurality of n oscillators having individual output frequencies, a plurality of n-1 modulators having their respective first and second inputs connected to individual pairs of adjacent oscillators of said plurality of n oscillators to form a series string of alternate oscillators and modulators beginning with a first oscillator and ending with a last oscillator of said plurality of n oscillators, the last of said plurality of n oscillators in said series string being connected directly to the second input of the demodulator of said n demodulators connected to the output of said receiver, .Iadd.means connecting said phase detector with said first oscillator in an automatic phase control loop for establishing phase lock with the carrier pilot signal, .Iaddend.and means connecting the outputs of individual ones of each of said plurality of n-1 modulators to the second input of individual ones of each of said plurality of n demodulators except the demodulator connected 
     
     
        to the output of said receiver. 5. A receiver for a frequency division multiplexed transmission system in accordance with claim 4 wherein each of said n demodulators is lower single sideband and the signal at the said output of each of said plurality of n-1 modulators is at a frequency which is the sum of the frequencies of the pair of oscillators connected 
     
     
        thereto. 6. A receiver for a frequency division multiplexed transmission system in accordance with claim 4 wherein each of said n demodulators is upper single sideband and the signal at the said output of each of said plurality of n-1 modulators is at a frequency which is the difference of the frequencies of the pair of oscillators connected thereto. .[.7. A receiver for a frequency division multiplexed transmission system in accordance with claim 4 wherein a carrier pilot signal is transmitted with said input signal modulated n times and a carrier recovery circuit including the first of said plurality of n oscillators in said string is connected to the input of said receiver, said first oscillator of said plurality of n oscillators having an output frequency substantially that of the received carrier pilot signal to correct for the frequency variations suffered during transmission over the transmission medium..]. .[.8. A receiver in accordance with claim 7 wherein said carrier recovery circuit additionally includes a phase detector coupled to said input of said receiver and means connecting said phase detector with said first oscillator of said carrier recovery circuit in an automatic phase control loop..]. .[.9. A transmitter for a frequency division multiplexed system comprising a first modulator having a first input connected to the input of said transmitter, a summing network having a first input connected to the output of said first modulator, a second modulator having a first input connected to the output of said summing network, the output of said second modulator being connected to the output of said transmitter, a first oscillator and a second oscillator, means connecting the output of said first oscillator to the second input of said first modulator and the second input of said summing network, the frequency offset accumulated in said transmitter in the signal at the output of said first modulator being due only to the frequency variations of said first oscillator, a third modulator having a first input connected to the output of said first oscillator to receive only the output frequency and variations thereof of said first oscillator at said first input and a second input connected to the output of said second oscillator to receive only the output frequency and variations thereof of said second oscillator at said second input, the signal appearing at the output of said third modulator including only the output frequencies and accumulated frequency offsets of said first and second oscillators, and means connecting said output of said third modulator to the second input of said second modulator so that the accumulated frequency offset in the output signal of said second modulator and said transmitter is due only to the frequency offset of said second 
     
     
        oscillator..]. 10. A receiver in a frequency division multiplexed system for demodulating a modulated input signal .[.comprising.]. .Iadd.having a carrier pilot signal, said receiver comprising: a phase detector coupled to the input of said receiver, .Iaddend.an input terminal, a first demodulator having a first input connected to said receiver input terminal, a second demodulator having a first input connected to the output of said first demodulator and an output connected to the output of said receiver, a first oscillator and a second oscillator, a modulator having a first input connected to the output of said first oscillator to receive only the output frequency and variations thereof of said first oscillator at said first input and a second input connected to said second oscillator to receive only the output frequency and variations thereof of said second oscillator at said second input, the signal appearing at the output of said first modulator including only the output frequencies and accumulated frequency offsets of said first and second oscillators, .Iadd.means connecting said first oscillator to said phase detector to form an automatic phase control loop for establishing phase lock with the carrier pilot signal, .Iaddend.means connecting the output of said first modulator to the second input of said first demodulator, and means connecting the output frequency and variations thereof at the output of said second oscillator to the second input of said second demodulator, whereby the frequency offset introduced by said first and second oscillators in said receiver is minimized. .[.11. A receiver for a frequency division multiplexed transmission system in accordance with claim 10 wherein a carrier pilot signal is transmitted with said modulated input signal and a carrier recovery circuit including said first oscillator is connected to the input to said receiver, said first oscillator having an output frequency substantially that of the received carrier pilot signal to correct for the frequency variations suffered during transmission over the transmission medium..]. .[.12. A receiver in accordance with claim 11 wherein said carrier recovery circuit additionally includes a phase detector connected to said input of said receiver and means connecting said first oscillator to said phase detector 
     
     
        to form an automatic phase control loop..]. .Iadd.13.  A transmission system including a transmitter and receiver, the transmitter having a chain of n successive modulator stages, each stage comprising a modulator and an oscillator, the output of each modulator--except the last in the chain--being connected to a first input of the next modulator, the first stage oscillator output being applied to a second input of the first modulator, the frequency applied to the second input of any subsequent modulator being derived by combining--in an interstage modulator--the output of the oscillator associated with that stage and the output of the oscillator associated with the previous stage, the output of the last modulator stage being a signal combining sideband frequencies and the carrier frequency (fp) of that stage; the receiver having a chain of n successive demodulator stages, each stage comprising a demodulator and an oscillator, the output of each demodulator--except the last in the chain--being connected to a first input of the next demodulator, the frequency applied to a second input of each demodulator, except the last in the chain, being derived by combining--in an interstage modulator--the output of the oscillator associated with that stage and the output of the oscillator associated with the next stage, the oscillator associated with the first demodulator stage in the chain being phase locked to the carrier frequency (fp), and the oscillator with the last demodulator stage in the chain being applied to the second input of the demodulator. .Iaddend.