Extracting Stable Clock Signals From AM Broadcast Carriers for AmateurS Spread-Spectrum Applications

To recover data from spread-spectrum transmissions, it is necessary to reintroduce, at the receiver end, locally generated signals locked in frequency and phase to the clock used in the original transmissions. At carrier frequencies of up to 50 MHz, the extraction of clock pulses from direct-sequence (DS) spreadspectrum transmissions can be achieved fairly easily, using readily available ICs.' The 1986 FCC decision to relegate Amateur Radio spread-spectrum transmissions to frequencies above 400 MHz, however, singularly complicated the design and realization of this type of eq~ipment.~ Although it will be eventually possible to find a way to extract the clock signal at those frequencies, it is a major stumbling block to the development of uncomplicated amateur spreadspectrum equipment. Another way to obtain a clock is to use an external reference signal readily available at both the transmitting and the receiving sites3 Since our amateur transmissions must take place at UHF, communications will usually occur when the two parties are within a fairly short distance of each other. Locally available reference signals are generated by TV, FM and AM radio stations. I have already explored the recovery of such signals, and have demonstrated the possibility of using vertical synchronization signals from local TV stations to generate reliable clock pulses for slow frequency-hopping spread-spectrum experiment^.^ DS spread-spectrum on the other hand, requires that clock signals be in the megahertz r e g i ~ n . ~ , ~ In DS, the clock