Pulse coherent transponder with precision frequency offset
First Claim
1. Radar transponder apparatus for receiving a sequence of interrogation pulses and generating reply pulses that are phase coherent with the interrogation pulses but delayed in time and offset in frequency, by a precise amount, including in combination:
- means for providing a reference signal at a first predetermined frequency, said means being of continuous operation;
means for down-converting the interrogation pulses to intermediate frequency signal information in the presence of said reference signal at said first predetermined frequency;
means for delaying said intermediate frequency signal information for a predetermined time interval;
means receiving said intermediate frequency signal information from said down-converting means, which is coupled to said means for providing a reference signal, for causing the frequency of said reference signal produced thereby to be incremented by a fixed, predetermined amount, the instantaneous phase of said incremented frequency reference signal being related to the instantaneous phase of said reference signal at said first predetermined frequency with said relationship being substantially invariant from pulse-to-pulse of the sequence of received interrogation pulses; and
means for up-converting said delayed intermediate frequency signal information in the presence of said reference signal at said incremented frequency, said up-converted signal information being a sequence of reply pulses offset in frequency but phase coherent with said received interrogation pulses.
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Abstract
A radar reply transponder for use with existing radar tracking systems wherein a reply pulse is generated and trasmitted in response to an associated interrogation pulse. The reply pulse is delayed in time and offset in frequency by a precise amount with respect to the interrogation pulse, but it is nevertheless phase coherent therewith so as to preserve the required Doppler frequency information and thereby permit accurate velocity measurements. Phase coherence is effectively maintained by utilizing a single continuously operating local oscillator for down-converting the received interrogation pulse to an intermediate frequency as well as up-converting the signal information to provide the delayed and frequency offset reply pulse. The precision frequency offset is obtained by switching in an additional reactance element in the oscillator-determining circuitry during the time up-converting is being effected. The radar transponder may thus be used in a high-vibration environment with no substantial introduction of error in velocity measurements caused by using two independent oscillators to formulate the frequency offset.
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Citations
13 Claims
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1. Radar transponder apparatus for receiving a sequence of interrogation pulses and generating reply pulses that are phase coherent with the interrogation pulses but delayed in time and offset in frequency, by a precise amount, including in combination:
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means for providing a reference signal at a first predetermined frequency, said means being of continuous operation; means for down-converting the interrogation pulses to intermediate frequency signal information in the presence of said reference signal at said first predetermined frequency; means for delaying said intermediate frequency signal information for a predetermined time interval; means receiving said intermediate frequency signal information from said down-converting means, which is coupled to said means for providing a reference signal, for causing the frequency of said reference signal produced thereby to be incremented by a fixed, predetermined amount, the instantaneous phase of said incremented frequency reference signal being related to the instantaneous phase of said reference signal at said first predetermined frequency with said relationship being substantially invariant from pulse-to-pulse of the sequence of received interrogation pulses; and means for up-converting said delayed intermediate frequency signal information in the presence of said reference signal at said incremented frequency, said up-converted signal information being a sequence of reply pulses offset in frequency but phase coherent with said received interrogation pulses. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. Radar transponder apparatus for generating, in response to a sequence of received interrogation pulses which are periodic at a fixed pulse repetition frequency, reply pulses that are phase coherent therewith but delayed in time and offset in frequency by a fixed, precise amount, including in combination:
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oscillator means for generating a radio frequency carrier signal; means coupled to said oscillator means for modulating the same to provide harmonically related frequencies; frequency selection means coupled to said oscillator means for selecting two reference signals from said harmonically related frequencies, said reference frequencies differing by an integral multiple of the received pulse repetition frequency; means for down-converting the received interrogation pulses to signal information of an intermediate frequency in the presence of the first of said reference frequencies; means for delaying said intermediate frequency signal information by a predetermined time interval; means for up-converting said delayed intermediate frequency signal in the presence of the second of said reference frequencies, said up-converted signal information being offset in frequency from that of said interrogation pulses; and means for amplifying and transmitting said up-converted signal information, said up-converted signal information being a sequence of reply pulses which are phase coherent with said sequence of received interrogation pulses. - View Dependent Claims (10, 11)
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12. In a radar transponder apparatus for receiving a sequence of interrogation pulses and generating reply pulses wherein the combination includes a continuous operating oscillator for developing a reference signal of a predetermined frequency;
- wherein said interrogation pulses are down converted to signal information at an intermediate frequency in the presence of the reference signal;
the intermediate frequency signal being delayed; and
wherein the delayed intermediate frequency signal information is up-converted to reply pulses in the presence of the reference signal, the improvement, which comprises;the continuous running oscillator including a resonant circuit at the reference signal frequency; means for incrementing the frequency of the continuous running oscillator by a predetermined amount, said means for incrementing the frequency including switching means for alternately switching at least one additional reactance element into said resonant circuit of the oscillator to alter the resonant frequency of the resonant circuit without significant alteration of the stored energy of the resonant circuits, the phase of the incremented frequency reference signal being related to the phase of the predetermined frequency reference signal and being substantially invariant from pulse-to-pulse of the interrogation pulses; and the delayed intermediate frequency signal information being up-converted to reply pulses in the presence of said incremented frequency reference signal, said reply pulses being offset in frequency and phase coherent with the interrogation pulses. - View Dependent Claims (13)
- wherein said interrogation pulses are down converted to signal information at an intermediate frequency in the presence of the reference signal;
Specification