Generation of wideband linear frequency modulation signals
AssigneeHughes Aircraft Company
InventorJames L. Blanton
Life of this patent
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Abstract
A low-frequency low bandwidth Linear Frequency Modulation (LFM) waveform, nominally a 1 MHz to 10 Mz swept frequency analog sinusoid or digital square wave, is produced by direct digital synthesis. This waveform is upconverted in frequency and expanded in bandwidth, nominally to microwave frequencies with bandwidths of nominally 160-360 MHz, in a multiplying offset phase locked loop. The phase locked loop also linearly frequency modulates a X-band carrier with the microwave frequency LFM waveform to produce an output signal suitable for Synthetic Aperture Radar. The phase locked loop induces low phase error, and may be closed around the radar transmitter to remove phase errors induced by that unit.
Claims18
An apparatus for generating a wideband Linearly Frequency Modulated, LFM, carrier signal comprising:
- a generator of a low frequency, low bandwidth, baseband LFM signal;
- a variably offset phase locked loop (i) receiving the baseband LFM signal and also a fixed frequency reference signal in the band of a carrier signal, (ii) upconverting in frequency and expanding in bandwidth the received baseband LFM signal, to (iii) produce a wideband high frequency LFM signal.
An apparatus receiving a Linear Frequency Modulated, LFM, signal and concurrently (i) upconverting in frequency the received LFM signal (ii) expanding the modulation bandwidth of the upconverted LFM signal to produce a wideband LFM carrier signal, the apparatus comprising:
- a Voltage Controlled Oscillator, VCO, controllable to produce a frequency modulated carrier wave output signal in accordance with a received modulation control signal;
- a reference signal source for producing a fixed frequency reference signal in the band of the carrier signal;
- a mixer for mixing a portion of the frequency modulated carrier wave output signal received from the VCO with the reference signal received from the reference signal source to produce a difference frequency signal;
- a frequency divider circuit for receiving the difference signal from the mixer and reducing it in frequency to a frequency range of the received baseband LFM signal;
- and
- a phase detector for producing the modulation control signal responsive to a detected phase difference between a reduced frequency difference signal received from the frequency divider circuit and the received baseband LFM signal;
- wherein the VCO, reference signal source, mixer, frequency divider, and phase detector operate as a phase locked loop.
A phase locked loop for a transmitter circuit CHARACTERIZED IN THAT
- the phase locked loop is variably offset in frequency for modulating a transmitted carrier signal to be Linearly Frequency Modulated (LFM) in response to a modulation control signal;
- and
- the modulation control signal is developed within the phase locked loop from (i) a fixed frequency reference signal in the band of the carrier signal, and from (ii) a received LFM signal that is both of lessor frequency and lessor modulation bandwidth than the control-LFM signal, to effect (1) a frequency upconversion and (2) a bandwidth expansion of the received LFM signal.
Dependencies are inferred from the claim text.