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Photonic Generator of Arbitrary RF Waveforms

IP.com Disclosure Number: IPCOM000201129D
Publication Date: 2010-Nov-08

Publishing Venue

The IP.com Prior Art Database

Abstract

This invention is a generator of wideband arbitrary RF waveforms according to a set of lower bandwidth digital control signals. The generator produces a time-sequence of picosecond duration optical pulses whose intensities are specified by the digital control signals. These pulses are supplied by optical fiber to an optical-to-electrical (O/E) transducer, such as a photoconductor, that has picosecond response. The optical pulses, when incident on the photoconductor, produce pulses of electrical charge that momentarily changes the resistance of that photoconductor. The photoconductor is part of the bias circuit of an electronic amplifier that has a low-pass filter response. These optical pulses act like the sampled impulses of an ideal digital-to-analog converter (DAC) based on the Shannon sampling theory. Thus, the generator is capable of forming any arbitrary waveform consistent with the sampling rate or the frequency of the optical pulses and with the resolution of the digitally specified intensities of the optical pulses. Use of short optical pulses allows the timing of the digital control signals to be less precise and also alleviates the constraints on the sharpness of the low-pass filter of the DAC.

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Figure 2 illustrates a photonic generator capable of producing wideband arbitrary RF waveforms. This photonic generator makes use of the Shannon impulse-sampling concept of waveform reconstruction. Any RF waveform can be represented as a time sequence of impulses whose intensities match the amplitude of RF waveform at those instances of time corresponding to the occurrence of the impulses. The frequency of these impulses or samples must be at least as great as the Nyquist rate. Thus, for an RF waveform with a bandwidth extending from DC to 40 GHz, the Nyquist rate is twice the maximum frequency of the RF waveform, i.e., 80 GHz. In practical implementations, the actual frequency of the samples typically is somewhat greater than the Nyquist rate, e.g., 100-120 GHz sampling rate for a 40 GHz signal bandwidth, so that the low-pass filter used in reconstructing the RF waveform can have a more gradual drop-off at the edge of its passband while still avoiding aliasing.

DC power

RF antenna element

Figure 2. Photonic generator of arbitrary RF waveforms.

The photonic generator comprises a laser source that generates a train of optical pulses. These optical pulses are delivered to an optical pulse-pattern generator. This pulse pattern generator produces a sequence of optical output pulses whose intensities are controlled by sets of digital control signals that have a clock frequency much lower than the frequency of the output pulses. The width of these output pulses preferably is 3 psec or less, but the pulses can be wider if the bandwidth of the RF waveform to be generated is smaller. The width of the optical pulse preferably is much shorter than the desired sampling interval. For example, a sampling rate of 100 GHz would correspond to a sampling interval of 10 psec. Preferably, the optical pulses are short enough that they almost be considered as impulses. The crosstalk between adjacent output pulses or samples limits the achievable dynamic range of the waveform generator. Thus, it is preferable to have each output pulse decayed by many tens of dB before the occurrence of the next output pulse.

The optical pulse-pattern generator can supply the output pulses to an optical fiber that carries those pulses to an optical-to-electrical (O/E) transducer. A preferred O/E transducer illustrated in Figure 2 is a picosecond photoconductor that produces electrical charge carriers according to the intensity of the incident light. Thus, this photoconductor produces a pulse of electrical carriers, with a corresponding pulsed reduction in the electrical resistance of this photoconductor, according the number of photons that are in

Short-pulse laser

Multi-channel, low-frequency digital control signals

 Optical pulse- pattern generator

 Pico-second photoconductor & RF mixer

RF power amplifier

Optical fiber


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the pulse of incident light. The photoconductor changes the bias of the input stage of the RF power amplifier to which it is connected...