A photodetector circuit has been built for use in the receiver portion of a continuous-wave infrared Doppler lidar system for atmospheric research. As in nearly all Doppler lidar systems, the detection of the return signal in this system involves heterodyning, in a photodetector, with a signal at the same frequency as that of the transmitted laser beam. Thus, the Doppler frequencies manifest themselves as beat frequencies (typically of the order of 40 MHz in this system) that appear in the photodetector output. The reason for developing the present photodetector circuit instead of using one of the many commercially available photodetector circuits is that a unique combination of high gain, high bandwidth, and low noise is needed for detection of the faint (power as low as 10-15 W) return signal in this system.
The present photodetector circuit includes a photodiode, a load resistor, and amplifier circuitry. Ordinarily, a photodiode would be connected to a 50-Ω load resistor because radio-frequency amplifiers capable of handling the Doppler-frequency components of the photodiode output are typically designed for an input impedance of 50 Ω. The load resistance establishes a noise floor in that Johnson noise (expressed as current noise because the photocurrent from the diode is what is to be measured) is inversely proportional to resistance. Thus, if one could use a greater load resistance with the photodiode, then one could lower the noise floor. Accordingly, in this circuit, the photodiode is connected to a 500-Ω load resistance.
The amplifier circuitry comprises an electrically switchable array of commercial two-channel, low-noise video multiplexer/amplifiers that are compatible with the increased input impedance of 500 Ω. Each multiplexer/amplifier has a nominal gain of 4 and a bandwidth > 100 MHz. Through a 4-bit control word, the number of amplifier stages can be set at 3, 4, 5, or 6. In addition, a factor-of-two attenuator can be interposed between the next-to-last and last amplifier stage. The net result is that the gain can be electrically switched (or, equivalently, digitally programmed) to values from 8 to 1,024 in eight factor-of-two ( ≈ 6-dB) steps to accommodate various input signal levels.
To obtain a required overall bandwidth of 60 MHz and to minimize coupling of noise, the circuit must be designed to minimize parasitic capacitance. This is especially true for the node that includes the anode of the photodiode, one end of the load resistor, and the input terminal of the first amplifier stage; every additional picofarad of capacitive loading of this node reduces the overall amplifier bandwidth by about 20 percent. In the design of the circuit, the parasitic capacitance is minimized through optimal choice of the geometry of the circuit, including the placement of components.
Two other outstanding features of the circuit are the following:
- An operational-amplifier-based servo loop actively regulates the photodiode bias current to prevent saturation of the first amplifier stage in the presence of bright background light, and
- An inverter chip is placed in the digital gain-control pathway to prevent entry of noise on the gain-control lines.
At the time of reporting the information for this article, the circuit had been constructed but not yet tested. The combination of high load resistance, low capacitance, and low-noise amplifiers with high frequency response is expected to yield (1) a noise floor 8 dB below the Johnson noise in a 50-Ωresistor and (2) gain-bandwidth as much as 400 GHz.
This work was done by Greg Cardell of Caltech for NASA's Jet Propulsion Laboratory.
This Brief includes a Technical Support Package (TSP).
High-Preformance Photodetector Circuit for Doppler Lidar
(reference NPO20558) is currently available for download from the TSP library.
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