Sampling technology has evolved into a powerful tool for microwave engineers. Effective sampling circuitry requires knowledge of time-domain and broadband techniques, both of which have been advanced at Picosecond Pulse Labs (Boulder, CO) in support of several product lines, including pulse generators, comb generators, and high-speed sampling modules. The firm has succeeded in pushing microwave sampling technology beyond 100 GHz with the help of its innovative nonlinear-transmission-line (NLTL) technology (Fig. 1).

The history of microwave sampling extends back 50 years.¹ It is essentially a simple process, in which a switch controlled by a local-oscillator (LO) strobe opens a path between the RF input port and an output intermediate-frequency (IF) port for an extremely short interval. In that short time, a portion of the RF signal is captured as the frequency-translated IF signal. Much of the effort to improve microwave sampling has involved work on the LO strobe drive and the sampling switch. The innovations at Picosecond Pulse Labs stem from work at its Oregon-based GaAs and thin-film facility (see sidebar). The GaAs fabrication facility creates high-performance air-bridged Schottky diodes with multi-THz cutoff frequencies that can be used as both varactors for millimeter- and submillimeter-wave frequency multiplication and as ultra-fast switches in mixers and samplers.

The firm's NLTLs are synthetic transmission lines, in which Schottky varactor diodes are distributed along a high-impedance transmission line. The lumped capacitances of these diodes are also distributed along the line (Fig. 2). The capacitance of a reverse-biased Schottky diode is nonlinear (voltage dependent) such that the capacitance at low reverse bias is much greater than the capacitance at high reverse bias. Because of this, the propagation velocity of a Schottky varactor based NLTL is voltage dependent. A large step signal that transitions from low to high voltage will be compressed in time as the initial low-voltage portion of the step travels down the line slower than the later, higher-voltage portion of the step. Consequently, the higher-voltage portion of the waveform "catches up" with the lower-voltage portion of the step, resulting in "edge compression," increasing the edge speed of the low-to-high transition. The high-speed diodes used in these NLTLs are capable of processing signals with subpicosecond transition times. To avoid aberrations due to shockwave formation, the NLTLs used in the company's flat-top step pulse generators are carefully tailored by "chirping" the diode size and line parameters along the length of the line.

Sampler performance depends on the qualities of the strobe pulse used to open and close the switches, the speed of the switches, and the circuit interconnections. An ideal switch has no series resistance and no parasitic capacitance. In other words, ideal switching diodes must have a very high cutoff frequency relative to the signal being sampled. The GaAs diodes fabricated in the Oregon facility exhibit cutoff frequencies in the THz range.

Even at lower bandwidths, other factors such as sampling efficiency (which affects noise), the linearity of the sampling process (which determines dynamic range), and isolation are all important parameters that critically depend on the strobe drive characteristics. Both the pulse shape and duration are key to critical real-world sampling circuits. A fast, square pulse will turn on the sampling diodes quickly, minimizing the time that the diodes operate in the nonlinear region of their transfer curve. A square strobe will also minimize input and bias dependant sampling efficiency by preventing input voltage-dependent aperture duration. The strobe must also be able to deliver enough current to drive the backshort. The reflection of the strobe drive waveform from the backshort turns off the sampling diode and determines the duration of the sampling aperture. Very narrow apertures are needed for high bandwidth, and therefore very fast edges are required for square apertures in the highest bandwidth samplers.

Traditionally, discrete step-recovery diodes (SRDs) have been used in strobe drive circuits. The fastest SRDs currently available have transition times of many tens of picoseconds. As a result, traditional samplers with 5 to 7 ps apertures (bandwidths of 50 to 70 GHz) have triangular or Gaussian apertures, and are necessarily very nonlinear since their aperture durations are input voltage dependent. Changing signal slew rates modulate the sampling aperture for Gaussian-shaped strobe pulses, resulting in dynamic distortion, while square-shaped apertures are relatively unaffected.

This nonlinearity can amount to as much as 30 percent over a modest 500-mV input range, and must be corrected in software. Dynamic nonlinearity, caused by input slew-rate aperture dependence, is not easily correctable and leads to significant intermodulation distortion (IMD) in high-bandwidth samplers. For this reason, Picosecond Pulse Labs uses NLTLs monolithically integrated with the sampling structure to produce the strobe drive waveforms, resulting in dramatic improvements in dynamic range and linearity. The company's advanced sampling technology is essential to a recent line (the Wave Expert series) of digital sampling oscilloscopes from LeCroy Corp., including one model with a sampling bandwidth of 100 GHz.

Getting the RF input signal to the sampling diodes is currently the main limitation to sampler bandwidth. Traditional approaches have involved the use of resistive-capacitive (RC) "peaking" to extend the bandwidth of a 50-GHz module to 70 GHz, at the expense of input matching. For the 70- and 100-GHz sampler modules employed in the LeCroy scopes, major improvements were obtained in the 1-mm and 1.95-mm coaxial interconnects. As outlined in US Patent No. 6,900,710, the GaAs sampler die penetrates a coaxial 1-mm airline cavity. The center conductor of the coaxial airline contacts the GaAs sampler die through a gold bump formed on the sampler die. This creates a "through" sampler, in which the coaxial signal line is passed through the sampler to be reused or terminated externally. This feature was also exploited in a prototype 100-GHz sampler module specifically designed to ride atop 1-mm wafer probes to facilitate on-wafer measurements.