Accurate and reliable signal sources are essential for research and development work at mmWave frequencies. Approaching THz frequencies, the available options for signal generation are limited but, thankfully, growing. The newest offering from Eravant is the model STE-KF1803 frequency extender, which combines Eravant’s high power amplifier expertise with multiplier diode technology from ACST GmbH. This x18 multiplying source extender accepts input signals from 12.2 to 18.3 GHz and delivers full-band frequency coverage from 220 to 330 GHz with a nominal output power of +5 dBm (see Figure 1).
In many applications, power levels greater than 0 dBm are required to provide adequate signal amplitude for driving the mixers and modulators used in radar, communication, instrumentation and remote sensing systems. When testing passive components such as filters, couplers and isolators, higher signal levels provide greater measurement dynamic range.
Used with a low frequency programmable signal generator or sweeper, Eravant’s frequency extender preserves the inherent switching speed, frequency accuracy and amplitude stability of the frequency source. With its combination of full-band frequency coverage and ample output power, the STE-KF1803 is a versatile addition to the lab for research and development at sub-THz frequencies. Designed for benchtop use, the unit measures 4.9 × 5.0 × 1.9 in.
The nominal input power level is +3 dBm, provided through a 2.92 mm female coaxial connector. The output uses a WR03 anti-cocking waveguide flange, with the output level adjustable over a 25 dB range using an integrated manual attenuator. Harmonic content in the output is -15 dBc or lower, and typical spurious signals are -60 dBc or below. The input return loss is 10 dB or better.
WHY FREQUENCY MULTIPLICATION
Several approaches can be used to extend the frequency range of a tunable RF signal source. Up-conversion using a mixer and fixed frequency or tunable local oscillator (LO) may seem like a reasonable approach for covering a narrow range of frequencies; however, this technique has disadvantages. To preserve the frequency accuracy and phase noise characteristics of the low frequency source, the frequency and phase noise of the LO must be low, which typically requires a phase-locked or synthesized LO. Up-conversion may require an LO frequency relatively close to the desired RF output. A sub-harmonically pumped up-converter can be used, although it may not yield sufficient RF output power. Another challenge for up-converting to sub-THz frequencies is suppressing unwanted LO, sideband and spurious signals.
Compared with up-conversion, frequency multiplication with amplification is the preferred approach for generating tunable test signals at mmWave and THz frequencies. However, the frequency of the phase noise sidebands of the low frequency source will be expanded in proportion to the extender’s multiplication factor. For an ideal noiseless multiplier, the phase noise power density at a given offset from the carrier, relative to the carrier amplitude, is increased by 20log(N), where N is the multiplication factor. The STE-KF1803 frequency extender employs a cascade of multiplier stages and matching circuits, filters and amplifiers to achieve a net multiplication factor of 18.
ACST DIODE MULTIPLIER
At mmWave frequencies, varactor diodes are often used for frequency doublers and triplers as they provide nonlinear impedance characteristics that yield good multiplication efficiency and low RF loss. The final multiplication stage in the STE-KF1803 extender uses a pair of Schottky varactor multiplier diodes fabricated using a film diode process developed by ACST GmbH (see Figure 2). ACST GmbH’s device capabilities include square-law detectors up to 2.5 THz, resonant tunneling diode oscillators up to 1.1 THz and frequency multipliers up to 600 GHz.
The diodes are fabricated with a planar process on thinned, semi-insulating GaAs substrates, with metal-insulator-metal capacitors and conductors monolithically integrated with the diodes to form an efficient multiplier circuit. The diode terminals and conductor traces are formed on both sides of a 5 μm membrane substrate, resulting in low parasitics and RF loss, with air bridge interconnected mesas reducing the parasitic capacitance. A diamond substrate transfers heat so the diodes can operate at higher power levels with greater conversion efficiency. The size of a typical three-element varactor multiplier diode is 60 × 240 µm.
FREQUENCY EXTENDER FAMILY
The STE-KF1803 frequency extender from Eravant complements the other members of the STE series (see Table 1). The extender family provides frequency coverage from 40 to 330 GHz with output power to +20 dBm, using multiplication factors from 2 to 18. Over 30 standard models are available, and custom configurations are possible.