In scientific research, particularly in the high-frequency end of the millimeter wave band, researchers have been working on developing and designing equipment for experimental studies. The signal source used in these studies typically involves a multiplier, with the drive signal coming from a Gunn diode oscillator or a backward wave oscillator capable of operating above 110 GHz. Signal detection is achieved using custom narrowband detectors or resonant mixers. However, during the research process, scientists often face challenges due to the limited bandwidth of the testing instruments.
Studies in the millimeter wave frequency range commonly involve spectral line analysis, molecular particle identification, and material characterization—fundamental tasks in this field. Due to atmospheric absorption and other environmental factors affecting millimeter wave propagation, new applications are emerging in areas such as communications, transportation, scientific exploration, and homeland security.
In the early 1980s, a full-waveguide bandwidth vector network analyzer (VNA) system was introduced, capable of measuring signal absorption, reflection, and scattering up to 110 GHz. By the late 1990s, this capacity had expanded to 220 GHz, and by 2002, systems covering the 220–325 GHz range were developed. With the introduction of the 325 GHz waveguide VNA system, researchers began seeking even higher frequency bands, which led to the development of frequency expansion modules beyond 500 GHz.
The 325–500 GHz VNA frequency expansion module described here represents the highest frequency at which harmonic interference can be suppressed using a 20 GHz synthesizer. As harmonic interference within the waveguide passband has become increasingly difficult to filter, the feasibility of extending the frequency range further using a practical multiplier scheme has been impacted.
Figure 1 illustrates the structure of the WR-02.2 frequency expansion module. This design follows the concept of using a 20 GHz synthesizer as both the local oscillator and RF input. At frequencies of 20 GHz or higher, the synthesizer employs double or triple multipliers to extend its frequency range, while maintaining phase noise attenuation of 20 log(n). Although this architecture does not offer significant advantages over traditional multiplier/amplifier setups, it remains a viable option for certain applications.
To achieve the 325–500 GHz range, the RF input frequency is amplified and multiplied by a factor of 30. To minimize amplitude fluctuations caused by mismatches between the RF cable and connection interface, an isolator is added before the multiplier/amplifier. The output signal from the multiplier/amplifier drives a 15x multiplier chain, producing an output frequency in the WR-02.2 band. The original design optimized the 15x multiplier chain to reduce in-band harmonic interference, using available filters. While lower multiplication factors like 2x or 3x can help avoid some harmonic issues, they require interstage amplification, which increases complexity. Commercial W-band or higher amplifiers are rare and often problematic, making the 15x multiplier chain a more practical solution. This setup achieves an average output power of -32 dBm, measured using a calorimeter.
After being amplified and multiplied by a net factor of 4, the LO input frequency is fed into the millimeter wave mixer as the local oscillator signal. An isolator is placed at the input of the local oscillator amplifier to reduce fluctuations caused by mismatched cables. The output from the multiplier/amplifier is split equally and drives the next multiplier chain, providing a reference signal for the harmonic mixer. To optimize the match between the splitter and the multiplier, an isolator is also placed at the multiplier’s input. The frequency multiplier produces a minimum output power of 10 dBm in the WR-15 band, sufficient to bias the harmonic mixer effectively. This local oscillator topology is simple, well-proven in low millimeter wave bands, and ensures excellent phase continuity, leading to optimal noise performance.
As shown in Figure 2, the 325–500 GHz signal is coupled to the RF input of the millimeter wave harmonic mixer via a 10 dB coupler. The IF output of the mixer is optimized for a frequency range of 5 to 300 MHz. The multi-stage IF amplifier provides a gain greater than 50 dB, increasing the peak IF output to -13 dBm. This power level is chosen to prevent saturation of the vector network analyzer's internal links while maximizing the dynamic range of the system. Depending on the specific millimeter wave VNA system in use, it may be necessary to further reduce the IF output power to avoid saturation of the test controller.
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