In scientific research focused on the high-frequency end of the millimeter wave band, researchers have been developing specialized equipment and conducting experiments. The signal source in these studies typically involves a multiplier, with the drive component being a Gunn diode oscillator or a backward wave oscillator capable of operating above 110 GHz. For signal detection, custom narrowband detectors or resonant mixers are commonly used. However, during these investigations, researchers often faced challenges due to the limited bandwidth of their testing instruments.
Studies in the millimeter wave frequency range encompass fundamental tasks such as spectral line analysis, molecular particle identification, and material characterization. Due to atmospheric effects that impact millimeter wave transmission, new applications for this technology have emerged in areas like communications, transportation, scientific exploration, and homeland security.
In the early 1980s, a full-waveguide bandwidth vector network analyzer (VNA) system capable of measuring absorption, reflection, and scattering characteristics up to 110 GHz was introduced. By the late 1990s, the system's capability had expanded to 220 GHz. In 2002, a VNA system covering the 220-325 GHz band was developed. With the introduction of the 325 GHz waveguide VNA system, researchers began to demand even higher frequency bands, which led to the development of frequency expansion modules reaching 500 GHz and beyond.
The 325–500 GHz VNA frequency expansion module described here represents the highest frequency at which harmonic interference can be effectively suppressed using a 20 GHz synthesizer. However, as harmonic interference within the waveguide passband has become too strong to filter out, future plans for extending the frequency range beyond 500 GHz using practical multiplier schemes face significant challenges.
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 20 GHz or higher, the synthesizer employs double or triple multipliers to extend its frequency range, while its phase noise is attenuated by 20 log(n). This configuration does not offer significant advantages over traditional multiplier/amplifier setups in millimeter wave frequency expansion modules.
To reach 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 the connection interface, an isolator is added at the input of the multiplier/amplifier. The output 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. Using lower multiplication factors like 2x or 3x may help avoid some harmonic issues, but inter-stage amplification is still required. Commercial amplifiers for W-band and higher are scarce and often problematic, adding complexity to the system. The 15x multiplier chain was measured using a calorimeter, achieving an average output power of -32 dBm.
After being amplified and multiplied by a net factor of 4, the LO input frequency is sent to the millimeter wave mixer as the local oscillator input. An isolator is placed at the input of the local oscillator amplifier to reduce amplitude fluctuations caused by mismatches between the LO cable and the interface. The output from the multiplier/amplifier is split equally, driving the next multiplier chain. The output is then used as a reference signal for the millimeter-wave local oscillator port and for testing the harmonic mixer. To optimize the match between the splitter and the multiplier, an isolator is also placed at the multiplier input. The frequency multiplier produces a minimum output power of 10 dBm in the WR-15 band, sufficient to properly bias the millimeter-wave harmonic mixer. This local oscillator chain topology is simple and has been proven effective in the low millimeter wave band, ensuring optimal noise performance and phase continuity.
As shown in Figure 2, the 325–500 GHz signal is coupled into 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 of more than 50 dB, allowing the peak IF output to reach -13 dBm. This power level is chosen to prevent saturation of the internal network link in the vector network analyzer while maximizing the dynamic range of the system. Depending on the specific millimeter wave VNA system used, it might be necessary to further reduce the IF output power to avoid saturation of the test setting controller.
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