How do I design a frequency extender module for extending VNA measurements above 110 GHz?

Designing a frequency extender module for extending VNA measurements above 110 GHz converts the VNA's microwave test signal (typically 10-20 GHz) to the target mmW or THz frequency band using frequency multiplication and harmonic mixing, enabling S-parameter measurements at frequencies far beyond the VNA's native range. The frequency extender architecture consists of: a transmit path (the VNA's source signal at frequency f_LO (approximately 10-20 GHz) is multiplied by a factor N (typically 6, 12, 18, or 24) using a Schottky diode multiplier chain to generate the test signal at N x f_LO; for a WR-3.4 (220-330 GHz) extender: 18× multiplication from a 12-18 GHz source), a receive path (the reflected or transmitted signal from the DUT returns to the extender and is downconverted to an IF signal by mixing with a harmonic of the LO signal; a subharmonic mixer uses the LO at f_LO and mixes with the RF signal at approximately N x f_LO, producing an IF at a few MHz that is sent back to the VNA for magnitude and phase measurement), and the waveguide test ports (the extender module has a waveguide port (WR-X flange) that connects to the DUT; the waveguide size determines the frequency band). The design challenges are: generating sufficient signal power (each multiplication stage has conversion loss of 8-15 dB; for 18× multiplication through three 6× stages: total conversion loss approximately 30-45 dB; the output power at 300 GHz is typically 0.1-1 mW), achieving adequate dynamic range (the system dynamic range must be sufficient to measure both the DUT's insertion loss and return loss; typical extender dynamic range: 50-90 dB depending on the frequency; this is reduced from the VNA's native dynamic range (120+ dB) due to the multiplication loss and mixer noise figure), and calibration (the extender introduces its own systematic errors (source match, load match, directivity) that must be removed through VNA calibration; calibration at mmW/THz frequencies uses: waveguide calibration standards (short, offset short, load, thru), TRL (Thru-Reflect-Line) calibration, or on-substrate calibration for on-wafer measurements).