50-to-110-GHz high-performance millimeter-wave source modules

Hewlett-Packard Journal, April, 1991 by Mohamed M. Sayed, Giovonnae F. Anderson

V-Band and W-Band Coupler Detectors

For maximum flexibility in using the source with or without leveling, the coupler detector was designed to be separate from the V-band or W-band multiplier. (In the earlier HP 83554A, HP 83555A, and HP 83556A source modules, the coupler detectors were integrated with the multipliers). The coupler was designed and is manufactured in-house to obtain superior performance, ruggedness, and mechanical strength. (Special versions of the HP V752C and HP W752C are used). The schematic diagram and the mechanical layout for the V-band and W-band detectors are shown in Fig. 5.

The RF energy travels through a waveguide taper and impinges on a beam-lead diode at the end of the guide. One end of the diode is attached to a 6-pF capacitor, which rectifies the RF signal. The other end of the diode is attached to ground. The device used for this detector is a beam-lead planar doped barrier diode fabricated on a GaAs substrate with a 40_/AM2 active area.

The waveguide taper acts as an impedance matching circuit. It reduces the height of the guide from 0.074 inch to 0.006 inch for the V-band detector and from 0.050 inch to 0.005 inch for the W-band detector. The V-band detector employs a linear (straight line) taper, but the W-band detector uses an exponential cosine taper for improved input match.(4) The taper in the W-band detector also contains a high-pass filter, which attenuates below-band harmonics which would have degraded flatness. Fig. 6 shows the V-band and W-band coupler detector output voltages as functions of frequency.

W-Band Amplifier Tripler

Odd harmonics are typically generated by symmetrical distortion of an incoming waveform. For example, if an ideal sinusoidal waveform is clipped symmetrically by a circuit, then the output of that circuit will contain only odd harmonics. The circuit used for the W-band tripler is shown in Fig. 7. The diode used is a 20-[micro][.sub.2] beam-lead device fabricated on an n epitaxial layer on a GaAs substrate.

The filters are designed for proper signal routing. The design of the low-pass filter is crucial to the multiplier design since it provides the impedance match to the diode and a short circuit to the higher harmonics generated in the multiplier.(6) For more discussion on the design of this filter, see "The Use of the HP Microwave Design System in the W-band Tripler Design," page 53. The final design is realized as a nine-element modified Chebyshev filter with a cutoff frequency of 45 GHz. The last element of the filter is a chip capacitor fabricated in-house with a value of 0.006 pF. This chip capacitor gives the filter excellent out-of-band response because it presents an excellent short circuit to the higher harmonics. The output of the tripler is ac-coupled to the waveguide. The high-pass filter is an exponential cosine taper in the waveguide, similar to the one used in the detector; it provides an open circuit at the plane of the diode to frequencies below 70 GHz. The taper also reduces the height of the guide at the diode end to lower the impedance to 45 ohms to provide a better match to the third harmonic. The physical layout of the multiplier is shown in Fig. 8.