R

from Audio Phase Shift Network

R3 rfi Phase Adjust

R3 rfi Phase Adjust s-

from Audio Phase Shift Network

W\A Amplitude Adjust

Fig 10—Audio amplitude and phase-shift adjuster.

phase shifter or the quadrature hybrid for connection to an RF port. Don't use a tee network because its inductive output—which was advantageous for driving an LO port—can forward bias the mixer diodes during portions of each LO cycle when there is little, or no, diode current from the LO.

The big disadvantage of placing a phase shifter in the RF path is that you lose the benefit of the DBM's insensi-tivity to LO-drive level. Thus when you change frequency, the level of sideband rejection will change more rapidly than with the phase shifter in the LO path.

Phase Adjustment at Audio Instead of at RF

In any phasing exciter or receiver, you must adjust both the amplitude and phase to get good opposite-sideband rejection. Conventional wisdom makes the amplitude adjustment in the audio circuit and the phase adjustment in an RF phase shifter. To adjust the phase at RF requires adjusting C or L in the discrete-component phase-shift networks or the physical length of the transmission-line phase shifter.

Phase can be adjusted in the audio circuits by summing a small quadrature signal with the in-phase signal. This can be accomplished in the por tion of the audio circuit between the IF ports of the DBMs and the audio quadrature phase-shift networks, for either receiving or transmitting. Fig 10 shows an audio phase-shift trim circuit for an exciter. One for a receiver would be similar.

Simulation Details

The mixer modeling uses the full nonlinear model for MBD101 Schottky diodes along with ideal transformers. The DBM is connected as a modulator, with 10 mV dc behind 50 Q, applied to the IF port. The RF port is terminated with 50 £2. Transient analysis is used, followed by Fourier analysis to extract the fundamental amplitude and phase from the RF output waveform.

For analyses of the quadrature hybrid alone, not connected to the DBM, ac analysis is used to show amplitude and phase versus frequency. PSpice Evaluation Version 5.3 was used for the simulations. To produce the published curves the .PROBE/CSDF option of PSpice was used to produce text-format output. These files were then reformatted using gAWK1 3.0 into a form that could be imported into Borland Quattro Pro 4.0 and printed.

Acknowledgements

Rick Campbell, KK7B, inspired the project through his QST articles and helped via encouraging letters over a period of many months. Walter Banzhaf, WB1ANE, introduced me to PSpice.8 Nat Sokal, WA1HQC, an ARRL TA, gave me information on quadrature hybrids from his library

Motes and Bibliography

1 R. Campbell, "A Multimode Phasing Exciter for 1 to 500 MHz," QST, Apr 1993, pp 27-31.

2 R. Campbell, "High-Performance, SingleSignal Direct-Conversion Receivers," QST, Jan 1993, pp 32-40.

3 G. Breed, "A New Breed of Receiver," QST, Jan 1988, pp 16-23.

4 PSpice 5.3, Evaluation Version, MicroSim Corporation, 20 Fairbanks, Irvine, CA 92718.

5 J. Cappucci and H. Seidel, "Four Port Directive Coupler Having Electrical Symmetry With Respect to Both Axes," US Patent 3,452,300, June 24, 1969.

6 R. Fisher, "Twisted-Wire Quadrature Hybrid Directional Couplers," QST, Jan 1978, pp 21-23.

7 R. Withers, "gAWK", C Users Group Disk #333, R&D Publications, PO Box 3127, Lawrence, KS 66046.

8 W. Banzhaf, "Applications of PSpice for the

Radio Amateur," Proceedings of the Eighteenth Eastern VHF/UHF Conference, West Hartford, CT. May 23, 1992, published by ARRL. CD

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