23 January 2012

App Note 85

"Low noise varactor biasing with switching regulators: Vanquishing villainous vitiators vis-à-vis vital varactors." 24 pages.

This app note discusses low-noise bias-voltage generators for varactor diodes. The main application, as shown in Figure 1, is VCO tuning for phase-locked loops. This application is extremely sensitive to power-supply ripple, as any corruption of the bias voltage will cause spurs in the VCO output. In the circuit shown in Figure 6, careful power-supply design and appropriate filtering produce the varactor-bias voltage shown in Figure 11, which exhibits only 20 microvolts of ripple and noise.

However, the major emphasis of this app note isn't the circuitry, but the instrumentation (this topic is well-worn territory for Jim, of course). Measuring 20 microvolts of ripple and noise is HARD, and Figures 12 through 17 show several ways that improper measurement technique ruin the measurement. Figures 19 through 24 show frequency-domain measurements of the VCO output using a spectrum analyzer. Again, it is shown that improper measurement technique, or careless construction and layout, degrade performance significantly.

Appendix A is a primer on varactor diodes written by Neil Chadderton of Zetex.

Appendix B discusses amplifier and oscilloscope selection in order to facilitate the 20-microvolt sensitivity needed for Figure 11. This appendix is borrowed from Appendix B in App Note 70. I can't resist quoting the footnote again:
In our work we have found Tektronix types 454, 454A, 547 and 556 excellent choices. Their pristine trace presentation is ideal for discerning small signals of interest against a noise floor limited background.
Appendix C is copied from App Note 70 Appendix C (as he explains in the footnote).

The app note ends with a varactor cartoon. Probably the only varactor cartoon in the world. "I never had it so good."

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