23 September 2011

App Note 43 part 2

The second major part of this app note discusses low-distortion sine-wave oscillators, inspired by his obsession with the HP200A oscillator.

He has discussed this topic before, of course. In 1981, he wrote National Semiconductor App Note 263, "Sine Wave Generation Techniques", which includes a wide variety of schemes, including the Wien-bridge approach (see NSC App Note 263, Figures 2 and 3). He uses a Wien-bridge oscillator, without much comment, in Figure 10 of LT App Note 3 (compare LT-AN3 Figure 10 with NSC-AN263 Figure 2b). In App Note 5, Figures 12, 13, and 14, he discusses a Wien-bridge oscillator with the light bulb for amplitude control (these figures are reproduced in the present app note). He also references Bill Hewlett's thesis (see App Note 5, Reference 5).

In this app note, the discussion of oscillators starts on page AN43-27, with a simple multivibrator in Figure 32, and a quartz-stabilized version in Figure 33 (quartz-stabilized oscillators are discussed in App Note 12, but not using bridge circuits). A #327 lamp is added in Figure 35 for amplitude control, and a common-mode-suppression loop is added in Figure 37.

The Wien bridge is introduced in Figure 39 (compare to Figure 2a in NSC App Note 263). Figures 40, 41, and 42 and copied from App Note 5, Figures 12, 13, and 14, using multiple lamps to increase the thermal time constant and reduce the distortion at low frequencies. This approach, as explained in Appendix C, is the basis of the HP200A oscillator.

Figure 43 begins the hunt to replace the light bulb. Using a FET as a simple voltage-controlled resistor increases the distortion by fifty times. As he says in the footnote, "What else should be expected when trying to replace a single light bulb with a bunch of electronic components? I can hear Figure 39's #327 lamp laughing." Figure 45 corrects the error (due to channel-length modulation), and Figure 47 replaces the FET with an optically driven CdS cell. Figure 48 further improves the distortion with a common-mode-suppression loop (a similar improvement was seen between Figures 35 and 37). The distortion is now below 3 ppm, below the uncertainty floor of his distortion analyzer. (This section is the basis of one of his book chapters, so we'll see this material again, soon.)

The app note concludes with a synchronous-rectifier AC-to-DC converter in Figure 50 and 51, which are copied from App Note 13, Figure 36 and 37.

Best quote (from page AN43-29): "History records that Hewlett and his friend David Packard made a number of these type oscillators. Then they built some other kinds of instruments."


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