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How to injection-lock a Wien-bridge oscillator

03 Dec 2012  | Glen Brisebois

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Although this FFT is accurate in some ways, a closer inspection reveals some problems. For example, the input signal is ?1 dBFS, but it certainly looks graphically lower than ?1 dB down. The reason is that even an excellent windowing function leaves some of the fundamental power in the frequency bins adjacent to the main spike. The software includes these bins in its power calculations, and rightly so, but the fact is that the spike looks too low to make a good photograph.

The same can be said about the height of the harmonics; although they are calculated correctly and are accurate relative to the fundamental, they also look too low in absolute terms. So windowing is no substitute for a coherent phase-locked system.

Figure 4: With the generators phase locked through the 10MHz reference, the low-noise and -distortion Wien-bridge oscillator is gently nudged into coherence through the high-impedance, 200k resistor.

When those objections were raised, I despaired that I was going to have to return to the drawing board and maybe stay there, or find a locked oscillator with low distortion and noise or with awesome postfiltering. How could I ever make such a fundamentally analogue oscillator coherent to an FFT bin in such an overwhelmingly digital environment? At 5kHz, a passive filter with notches would be large and fussy. I thought of detuning the Wien-bridge oscillator by reducing the gain, thereby converting it into a filter.

Figure 5: A more accurate FFT is obtained using the same Wien-bridge oscillator but with the frequency injection locked to a coherent 5.157kHz using an HP33250A driving the 200k resistor at the "new input." Note that the peak is now visibly a believable –1 dBFS and that there is almost no power in the bins adjacent to the peak.

But then it occurred to me that a gentle, analogue sinusoidal nudge from a distorting but well-locked external oscillator might be enough to tweak the Wien-bridge frequency to where it needed to be. I decided to try injecting a sinusoid into the input of the Wien-bridge op-amp circuit, and opted to use a high series impedance to avoid simultaneously injecting noise and distortion. I came up with 200k—about 1000× the impedances already there—and put it in as shown on the left side of figure 4 (the "new input"). I set up the Agilent 33250A for a 5kHz sine wave and applied it to the new input. Looking at both the 33250A and the Wien-bridge outputs with an oscilloscope, I slowly dialed up the 33250A frequency and was thrilled to finally see the sinusoids come "close" and then snap into lock.

I connected the 10MHz back-panel references and changed the 33250A frequency to 5.157kHz, the nearest coherent bin in the FFT. The sinusoids remained in lock, and the programmable 33250A generator successfully pulled the Wien-bridge oscillator slightly away from its natural frequency and into the desired frequency. The result was a nearly ideal FFT; all of the pertinent fundamental and distortion powers were situated in unique bins and were accurately represented (figure 5).

Programmable sinusoidal generators often have excellent phase-noise characteristics and 10MHz locking capabilities, but they also have high-output wideband noise floors and distortion. An FFT is sensitive to all of these forms of source corruption and also has a finite number of output bin frequencies. To test high-performance analogue and mixed-signal systems, the right combination of classical Wien-bridge oscillators with programmable generators can provide a nearly perfect source with synchronous sampling, generating accurate FFTs.

About the author
Glen Brisebois is an Applications Engineer with the Signal Conditioning group at Linear Technology in Silicon Valley. He attended the University of Alberta in Canada, achieving Bachelor's degrees in both Physics and Electrical Engineering. He attempted monastic life for several years with both the Trappists and the Carthusians, but couldn't stop thinking about circuits.

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