For my frequency heterodyning circuit concept, I needed a reference oscillator that would output a clean sinusoid at a precise fixed frequency above the audio range. Obviously, a crystal oscillator makes a lot sense, but the main problem was how to get sinusoidal output, and then at a calibrated voltage/power level. Mainly because this signal would be fed into a mixer, and the power level affects the amplitude of the mixing products.
While numerous direct waveform generation techniques were possible, and even phase-lock from a higher frequency crystal reference oscillator, all of these schemes were rather complicated. In particular, I wanted a simple circuit to exploit readily avalible 32.768 KHz tuning fork crystals, as this was a perfect frequency for heterodyning to audio. The heterodyning frequency synthesis concept is clearly RF-oriented, but I did not necessarily develop designs that required inductors for operation, esp, in the ≈︎30-50 KHz range. After studying all the classic crystal oscillator circuits, digging much deeper, I learned of circuits that used bridges in their design. This was pay dirt. After applying the Meacham bridge, I then found a variant that exploits the properties of a differential amplfier to reject common-mode signals. The following circuit used 1/2 of a OPA2227A for bench testing, because it was handy on my breadboard. But at ≈︎33 KHz, any high-gain op amp will do, DC performance is uncritical.
The circuit has rising sinusoidal oscillations that take about 30 seconds before leveling off to a calibrated level set by the potentiometer in the bridge. The bridge arms were designed around the series resistance of the crystal, as given it its data sheet.
While numerous direct waveform generation techniques were possible, and even phase-lock from a higher frequency crystal reference oscillator, all of these schemes were rather complicated. In particular, I wanted a simple circuit to exploit readily avalible 32.768 KHz tuning fork crystals, as this was a perfect frequency for heterodyning to audio. The heterodyning frequency synthesis concept is clearly RF-oriented, but I did not necessarily develop designs that required inductors for operation, esp, in the ≈︎30-50 KHz range. After studying all the classic crystal oscillator circuits, digging much deeper, I learned of circuits that used bridges in their design. This was pay dirt. After applying the Meacham bridge, I then found a variant that exploits the properties of a differential amplfier to reject common-mode signals. The following circuit used 1/2 of a OPA2227A for bench testing, because it was handy on my breadboard. But at ≈︎33 KHz, any high-gain op amp will do, DC performance is uncritical.
The circuit has rising sinusoidal oscillations that take about 30 seconds before leveling off to a calibrated level set by the potentiometer in the bridge. The bridge arms were designed around the series resistance of the crystal, as given it its data sheet.
Comments
Post a Comment