Circuit Crystal Oscillator/Amplifier

  
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This oscillator is a variation of the oscillator presented by Ulrich L. Rohde, DJ2LR, in his article `Evaluating Noise Sideband Performance in Oscillators`, Ham Radio, October 1978, Page 51. The original circuit is at the bottom of the web page `4 MHz Oscillator` in the Super Receiver section of this web site. The article states the above oscilla
Circuit Crystal Oscillator/Amplifier - schematic

tor is a "suitable low-noise crystal oscillator circuit with a wideband postamplifier that delivers the required +17 dBm output level or slightly more. Any inductive-mode crystal between 400 KHz and 30 MHz can be plugged into this circuit and give useful output without any adjustments. " "The noise performance of the crystal oscillator circuits. is better than 120 dB/Hz at 1 KHz from the carrier, and 150 dB/Hz or more at 20 KHz from the carrier. Because of their excellent noise performance, these circuits can be used as local oscillators without degrading receiver performance; very few oscillators and practically no frequency synthesizers achieve their low-noise sideband levels. " Update: Email correspondence with Dr. Rohde (April 2008) shows that even better performance is now available. Dr. Rohde wrote that ". at 5 MHz my oscillator with a 2008 quality crystal, Q=2. 6 E6, Cs=8ff, and some modifications gives a superb phase noise. " See curve below, much better noise now. Diode switching is used to switch the crystal frequency. Regular LEDs were found to work just as well as 1N914s, used in almost every crystal switching circuit in Ham radio literature. The advantage of using LEDs is the visual indication of which oscillator is on, helping with diagnosis in case the switching circuit is not operating properly, or in the case of this receiver, ambient light is interfering with the proper operation of the phototransistor....



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