CXO: Low distortion  oscillator

2011/8/2

both <<EMRFD>> and <<ARRL hand book 2006>> emphasis  one crystal oscillator which get signal direct from the crystal.  it's worth to try.   i setup a following circuit with  NO L1, R1=10k, C1=220pF, C2=470pF. Q1=2sc9018, cystal 7.47Mhz, NO buffer Q2.

http://www.techlib.com/files/xtalosc.pdf

then we get a prety good output from crystal, at first glance. after very carefully exam, by align the peak to Y axis, move wave up and down, check the  distance between 2  X axis cross point to the zero point, there are about 1ns different between the raise edge and  falling edge.

Such asymmetric due to the even order harmonics. there's another very good experiment done by Jane, http://www.qrp.pops.net/crystal-osc-2010.asp   Jane point out if the harmonics more than -50dBc, there are visible asymmetric on Oscope.

another wave, the emitter output is very distortion(below):

fist of all, check the biasing. Fig1's basing method is not very predicable, with Q1, the 2SC9018. base get only about 1V. so i re-biasing  it  ( Fig 5. )

Just change bias didn't change anything. another noticeable configuration in Fig5 is the feed back caps, C1=2.2nF, C1=390pF.  use  such big C1/C2 value  dramatically  reduce the positive feedback,  get close to unity gain, easy to get more clean signal. I finally use C1=2200pF, C2=56pF to get a pretty good emitter output , Fig 6.

What about the Crystal output now? , still not see any observable change, 1nS distortion still there. Call for a SA.  low pass filter is mandatory to get more clean clear sine wave.