Crystal: checker

2013/7/14

i have bunch of  crystal which don't have any  mark on it.   Or some time i need to know  if it is a overtone crystal.

so i need a crystal checker use fundamental mode oscillator.  The requirement is, oscillation from 1Mhz to 40Mhz ( i did find a 36Mhz fundamental crystal, may be it's not, but always start at 36Mhz on a fundamental oscillator)

Choose a Oscillator

<<EMRFD>>   given several oscillator of crystal.  i think the popular  Colpitts version is suit for such a simple device.

The Fig4.23  is identical to right one if you remove L, CV from Fig4.30.  

Fig4.23: 2-20Mhz fundamental mode.  Q1 2N3094.

Fig4.25: 2-20Mhz fundamental mode.  Q1 2N3094. C1: 10-20pF   output: few milliwatt.  a variation of Colpitts.

Fig4.30: VXO

Build the Tester

I actually built my version start from Fig.4.30, but NOT a VXO,  remove L,CV,and no buffer. but insert a 16:1 impedance transformer(8T:2T) to emitter of oscillator. I use a very cheap ferrite Toroids: which extract from a fluorescents light, this just a checker, so such a toroids good enough work from 1Mhz to 30Mhz.   8T in primary about 170uH. 

for high frequency: 4-40Mhz

change 2k emitter resistor to 330R(1k might work not test), which about 10mA.  C1, C2: 200pF

for 1Mhz to 8Mhz

to reliable start, around 1Mhz,  this emitter resistor must at least 1k, or more, otherwise low frequency not work.  C1, C2: 1000pF