I'm trying to make an 8.192 MHz VCXO using a JFET Colpitts circuit. I had a
100pF trimmer and a varicap in series with the crystal between gate and ground, and I was expecting the tuning range to centre on the crystal frequency; but the output frequency was too high. I tried it with 6 MHz and 7.3728 MHz crystals from my junkbox and these worked fine: I could adjust a few hundred Hertz either side of the nominal (marked) crystal frequency e.g. 5.999900 to 6.0000100. I tried changing the Colpitts capacitors and even removed all other series capacitance, but the 8.192 MHz crystal refused to oscillate below 8.193 MHz; so I tried series inductance and finally it worked. Is this because I have a mixture of series and parallel cut crystals here? The hobby supplier I bought the crystal from doesn't even specify load capacitance, let alone cut, so it's just as well that I prototyped the oscillator before doing my board layout. Any tips?TIA