Help with Changing Crystal Frequency

Hi there guys,


I need to use a crystal at 4.26 MHz to 4.38 Mhz,however I only have 4.00,4.19 and 4.93MHz available.Is there a way I can change this easily?
LO schematic is here(scroll down to see circuit):Softrock Lite II Builders' Notes

Thanks
A

Hi,

It is possible to pull crystal oscillators, by changing the load capacitance (CL). This exploits the fact that most oscillators use the parallel resonance rather than series resonance of the crystal. Not sure it is possible with your range of frequencies, but perhaps worth a look.

The pull range of usual crystals is in a few 100 ppm order of magnitude. So it's effectively impossible to pull one of your available crystals to the intended frequency range, also a range of 4.26 to 4.38 MHz isn't feasible with a crystal oscillator.

Hi A,

FvM is correct on both issues. Changing the load capacitor can be useful for moving the frequency a few hundred ppm. The direction in which it changes and amount in each direction will depend on the load capacitance in the circuit you have. Changing the load capacitor has the undesirable effect of changing the oscillator negative resistance - which can lead to no oscillation.

The only way I can think of to get 4.26 MHz or 4.38 MHz is to use two separate crystal units tuned to the appropriate frequency (allowing for the load capacitance of the oscillator). However, I noticed that the circuit note you provided details a mixer following the crystal oscillator in order to mix two tones and filter a harmonic. If that is the case, you might be able to mix the tones obtained using a 4.00,4.19 and 4.93MHz crystal unit with an appropriate second tone, filter out the undesired harmonics, and get 4.26 MHz or 4.38 MHz. For example, if you mix 4.00 MHz with 8.26 MHz, you will get tones at 12.26 MHz and 4.26 MHz...just a thought based on my reading