Saturday, January 11, 2014



Xtals are used from the begining of radio era, as a very stable fixed frequency oscillators. They are easy to use, but when time comes to park them within a few KHz of their frequency, it is a different matter. Now, we have a method to shift the frequency without affecting the stability of it. It is called variable xtal oscillator or VXO. And there is another sub variety also, which is called Super VXO -with greater shift in the frequency.

Our concerne here is to shift the VXO frequency, so that we are able to receive the CW and SSB signal in the receiver when Xtal (ladder) filter is used. 

I have used the collpitt oscillator.  The ckt are given in figure below. fig-1. Only difference between normal collpitt oscillator is that a trimmer or a varable capacitor is put there. wchich shifts the freq to upper side. can be used as a LO of USB. Lower the capacitance higher the frequency. increase the capacitance and frequency decreases. 

In low freq range generally shift of around 400Hz is observed for 4Mhz xtal . High freq xtal of10Mhz and up have 4000 Hz shift.  For the 28 mhz fundamental xtal shift might be much higher. With super vxo these shift can be increaed further.
In fig.2 . Inductance is used to pull the frequency a little lower. (Lsb).
If we are using this as BFO without inductor and freq is 10000 kc we can pull it to 09999 kc using inductor, so we got a 1 Kc shift for cw receiving. By using the variable capacitor (generally called gang) or trimmers osc freq can be increased in updirections. (Usb) Please not that if you have xtal of 14318kc it is oscillating between 14313.6 kc and 14316.6 kc. now lower limit can be made 14313kc and higher limit can be around 14316 kc. see the difference of freq remain same but only shift in the range usable. Please not that it can not increase beyound the freq marked on Xtal .

How much your xtals will pull ? Test it by connecting the vc and xtal and no inductance and see FIG.1 and measure the shift in freq. This is the range you will get from the design and nothing more. If use two xtal in parallel freq. range will increase slightly. Use of the inductance has the limit, if you use large inductance, unshielded inductance then is does not remain vxo it become LC oscillator. You can use any Coil without internal capacitor. i used 10MHz if can, by crushing its internal capacitor. Please ground the xtals and cans body. Also use use the some knob for variable capacitor if using gang. due to close proximity to the gang there was slight change in freq observed in counter.

parts list:-R1-100k, R2-1k, R3-100 ohm, C1 &C2 about 100pf,it depends on the type of xtal holder hc-69 and halfsize chinese hc69 likes the 100pf and on lower freq say 3.579, 4.000 mhz xtal you can use 1000pf if xtal is big one. C3,C4-0.1mfd (104). regulated 12 v dc from LM7812 is used. L is generally found SIF of tV or you can try anything which has small inductance. air core is not tried. This osc stage feeds its output from emmiter to the buffer amplifier which is not shown here but can be found in blog somewhere.You can measure freq at emitter of this oscillator. Xtals are fundamental range over tone may not work.

thats it folks.
wheel is re-invented ! Now, enjoy.