The Super VXO uses two X-tals of the identical nominal frequency in
parallel instead of a single X-tal of a conventional VXO. Nothing else
is special. It can pull considerablly more frequencies than the
conventional one. About 40kHz for a pair of 10.15MHz X-tals, and 90kHz
for 13MHz, according to my experiences.
The Super VXO was invented and named by JA0AS(Mr. Shimizu, Silent Key) and JH1FCZ(Mr. Okubo). An article on experimental results of the Super VXO first appeared(in Japanese) in the August 1980 issue (Number 64) of "Fancy Crazy Zippy", an HB- and QRP-related periodical magazine published by JH1FCZ. The story of their invention of the Super VXO is reported in a book(written in Japanese) entitled "Textbook for Homebrewing of Electronic Circuits" (the original title is in Japanese.) written by JH1FCZ.
The followings are my experimental results.
(1)Two 10.15MHz X-tals in parallel + 15uH inductor + 20pF poly-variable cap. covered the range of 10.10 - 10.14MHz. I built a 30m direct conversion QRP XCVR with it. Frequency is very stable. The transistor used is 2SC1815, a common general purpose small signal TR with fT=80MHz.
(2)Two 13MHz X-tals in parallel + VXO-50 coil(see below) + 30pF air variable cap. gave the range 12.91 - 13.00MHz. The transistor used is again 2SC1815.
The VXO-50 coil is designed specially for VXO for 6 meter rigs and sold by the FCZ-lab, a company owned and run by JH1FCZ(they sell many kits and special parts for HAMs). This coil is slug-tunable with a core of small temperature coefficient. The inductance ranges between 7 and 11uH.
This Super VXO is used in my 6m QRP SSB super-het XCVR. The VXO freq is multiplied by 3 and used with IF of 11.2735MHz to cover the freq range of 50.00 - 50.27 MHz (270 kHz !). Frequency is acceptablly stable but not so stable as the above example of 10.15MHz. A change in voice pitch is audible in a long QSO.
I have results also on 20m CW XCVR and 40m CW XCVR. Refer to descriptions for those rigs.
A X-tal with a frequency below 10MHz is hard to pull by more than 50kHz even with this method if you need stable freq. The lower the frequency, the narrower the range. Therefore, for lower bands, you need to choose appropriate IF to cover the wide freq range with Super VXO of higher freq.
A 3rd over tone X-tal can be used at its fundamental frequency.
The frequency range could be made wider with larger inductance value, but the frequency stability gets worse rapidly with increasing inductance. JH1FCZ recommends in his book mentioned above that one should keep the freq range within 0.5% of the nominal freq to maintain the freq stability, namely 50kHz for 10MHz for example.
I tried super VXO's with various types of inductors of the same value, and found that inductors with larger physical size seem to work better. Those who want to experiment this method may need to try with as many types of inductors as possible. Increasing the bias current of the TR may also help in case it does not oscillate.
As you vary the frequency, you might observe a sudden skip of the frequency with hysteresis. This phenomenum can be cured by putting a 10- to 30- kiloohm resistor in parallel to the inductor.
You might want to use a surplus square canned slug-tunable coil of an appropriate inductance for Super VXO, but the freq stability depends on the temperature coefficient of the core material used. Commonly used core materials seem to have large temperature coefficients.
I have no experience of Super VXO with an iron powder troidal core inductance, but proper choice of the core material is also the key in the sense of the freq stability.
Experiment, experiment, and experiment!
No reward without experiments.