Super VXO shifts 80 kHz on 80 metres

A demonstration of how 3.579 MHz TV colourburst crystals can be pulled to cover the CW end of 80 metres, all the way down to 3.500 MHz. Use is made of a 'super VXO' circuit with multiple parallel crystals and carefully selected inductors to achieve this amazing range. The ideal could be useful to convert crystal locked QRP rigs to be more frequency agile and thus more versatile.
Frequency stability checks: tested on 3.530 MHz.
0 min 3.530.0
5 min 3.528.5
15 min 3.527.5
40 min 3.527.2
60 min 3.527.0
75 min 3.526.9
85 min 3.526.9
This VXO needs to be warmed up for 15 - 30 min until it settles down. Buffering also needs to be much better than with a crystal oscillator otherwise the keying will be chirpy.
To clarify, the zener in the diagram is 9.1 volt. Also note the error in the circuit diagram around the two 470 pF capacitors. The top 470pF capacitor shown should be replaced by a wire link with it connected between base and emitter instead.
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[@vk3ye]

@SzpakBarwnyPtak Did a test and put in in the text above. It drifts a lot in the first 15 min but settles down, espeically after about 30 min. A better internal box and voltage regulation would help. On key down it shifts by about 1.5 kHz which indicates significant pulling (it's too much for a proper tx offset). So an extra buffer is suggested.

[@vk3ye]

@Hogkat Good question - never tried series xtals. Just google 'super vxo' in images and you'll find many circuits of super VXOs using 2 or more crystals.

[@monarchham]

Well done, Peter! The 47pF parallel cap could be switched in and out of the circuit giving even more freq agility? I like to work between 3530 and 3560kHz. In my junk box is a bag of 100 color burst crystals so I will attempt a build. Thanks!! Herb/WR9H

[@poikaa3]

Nice Peter! Looks like careful selection will give good results.... Does the orientation of the inductors make a difference? 73 Rod KB8DNS

[@monarchham]

Thanks for the reply, Peter. Have you seen Jim's/K8IQY's new SAVXO oscillator kit sold by 4SQRP? 72 Herb/WR9H

[@vk3ye]

@poikaa3 Inductors are not polarised, so no difference.

[@vk3ye]

@monarchham Herb - let us know how you go - would be great to see a video when done.

[@raybuck9675]

I believe the schematic at 0:46 is incorrect. Shouldn't the top 470 pF capacitor connect to the base of the transistor? The center tap of the two capacitors would connect to the emitter putting the lower capacitor in parallel with the 1K resistor. I wired the circuit up the way it is shown with a 10 MHz crystal and it would not oscillate. I then turned to my copy of EMRFD to see how a Colpitts oscillator is supposed to be wired. I corrected the error and the circuit immediately started working. Ray, AB7HE

[@vk3ye]

@@raybuck9675 Thanks Ray. You are correct.

[@Hogkat]

Nice done Peter. Got a question though I see you use crystals in parallele. Now why not in series? I have looked all over for information about series and parallele connected crystals but can't find any thing. Maybe you can help? WIth a link or something............. 72 PA1TAS

[@af4k]

A drifting VXO can be worse than a regular drifting VFO. Not going for this. 50 year-old Drake VFOs don't drift much! ALSO - the SAVXO kit only covers 8 kHz. Not great. A DDS-VFO is what you need.