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Some experiments with Doubler Variable Frequency Oscillators (VFO)

Getting into HF is easy and challenging simultaneously for modern homebrewers ! The difficulty lies with the fact that almost all modern ham HF QSOs occur in SSB and homebrewing SSB rigs is a bit tricky . However it is easy to get a large amount of resource from internet regarding the various transceiver circuits , test gigs etc .
40m (7MHz)  is a ham band that is still very active . However all my attempts to make a rig for this band failed due to VFO problems. Browsing the web one of the circuits that grabbed my attention is an old one - Doubler VFO by VU2ITI - OM Mani .( See here www.qsl.net/vu2msy/homebrew/Ceramic_Resonator_VFO_VU2ITI.pdf)  It a great circuit and the concept is beautiful . Make a ~3.5MHz sine wave , use a multiplier (a nonlinear circuit) , get a 7Mhz output ( and perhaps some undesired harmonics Thanks to OM Joseph Fourier , HI !) . This sounded  beautiful as 3.58MHz ceramic resonators are available in plenty  in my hometown ( Payyanur) ; in stark contrast to 7Mhz resonators which are not available. Using 3.58MHz resonators to make VFO is easy as using a series variable capacitor , its frequency can be swung .
(Details on using capacitors and inductors to change frequency of crystals can be found in this video by KF5OBS  https://www.youtube.com/watch?v=_OsvXQe_l2E  It is similar for resonators but resonators are not as stable as crystals and thus can be pulled very much . For more theoretical work on this you can refer Modern Comm Circuits by Jack Smith ) . Multiply it by 2 , bravo ! you get a 7MHz signal .

However things were not easy for me . The circuit did not give me the required output straightaway , perhaps due to fault with my style of construction . Also my frequency counter was (and is ) not working properly. So I decided to modify the VFO circuit , modified it and it worked like a charm ! ( I will post the modified circuit diagram soon )
I hope to make a DSB rig out of it .To that end I already conducted some experiments with this VFO.  I added a single balanced mixer ( more about mixers http://www.qsl.net/va3iul/RF%20Mixers/RF_Mixers.pdf )to its output and connected a long wire antenna ( WITHOUT ANY FILTERING !) to the other end of the mixer  after a single stage of amplification using an untuned amplifier based on BC548  . The output of the mixer was connected to a computer speaker which has an amplifier built into it after a single stage of amplification ( again using BC548) . The output was encouraging ! I could receive some strong Chinese broadcasts and Vatican radio which happen to broadcast on harmonic frequencies of 3.5MHz ( no filtering !) . Also at one end i could copy the VWN net ( A 40m ham net in India ) . So far this is the progress that I made with doubler VFOs .
However,this VFO has some stability issues. The frequency drifts quickly.
Here are some pics of the experiments . I know that they don't look great but to add some color to this post I'm adding them :)  73s to all !

The VFO before adding the mixer

BFO after adding mixer and some mods

VFO+mixer


Comments

  1. DHANESH PALLIKARADecember 1, 2015 at 10:41 PM

    Nice work Jishnu.... Congrats... I am on the way to hf with the help of ke3ij... and i hope your support ... :)

    ReplyDelete

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