VCO without a varactor?

Discussion in 'Wireless & RF Design' started by BlackCow, Mar 25, 2012.

  1. BlackCow

    Thread Starter Active Member

    May 11, 2009
    65
    1
    I want to build a stable FM modulated transmitter and receiver with a VCO controlled with a PLL (some new concepts I am learning and quite excited about).

    My issue is all the VCO circuits I have read about use a varactor (which makes sense). My issue is that Jameco (where I usually get my parts) doesn't seem to have varactors and I don't know where else to buy them.

    So are there any VCO designs that don't require them or, better yet, can someone tell me where to get my hands on some varactors?

    Thanks!
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    Many common diodes also have the capacitance change.
    See this quote from the wiki: http://en.wikipedia.org/wiki/Varactor

    Bertus
     
  3. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
    any standard reverse biased PN junction can be used as varactor. before i could get my hands on proper varactor i was even using BC of general purpose BJT transistor (BC109b) as a substitute.
     
  4. BlackCow

    Thread Starter Active Member

    May 11, 2009
    65
    1
    Sounds like this will be fine for my experimentation needs.
    I'm still wondering, where do you get your hands on a proper varactor?
     
  5. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
    you are in usa, so you have many online suppliers at your disposal (digikey, newark, mouser...)
     
  6. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    I wouldn't use a varactor circuit in a PLL design anyway. Use a real VCO.
     
  7. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
    how do you build real RF VCO without varactor?
     
  8. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    There are any number of ways to build a VCO. A goggle search for "Voltage Controlled Oscillator" truns up many circuits as well as Integrated VCO chips. Like this very very simple example:

    http://www.falstad.com/circuit/e-vco.html
     
  9. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
    but that is not an RF circuit.
     
  10. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    It could be, with careful design. And it was but a sigle example. There are many, many more.
     
  11. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
    then i couldn't find them, every single one i came across ultimately uses varactor.
     
  12. BlackCow

    Thread Starter Active Member

    May 11, 2009
    65
    1
    This is my experience as well.
     
  13. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    A simple 555 is a VCO. It could be frequency converted after the fact. I list this as one possible example.

    I have seen simple FM transmitters that use the PN junction of the transistor. This is another.

    Define what you consider RF? 100Khz is RF under the right circumstances.
     
  14. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
    Bill, that is true, few articles such as this:
    http://en.wikipedia.org/wiki/Radio_frequency
    also consider mere 3Hz to be RF, but we all know this is much of a stretch and hence not answering my question. i think we can say 0.3Hz is also RF, it's just that nobody ever built an antenna for it.

    to me 'real RF' is what is commercially and commonly used in radios - 0.5MHz to tens of GHz (and beyond of course - even if it is not common).

    for example, i can walk into ANY electronics store and:
    - get various 'radio' products (common),
    - most if not all using PLL (common)
    - all of which are in 'my' RF range

    that includes receivers for AM, FM, TVs, cell phones, WiFi products, you name it. and i'd say it is a safe bet that none of them are using 555 as VCO. (any takers? ;) )

    one of earlier comments suggested that varactors should not be used because all real VCOs don't use varactors. i am still waiting to see an example of such real and commonly used VCO. everything i came across that says 'radio' and 'PLL' or 'VCO' is varactor based.
     
  15. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Here's a variable freq oscillator using magnetic frequency control;
    http://www.hanssummers.com/huffpuff/fast.html

    I quote his page;
    "When a toroid is placed in a magnetic field, its permiability decreases, causing a decrease in inductance. A varying magnetic field can be generated by applying a varying voltage to a relay coil. In this way a correction can be applied to the VFO by the stabiliser circuit, with the relay coil and magnetic field replacing the conventional varicap diode which you see in all the usual Huff & Puff circuits."
     
  16. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
    thank you,

    this looks better. it may not be as compact as VCO with varactor but this is viable solution. i like the fact that it is linear.
     
  17. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    You could probably also adjust frequency by puttin a second winding on an inductor and applying DC to the second winding (magnetic amplifier style).

    Or one that has interested me; if the oscillator is subject to change frequency as PSU voltage changes, then you could trim freq by making small adjustments to the PSU voltage. That gives VCO effect and also eliminates instability caused by normal PSU fluctuations.
     
Loading...