Voltage controlled oscillator - where is this hideous harmonic coming from?

Discussion in 'Wireless & RF Design' started by Spottymaldoon, Mar 17, 2016.

  1. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
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    The attached voltage controlled oscillator circuit uses a Hartley configuration and a NE85633 VHF NPN transistor. The tank circuit consists of two simple air-core coils of 33nH each and a 5-20pF varicap (load inductor L5 is 100nH not nF as shown) 2000 copy.jpg uses

    What I get is the horrible waveform shown! I am aiming at the 100MHz range and, sure.it's in there, but where-oh-where is that ~7.7Mz monstrosity coming from please? I can't see which components could be resonating down there!
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    That's not a harmonic...it's a parasitic....probably due to power line coupling. Try some different values of VCC decoupling capacitors.
     
  3. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
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    I thought I had it decoupled to kingdom come with that big choke and 10uF and 1nF capacitors thrown in! But I will definitely play around with the decoupling and report back either way - thanks!
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Hang a scope on the Vcc line. If it's not pure DC, you know your problem! :)
     
  5. KL7AJ

    AAC Fanatic!

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    If you don't get anywhere with decoupling, you can decrease the emitter bypass capacitor, reducing its gain at lower frequencies.
     
  6. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
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    I just did that - there is NOTHING getting past the capacitors but there is a gorgeous and perfect little 100MHz signal on the regulator side of the big choke - I shall put a capacitor before the choke which I think I should have done in the first place - thanks again! Will report.
     
  7. KL7AJ

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    I suspect you can drop the emitter resistor to 10 pF or so.
     
  8. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
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    I realize that in the above circuit I have left no path to ground following the 100nH load but instead the effective load has 68uH in series with 100nH!
    Tomorrow I shall pop a cap in to give a ground after the proper load - thanks for the suggestions, they forced me to review my decoupling and if the problem persists I shall reduce that emitter bypass capacitor as you suggest although I hate to drop the gain.
     
  9. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    By the way, what kind of oscilloscope are you using that you can see 100 MHz directly?
     
  10. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
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    It's a Hantek DSO4102B rated 100MHz but I have seen strong 200MHz on it - presumably with big attenuation? Thanks for your interest - I will report the outcome.
     
  11. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    I presume that the bottom end of the emitter resistor is grounded, along with that entire lower bus. It's shown floating on your schematic.....be sure that really IS ground.
     
  12. panic mode

    Senior Member

    Oct 10, 2011
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    can you post photo of how the circuit is actually built?
    where exactly is power connected? and L5 is interesting, where can you buy 100nF inductor?
     
  13. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
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    Thanks for lending a hand - sometimes I overlook really obvious things but in fact that bottom rail is grounded. I agree a 100nF inductor would be a pretty useless item for most purposes - luckily it is actually marked 100nH is an air-cored coil and comes from Digikey. The power comes in at the regulator terminal marked "TP1"(!) and I've confirmed the regulated voltage is right. I should be more careful when I post my schematics.
     
    Last edited: Mar 17, 2016
  14. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
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    circuit for forum.jpg


    This is the circuit
     
  15. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The following PDF might be usefull for designing RF circuits:
    rf_proto-1.pdf

    Bertus
     
  16. SLK001

    Well-Known Member

    Nov 29, 2011
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    I would say that your transistor has too much gain for the frequency.

    Also, your layout is hideous. Is this a two sided board?
     
  17. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    As SLK001 asked, is it a double sided PCB?
    I have an other remark on the PCB:

    Spottymaldoon_circuit_for_forum_with_comments.jpg

    Bertus
     
  18. alfacliff

    Well-Known Member

    Dec 13, 2013
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    you might try a lower frequency rated transistor, those micorwave transistors have termendous gain at lower frequency and can be a beaar to stabilise paracitics out.
     
  19. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    That's why I suggested reducing the emitter bypass capacitor, to reduce the LF gain. Increasing the emitter resistance too, might help
     
  20. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    51
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    Gentlemen .. First of all, many thanks for your various and stimulating inputs.

    KL7AJ, blessings be upon you: your very first suggestion (which you edited away?) was to add some capacitance to ground from the base and this, combined with your second suggestion to reduce the gain (I changed the emitter bypass capacitor to make it 10x) has given me a brilliant sinusoidal output, with the detested parasitic ground into the dust, and adjustable between 75 and 135 MHz with remarkably uniform amplitude.
    You were all right too much gain.

    Nothing happened however (not even parasitic) until I added a 12pF base-to-ground capacitor (alternatively my finger, which was a clue) but HOW does that work exactly?

    This circuit was a quick feasibility thing and I apologize for offending you with the ugly board which was, in fact, single sided with an accessible ground plane behind - be assured the final version will be more politically correct.

    Also, Bertus, thanks for the referral to the RF circuit design pdf.

    I now plan to re-do this same circuit coupled (God help me) to a PD5700 hoping to boost the power to a quarter Watt or so and from that to a 5W power stage, likely using a 2N3375. You may see me here again, cap in hand - please continue to drop in a few gems!

    After that I hope to make a graceful exit from VHF power design and resume my normal life - happy to have met you all!

    Kind regards and thanks again
    Dick
     
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