HF oscillation

Discussion in 'Wireless & RF Design' started by Shagas, Jun 30, 2014.

  1. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Hello

    So now that I have a decent oscilloscope I've been experimenting
    with higher frequency colpitts oscillators.
    I've breadboarded the following arrangement (with lower RC values in the tank circuit)
    LC-Colpitts-osc-01.gif

    I get a nice 50Mhz sine wave (after a few hours of tuning) and I have a few questions after my experimentation:

    1) How can I tell if I'm emitting illegal amounts of RF radiation? (I don't have an antenna attached)
    2a) I'm probing at the collector so is the actual frequency of oscillation (without the probes attached) higher by a few Mhz?
    2b) How do I avoid adding that probe capacitance to the circuit? A buffer stage? That adds capacitance to the first stage aswell.
    3)I'm using 5 turns of insulated 25 guage wire as the inductor with turn diameter of about 1cm. When I squash the coil together the frequency goes down and vice versa. Does squashing it together increase parasitic capacitance or the inductance?

    Thanks in advance
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    squeezing the coil increases inductance. measure the input power, suppoly voltage times current to measure the power level involved, the output will be lower due to efficency. a buffere stage or high impedance probe will help with measurements.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    When doing experiments with RF circuits, a breadboard is far from ideal.
    The capacitance of the metal strips in the breadboard will have great influence on the desired outcome.

    For good practical information on RF prototyping, read the following PDF:
    rf_proto_1.pdf

    When you use a buffer stage, the capacitance will be fixed and can be compensated.
    The output of the buffer stage will be more frequency stable.

    As for the coil, read the following page:
    http://info.ee.surrey.ac.uk/Workshop/advice/coils/air_coils.html

    Bertus
     
  4. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Thanks for the answers. I actually initially thought that it was the capacitance between the windings that increased when I squashed the coil.

    Bertus:
    I am aware that breadboarding anything to do with RF is a bad idea due to parasitics.
    Thanks for the links. I'll definately check them out after I go get myself an ice cream.

    Concerning the RF power. I forgot to mention that was using +- 9V supply (I now changed it to 5V ). At those +- 9 volts most of the components heated up significantly so I'm not sure I can use that input power method even for a ballpark figure.

    Question 4a) When I attach my probes do they act as an antenna?
    4b) What happens If I'm using my first channel to measure the first stage collector and my second channel the output of the buffer stage , am I creating some sort of loop antenna?
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    The probes are quite short (a couple of centimeters) before entering a coax cable.
    They will hardy radiate any RF.
    Using two probes on the buffer will not create a loop antenna, as the cables of the probes are shielded.

    The following posts contains some links and a PDF on the manhattan style prototyping:
    Manhattan style PCB construction

    Bertus
     
    Last edited: Jun 30, 2014
  6. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    I agree that it is unlikely that whatever radiation you are producing is causing harmful interference. In the US these are called incidental radiators which are governed by Part 15 rules. In effect, if somebody complains, you have to do whatever it takes to mitigate the complaint. The wrong thing to do is to ignore the complaint and the letter. That will end you up in Federal Court. For an insight into FCC operations, reading the enforcement letters, one of my guilty pleasures, is both entertaining and informative.

    http://www.arrl.org/part-15-fcc-enforcement-actions
    http://transition.fcc.gov/eb/AmateurActions/Welcome.html

    One of the best ongoing sagas is that of Jack Gerritsen, ex KG6IRO

    http://www.interceptradio.com/wiki/index.php/Jack_Gerritsen_KG6IRO

    Enjoy
     
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  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I think that the workbench pages will also interest you.
    I posted some links in this post:
    RF workbench tutorials

    Bertus
     
    syed_husain likes this.
  8. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Thanks alot for the links , Bertus.
    Those workbench pages will indeed be usefull.

    That Jack fellow sounds like a pretty fun guy.

    So what you are saying is that even if I was somehow unintentionally causing small amounts of disturbances in some bands then I can get away with a warning?
    I live in the Czech republic in student dormitories.
     
    Last edited: Jul 1, 2014
  9. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    Your probe capacitance is indeed reflected into the tank and changes the frequency of oscillation a wee bit. Switching the probe to x10 will knock it down to about 2pF and it should then have little effect on the frequency of oscillation at HF, but becomes more significant at higher frequencies. At some point you would do well to make a little loop antenna to hook onto your probe and just hold it near the inductor in you tank.
     
    Shagas likes this.
  10. Papabravo

    Expert

    Feb 24, 2006
    10,148
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    I think you should research the situation in the Czech Republic. It is possible the rules are the same as in the US. Here the basic law dates from 1934. I don't have to point out to any student of European History that the 1930's was a very very different time. There could be old laws that are strictly and rigorously enforced, that have never been repealed.
     
    Shagas likes this.
  11. bertus

    Administrator

    Apr 5, 2008
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    Shagas likes this.
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