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RF Oscillator & Buffer Amp

Discussion in 'Homebrew and Kit Projects' started by KB1LQC, Jul 30, 2008.

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  1. NA5Z

    NA5Z Ham Member QRZ Page

    For a gate inverter at that frequency, I'd try a 74AC04. Put something between 100K to 1M resistance from the output to the input. Hang the crystal from output to input also. Then hang a shunt cap at each pin of the crystal with a value slightly under 2x the crystal's load capacitance spec. Should work like a champ.
     
  2. KB1LQC

    KB1LQC Guest


    I really dont have the capacitance to try this, I only have 1 or 2 PF caps or 30 pf caps and maybe one or two of each. and the EMRFD really does not explain how to properly bias a FET amplifier which is kind of discouraging. I was hoping to experiment with a MPF102 which I have on hand.
     
  3. WY6K

    WY6K Ham Member QRZ Page

    The coupling capacitor should be big, say .1uf, so that it does not attenuate the signal or add any source impedance to the input of the buffer amp. Ideally, it will have zero impedance (reactance) at the operating frequency.

    I have put up on my web site the circuit I built and simulations of a circuit that seems to accomplish your goal, along with my design rationale. It is not finished, but you can see the values I used etc. Check it out at:

    http://www.wy6k.com/wy6k/homebrew_stuff/small_signal_rf_amp.htm
     
  4. KB1LQC

    KB1LQC Guest

    WOW, thanks for writing up the whole webpage on this topic! I have only read through the first couple paragraphs (I am at work) so I will read the rest later tonight! 5Spice finally loaded in so judging by the page you made, it looks really nice to work with.
     
  5. WY6K

    WY6K Ham Member QRZ Page

    I'm enjoying it! I will expand it more tonight. There's a bit more to be done on it.
     
  6. KB1LQC

    KB1LQC Guest

    I am playing around with 5Spice and cant seem to find a library online for the PN918. Has anyone else found one? I am currently getting libraries for the 2N3904 and the PN5179.
     
  7. NA5Z

    NA5Z Ham Member QRZ Page

    I looked in my list and don't have one.

    You can try hunting in Google with a wildcard in front of 918 to see if another manufacturer made it with a different prefix like "MMBT", etc. Or try to dig up a cross reference chart that way?
     
  8. WY6K

    WY6K Ham Member QRZ Page

  9. KB1LQC

    KB1LQC Guest

    Wow, simply wow.... Awesome description! All the tricks and signs of experience with designing RF amplifiers within the web page really show! You are also getting me into simulating the circuit with 5Spice. I had access to Orcad Pspice during school when they are introducing it to us (I had played around with it prior to college, but nothing in depth). It simply will not install correctly for me and many of my classmates so I never really got into it. This program seems to be increadibly nice! The three transistor design certainly does look like it is robust, I am in the process of inputing the schematic into 5 spice and will play around with some values to see what happens. Once I order the parts I will definitly build an amp with these transistors. Now... what other parts do I buy to make a 5 doller mouser shipping fee worthwhile for 20 PN5179 transistors @ $0.08 a piece? HIHI!
     
  10. WY6K

    WY6K Ham Member QRZ Page

     
    Last edited: Aug 13, 2008
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