Keying transistor blues

Discussion in 'General Electronics Chat' started by solder_pat, Dec 2, 2010.

  1. solder_pat

    solder_pat Thread Starter New Member

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    Hello gang, I'm kind of stumped here. I put together a tiny little one transistor crystal oscillator. It's keyed by grounding the emitter and it works. So I decided I'd key it using an arduino micro controller and send some pulses to it via a 2N3906 transistor.

    I would then tie the arduino LED pin via a 1K resistor to the base of the 2N3906. It works, my system voltage is 7.2 volts and the arduino is clearly sending turning the transistor on grounding the emitter of the oscillator I get morse code.

    But when I up the entire system voltage to 12 volts, immediately the circuit begins oscillating. It's not drawing full current mind you only 10mA as opposed to the saturated 50mA, but it's turning on. I can't seem to figure out why, but it's as if the collector emitter junction is allowing current through regardless if the base is tied to ground or biased.

    Thanks,

    Pat
  2. thatoneguy

    thatoneguy AAC Fanatic!

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    Can you post the schematic as an attachment in .PNG format?
  3. solder_pat

    solder_pat Thread Starter New Member

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    Yeah sorry, my bad.

    And to answer anyone who has any legal questions, yes I am a licensed Amateur radio operator, yes the output is run into a 1/2w 50 ohm load.

    Attached Files:

  4. thatoneguy

    thatoneguy AAC Fanatic!

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    What frequency is it oscillating at?

    How close is the keying transistor to the RF circuit? If the wires are very long, it could be an antenna.

    What is the physical location of the 4.7uF cap in relation to the Transistor/Arduino?

    Also, your schematic shows an NPN transistor, but it is labeled 2N3096 which is a PNP?
  5. solder_pat

    solder_pat Thread Starter New Member

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    7.110 MHz

    The keying transistor is on the same board as the oscillator. It's on a double sided clad board, I'm building the circuit manhattan style, the leads of the switching transistor are less than .3 inches from the ground plane and other pads. I also grounded both sides of the copper clad board to get rid of any stray capacitance the double sided board might be introducing.

    Right now the arduino is disconnected as I'm trying to figure out why the circuit is oscillating without any of the arduino connected. I originally thought that maybe stray RF might be coupled into the base of the transistor turning the thing on.

    It's only about 1/2" away from the switching transistor, I had to reduce the value of the 4.7uf cap, because keying was very slow and wooopy, 1uf made the keying much more crisp minus the clicks.

    Yes, I'm sorry I'm using a 2N3904, not a 3906, It's been one of those days!
  6. thatoneguy

    thatoneguy AAC Fanatic!

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    If they transistor is electrically removed (short C to E w/no connection on base), does the oscillation still occur when at 12V and not at 7V?

    If not, try moving the (now) 1uF cap to be from collector of 2N3904 to GND/Emitter, it will electrically still be in the same position as shown on the schematic.
  7. solder_pat

    solder_pat Thread Starter New Member

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    Yes, oscillation occurs regardless of voltage once C to E is shorted.

    Hang on, it appears my link between the 1uF cap (formerly the 4.7 uF cap) to the bottom the emitter resistor is not there. Ugh.

    Let me warm up the iron, and fix this.
  8. solder_pat

    solder_pat Thread Starter New Member

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    Okay fixed the link, yet it exhibits the same results, 7.2 v battery no oscillation. 12 v battery it starts oscillating.

    I thought for a second, how could stray RF get into the base of the circuit if the transistor is not switched on for oscillation to occur? if I ground the base, and connect the 12 volts it will still oscillate. If I forward bias it, it starts drawing normal current.
  9. thatoneguy

    thatoneguy AAC Fanatic!

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    If you remove the transistor from the circuit, and use the standard key, does the circuit exhibit the same problem?

    We could be looking at the wrong thing here.
  10. solder_pat

    solder_pat Thread Starter New Member

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    If I remove the switching transistor, and ground the emitter resistor (of the oscillator) it keys wonderfully under both voltages.
  11. thatoneguy

    thatoneguy AAC Fanatic!

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    Do you have a 2N7000 or other FET to try in place of the BJT?
  12. solder_pat

    solder_pat Thread Starter New Member

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    Before you spend too much time on this thatoneguy,

    I think what I need to do is re-draw my schematic from scratch, as I have laid it out. I originally biased the transistor and scaled down the bias resistors of the original circuit to turn on at 7.2 volts, I've added another low pass filter to the output stage, and a varactor diode for the trimmer as connected to the crystal.

    Yet I fail to see how just a few volts higher would allow the switching transistor as it's inserted into the circuit to start passing current to the emitter resistor. I'll spend some time redrawing the schematic and we'll go from there.

    Sometimes you need another voice to help crowd out your own (ugh tunnelvision).
  13. solder_pat

    solder_pat Thread Starter New Member

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    I have a MPF 102 or two laying around? Are you thinking about the gate/base emitter junction resistance?
  14. thatoneguy

    thatoneguy AAC Fanatic!

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    Not sure yet. A 2N2222 would be nearly as valid of a test for swapping.

    I don't like 2N3094's personally, call it a grudge.
  15. solder_pat

    solder_pat Thread Starter New Member

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    Well you're in luck because I in a moment of fog stuck way more than six volts on the base of one while fidgeting around with the circuit and knocked it's everloving socks off. I'm now on 3904 number two.

    I'm most likely sure it's my brain at fault. I'll sit down after detailing the circuit schematic wise and we can all then take a good crack at it. The whole objective here is to build a QRSS 30meter beacon. I'm just toying with a 40meter beacon because that's the only crystal I have handy and I'm waiting on the 10.140 MHz crystal.

    That and the 3906/3904 confusion came from my eyesight. God what a difference 10 years makes.

    I opted originally for a 7.2 volt system because that was the NiMH pack I had originally designed the circuit around. But now as I'm seeing the pack is not living up to standards I'm opting for a 12V SLA contingency.

    Thanks for the help, I've already isolated about two ooopses since we've started!
  16. eblc1388

    eblc1388 Senior Member

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    It is possible that the keying transistor is being turned on via the RF oscillation.

    To counter this effect, you can try adding a 10K in parallel with a 0.02uF from the base of the keying transistor to ground.
  17. tom66

    tom66 Senior Member

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    I'm not certain but shouldn't the transistor be an 2N3904 instead of a 3906?
  18. BillB3857

    BillB3857 Senior Member

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    Just an off the wall suggestion. Try putting a resistor from the base of the 2n3904 to ground. A 1k or 2K should do it.
  19. BillB3857

    BillB3857 Senior Member

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    Just a suggestion. Try adding a resistor from the base of the 2N3904 to ground. Maybe a 1K or 2K.
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