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DAC and two channels — Parallax Forums

DAC and two channels

BluefishBluefish Posts: 4
edited 2013-09-22 08:01 in Accessories
I apologize if this question is terribly basic--I do not even know enough to be dangerous, yet.

I am trying to measure the resonant frequency of guitar pickups. What I was hoping to do with the Propscope is to use the function generator output, then have an x-y plot of the signal before and after the pickup. It appears that I can use either channel 2 OR the function generator, but not both. Is there any way that I can generate a signal with the function generator, and use both channels 1 and 2? Thank you for any suggestions.

Comments

  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2013-09-04 13:57
    Welcome to the Forum!!

    I'm not sure about using the DAC and both channels since I don't have a Propscope, but there are other ways to measure the resonant frequency by simply oscillating the coil and measuring the frequency directly. The attached circuit will cause the pickup coil to oscillate at it's resonant frequency. From there you can use the scope to directly measure the frequency.

    Simple Coil_Inductor Oscillator.jpg
    Simple Coil_Inductor Oscillator.jpg
    1024 x 719 - 103K
  • BluefishBluefish Posts: 4
    edited 2013-09-05 06:46
    Thank you for the information and for the welcome! I will give this a try. I really appreciate the diagram--very helpful!
  • kaoskidkaoskid Posts: 4
    edited 2013-09-08 07:37
    Neat circuit! My prototype of this works from about 1.7 to 4 volts, but stops oscillating at 5 volts for an inductor with a ferrite core (saturation?), and it won't oscillate at all with a guitar pickup. Freq. out is also dependent on supply voltage. Is this a transistor equivalent of a low voltage SCR? There are 4 semiconductor layers, but it's not a Darlington pair....
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2013-09-08 14:40
    "Neat circuit! ... Is this a transistor equivalent of a low voltage SCR? There are 4 semiconductor layers, but it's not a Darlington pair.... " - Thanks!, the circuit is actually a classic multivibrator in disguise. The two transistors on the left are part of the oscillator pair, while the one on the right is in a current mirror configuration with one of the oscillator pair transistors. I'm sorry that it does not oscillate with your guitar pickup, it should if it is a straight coil, but perhaps there is something else in the pickup element.
  • BluefishBluefish Posts: 4
    edited 2013-09-22 07:44
    I had the same problem. It seemed to have no effect on guitar pickups. Not sure what you mean by there being something else in the pickup element. It is just 8000 turns of 42AWG enamel coated wire around Alnico 5 poles (I wound them myself, so I know there is nothing else hiding in there!). To answer my own question on the Propscope, you either get frequency generator or channel two but not both. Not sure why this limitation, since other similar devices allow you to use both. So, my workaround was to download a frequency generator app for my Ipad so I could use both channels. I am intrigued by the circuit you suggested however. Is there anyway to use modify it for guitar pickups?
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2013-09-22 08:01
    'just 8000 turns' is a lot, it could be that the inductance is just too high for the circuit to start oscillating. Try increasing the current by lowering the 330 ohm resistor value. If that doesn't work, try increasing the resistor value. The circuit should work and start to self resonate, but if the inductor is too high the circuit could be in saturation.
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