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The metal detector project solution

Rock Shao
Expert 2975 points
Other Parts Discussed in Thread: THS4081, OPA810

Hi,

The metal detector project will need to design-in stable oscillator circuits with related discrete components(op-amp, inductor, capacitor, and resistor).
The oscillator’s basic specifications and requests are listed below.

1, The oscillator can generate a stable sinewave signal only with 100Hz ~ 1MHz bandwidth.

2, The oscillator needs to have stable/fixed sinewave amplitude output with 14.4Vp-p(peak to peak) or 5.1Vrms within the full band(100Hz ~ 1MHz) based on dc +/- 8V dual power supply.

3, The oscillator’s positive feedback networks must be designed with both inductors and capacitors only(NOT resistor and capacitor).

4, Please also recommend the single channel OP amp from TI.

And there are two additional key requested specifications of this oscillator design as below.

1, Condition one: If the oscillator is working under “200K Hz operating frequency”, please provide me all the component values/specifications settings of this oscillator.

2, Condition two: If the oscillator is working under “1K Hz operating frequency”, please provide me all the component values/specifications settings of this oscillator.

Regards

Rock

  • 0
    TI__Guru**** 152666 points

    Hello Rock,

    This sounds like a school project, perhaps it would be best for them to do their own work.

    Getting one frequency isn't difficult with a simple circuit. A 4 decade range sound like a piece of lab equipment (a whole system).

    I'll suggest using a hartley or colpitts oscillator design. For the wide frequency range required use a variable cap and switch the inductors.

  • 0 in reply to Ron Michallick
    Expert 2975 points

    Hi Ron,

    It is an OMM project. Did you have some ideas? thanks.

    www.sew.com.tw/.../home.phtml

  • 0 in reply to Rock Shao
    TI__Guru**** 152666 points

    Rock,

    The mandatory requirement for LC and other clues made me think of school design project. I did not find any TI reference designs that were compatible because the examples were RC based (not LC)

     I suggest them doing a web search for "sine oscillator reference design" to get some ideas for the initial circuit. 

    For the op amp, the minimum slew rate is Vpp*pi*f = 14.4V*3.14*1e6 = 45V/us, so I suggest at least 4X, 100V/us .  OPA810 or THS4081 are such op amps. These are high speed amplifiers , so I will get high speed op amp team to do further support for this e2e thread.

  • 0 in reply to Ron Michallick
    Expert 2975 points

    Hi Ron,

    [Comments]

    1, will design-in the TI OPA810 op-amplifier as the high-frequency sinewave oscillator’s core part.

    2, will design with the sinewave signal generator based on using Colpitts oscillator structure.

    3, The Colpitts oscillator structure could be referenced as an attachment.

    [Question]

    In general, the Colpitts oscillator output amplitude will decrease in an actual situation while the operating frequency is increasing within the full band(100Hz ~ 1MHz).

    So, if we want to keep the Colpitts oscillator output amplitude(Vp-p) with a fixed/stable value, we will need to add the auto gain control(compensation, AGC) functionality into the Colpitts oscillator.

    Please advise me on how to design it to integrate the Colpitts oscillator.

    Thank you very much.

  • 0 in reply to Rock Shao
    TI__Mastermind 25705 points

    Hello Rock,

       We have app notes and reference circuits with a RC based oscillator with an AGC such as these links: link1 and link2. There is also a simpler AGC reference circuit in the VCA810's datasheet which is Figure 36 and 38. 

    Thank you,

    Sima

  • 0
    Guru 140200 points

    Hi Rock,

    why a coil has to be used in the oscillator? Because the µr of metal to be detected will change the inductance of the measuring coil?

    What is an OMM project?

    Kai

  • 0 in reply to kai klaas69
    Expert 2975 points

    Hi,

    Thanks for the RC-based oscillator with AGC reference design. However, we will be forced to use an LC-based oscillator with AGC functionality since the measuring coil(inductor) is exactly a part of the LC oscillator.
    So, please advise me on how to design the Colpitts LC oscillator with AGC functionality integrated!
    I do need the integrated reference design exactly from TI.

    Thanks and regards