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my question seems to concern a really easy device. But since I am new to this field of the magical electrons I am very eager to be careful and thankful for any help.

My task is to power an ultrasonic senscomp 600 transducer with 150V DC and an added 150V AC at 40 and 50kHz. As stated on wiki I only have to dimension an inductor and a capacity, to fit following restriction:

\begin{align} X_C &= \frac{1}{\omega C} = \frac{1}{2\pi f C} \ll Z_0,\\ X_L &= \omega L = 2 \pi f L \gg Z_0,\\ \end{align}

I have troubles with following questions:

  1. As I understand, you can use a a bias tee to add an ac signal to a dc signal but also use the circuit to subtract the AC from the DC+AC signal. Is this correct?

  2. How do I choose a correct Z0? The ultrasonic senscomp 600 transducer does not list any impedance values of the transducer.

  3. Is there any possibility to protect my sources and the driven transducer from wrong doing? I was thinking of diodes, to protect the costly sources.

It is hard to imagine, that i only need besides the BNC connectors two more parts to accomplish this.

Some friend suggested to connect the DC and AC source in series, but i don't feel good about this idea.

Thanks a lot - I hope this question fits your requirements.

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  • \$\begingroup\$ How much current flows in each path? \$\endgroup\$
    – The Photon
    Jun 13, 2018 at 23:07
  • \$\begingroup\$ @ThePhoton The useless datasheet suggests that the impedance is around \$\frac{1}{2\pi 1000 × 450 pF}\approx353 kΩ\$ @ 1 kHz. So if we naively extrapolate we get ~ 7 kΩ impedance at 50 kHz. So the current will be roughly 15 mA RMS. \$\endgroup\$
    – Harry Svensson
    Jun 13, 2018 at 23:32
  • \$\begingroup\$ @LuisvanBomel, I don't think you want a "Bias Tee", a Bias Tee (according to wiki) splits up DC and AC to two different paths. You want to join those two paths together, so you should want a "Bias Tee backwards (with the schematic on the wiki in mind)". So on the wiki page, imagine your source coming from right and bottom and going left (backwards). \$\endgroup\$
    – Harry Svensson
    Jun 13, 2018 at 23:38
  • \$\begingroup\$ Don’t you think you should define the purpose? and some specs on the Tx pulse ? and the Rx design? I think you are over simplifying the complexity of SONAR. What substrate and acoustic port design is being used? \$\endgroup\$
    – Tony Stewart EE75
    Jun 14, 2018 at 3:02
  • \$\begingroup\$ @HarrySvensson, it's normal to use a bias tee either way. What you're calling "backwards" is how it's normally used to bias a laser diode, but nobody ever bothers to specify that this is a "backwards" application. Using it the other way is common on the output of some RF amplifiers. \$\endgroup\$
    – The Photon
    Jun 14, 2018 at 4:43

1 Answer 1

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  1. Yes. The one time I needed a bias tee circuit is when I built PA0RDT's capacitative antenna. There is a full supply schematic on pages 14-15 here that should give a sense of how it is made: http://dl1dbc.net/SAQ/Mwhip/Article_pa0rdt-Mini-Whip_English.pdf

  2. In the wiki page you linked there is a statement just above the formulas you quote: "Typically, the characteristic impedance Z0 will be 50 ohms or 75 ohms".

  3. I don't really know what "wrong doing" you imply, but I'd definitely go for a fuse in the bias supply to protect the line and the power supply from short circuit surges. I used resettable fuses for convenience.

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  • \$\begingroup\$ 2. I found two different definitions of "characteristic impedance" - One for the connecting coax cable which seems to be the correct one. An another source stated that you have to devide the voltage by the current. But as i don't know the flowing current or the impedance of the transducer the second option seems to be wrong. \$\endgroup\$
    – LuisvanBommel
    Jun 14, 2018 at 9:26

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