Impedance Matching for Echosounder

Discussion in 'General Electronics Chat' started by naumankalia, May 15, 2016.

  1. naumankalia

    Thread Starter New Member

    Feb 19, 2016
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    Hello all

    I am working on designing of depth finder (echo sounder). For that purpose, i have selected a power amplifier which can deliver 200 Vpp with 10 amp current in pulse mode. Its output impedance is 5 ohm. The sensor i selected has input impedance of 80 ohm @ 33 KHz. I need to deliver 200 Vpp at sensor input. My queries are:

    Do i need some impedance matching b/w sensor and power amplifier for delivering 200 Vpp at sensor input?

    If i do not place matching unit, what voltages i will get at sensor if i directly connect power amplifier and sensor?

    Any help is greatly appreciated.

    Thanks
     
  2. #12

    Expert

    Nov 30, 2010
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    100 Vp/80 ohms is 1.25 amps needed out of a 10 amp capability. Sounds good to me.
    200 volts peak to peak.
     
  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi n,
    What is the pulse width of the transmission period and water depth range being measured?

    E
     
  4. naumankalia

    Thread Starter New Member

    Feb 19, 2016
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    Thanks for reply. You mean i do not need matching even power amp output impedance is 5 ohm?
     
  5. naumankalia

    Thread Starter New Member

    Feb 19, 2016
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    Pulse width is around 11 ms and i have to design for depth up to 1500m
     
  6. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    With only 200Vppk into a 80R transducer at 33kHz for 11mSec you will have problems getting decent echo's from 1500 mtr depths. [is it the seabed profile that you are scanning]

    Do you have a datasheet for the 33kHz transducer that you could post.?

    I guess you know that your receiver will require TX pulse suppression and Timed Gain control.
    Do you have a circuit to post.?
    E
     
  7. naumankalia

    Thread Starter New Member

    Feb 19, 2016
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    Currently, my design is based on reson TC2122 transducer and Apex MP118 power amplifier whose data sheets are available on relevant sites (in case of failure, tell me i will post you). In your opinion, what is the maximum depth range i can achieve with this combination?

    With TX pulse suppression do you mean match filtering?
     
  8. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    I would say approx 180 metres depth, the power amp is basically designed to drive a piezo transducer in a print head application
    Can you tell us what is the actual application.
    E
     
  9. naumankalia

    Thread Starter New Member

    Feb 19, 2016
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    Hi

    Actual application is to design an echo sounder system able to get depths up to 1500 m

    Can you kindly tell how you come to this conclusion (i.e. 180 m depth) as i have to completely redesign my echo sounder?

    As you said "the power amp is basically designed to drive a piezo transducer in a print head application", do you mean it will not work with reson transducer at full power?

    Thanks for help
     
  10. Rafael B

    New Member

    May 15, 2016
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    Nauman Kalia, you do need impedance matching, otherwise you will waste power. Some of the power will be reflected. Probably you will need an transformer for that / Balun (?).

    To know how deep you can measure it's not gonna be simple manner, but not impossible also. You must know how much voltage you will get on the piezo element... And from the data sheet you must have the source level, probably in dB uPa/V at 1 meter.

    Than I would recommend you to find the element sensitivity in dBV re 1uPa. Tô know exactly how small the reflected pulse can be detected by the amplifier circuit, that oftenly have some gain.

    By measuring the pulse transit time you can have a good guess of the distance. I also would recommend to measure water temperature to make some adjustments on sound velocity formula to get more accurate results.

    Good luck
     
  11. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi n,
    Would you please post your existing design and say what is the application, why 1500 mtrs?
    The MP118 is a transmitter driver amp, have you designed a receiver amplifier.?
    E
     
  12. crutschow

    Expert

    Mar 14, 2008
    13,006
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    There is often a fundamental misunderstanding about impedance matching.
    It is needed for two situations:
    1. If you have a transmission line and you want to minimize high frequency reflections from the line then you need to match the source and/or load impedance to the transmission line impedance.
    2. If you have a fixed output impedance source and you want the maximum power out, then you match the load impedance to the source impedance.
    This is typically only done for special circumstances such as RF power amplifier outputs where maximum power out is more important than efficiency.

    Note that for maximum efficiency or maximum output voltage you don't want to match impedances.
    For those situations you want the source impedance to be as low as possible, compared to the load impedance.
    This is typical of audio power amps, for example, where the amp output impedance is often a fraction of an ohm (due to circuit negative feedback) as compared to the typical 8 ohm speaker load.

    A typical trick question on electronic exams, after the student has learned about the maximum power transfer theorem, is to ask:
    If the load resistance is Rld, what should the source resistance Rs be for maximum load power?
    Many will answer that Rs should equal Rld, but the correct answer is Rs=0, of course.
     
    Last edited: May 15, 2016
    naumankalia likes this.
  13. RichardO

    Well-Known Member

    May 4, 2013
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    Is your intent to drive the transducer with a pulse or burst?

    My experience is driving transducers with a pulse. In that situation, the pulse circuit drives the transducer for less than one cycle of its resonant frequency and then disconnects so that the pulse circuit does not load the transducer during the receive time. The receiver must be protected from the high-power/high-voltage transmit pulse. This is often done with a T/R switch as mentioned above.

    Your transducer is Piezoelectric so it will look like a capacitance while being pulsed. The transducer is actually much more complicated than this because it is acoustically impedance matched to the impedance of water. It also might have an inductor across the Piezo element to electrically tune it to the mechanical resonant frequency of the transducer.

    Note that the transducer is specified at a 1% duty cycle. This is much less than the 11 ms that you say you are using. I think you want a much shorter pulse width than 11 ms.

    As I remember, the attenuation in water is on the order of 1 dB/cm/MHz. I will let you do the conversions and verify that this is right...
    The gain of the receiver is increased with time to compensate for the loss of received signal versus depth.

    I hope that these somewhat random thought are of help.
     
  14. naumankalia

    Thread Starter New Member

    Feb 19, 2016
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    Thanks for help.

    Yes, i will certainly use Pulse mode.

    As far as 1% duty cycle is concerned, after 11 ms pulse duration, pulse transmission will be off for at least 2 sec before transmitting again so i think duty cycle will be less than 1%. what you say?

    I am using attenuation of 7 dB/Km @ 33 KHz in my case.
     
  15. naumankalia

    Thread Starter New Member

    Feb 19, 2016
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    This was exactly the confusion due to which i asked whether i need impedance matching in my case or not?
     
  16. naumankalia

    Thread Starter New Member

    Feb 19, 2016
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    Hi eric
    I have just started design with transmission part and am now in some doubt due to "180 m" figure pointed out by u.
     
  17. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi n,
    When I ask what is the application I am asking how you are going to use the echo sounder.
    eg: surveying etc .. Why 1500 mtrs.?

    You may find a TX On period of 11mSec at 33kHz too short for depths up to 1500 mtrs, you may have to make it adjustable up to say 3omSec . [in your design]
    Typically for 800mtrs depths, TX power would be a nominal 1kWatt at 30kHz.

    As Crutschow has pointed out, low output impedance from the TX driver output.

    The design of the receiver input protection and time swept gain is very important.
    Design for a 10microV signal in series with the transducer.
    How will the transducer be mounted in the hull of the boat.?

    Are you planning to digitise the depth readings.?

    The TC2122 transducer is dual frequency, 33kHz and 200kHz, do you plan to use the 200kHz section.?

    Eric
     
  18. naumankalia

    Thread Starter New Member

    Feb 19, 2016
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    Yes, I need to definitely digitize the raw data and do some processing like match filter/Correlation on it to improve S/N.

    I will use 200 KHz for short range depth like 100m.

    I have not done any work on its mounting technique yet.
     
  19. naumankalia

    Thread Starter New Member

    Feb 19, 2016
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    Eric[/QUOTE]
    Hi Eric

    As far as receiver side is concerned, according to my calculations, i will get 5 to 7 uvolt signals (or even lower) from maximum depth. I have to design a pre-amplifier having input noise level less than say 5 uvolt. I have studied some low noise variable gain amplifier ICs from "Analog devices" like AD600 series but i am not sure how to calculate, theoretically and practically, the overall input noise level of complete preamplifer (e.g. AD600 with some bandpass filter at its output). Do u have any idea?

    Secondly, selection of a T/R switch is also important which can withstand like 200 V signal as well as allow to pass few uvolt signals through it without any further attenuation.

    Can you kindly guide me more on these issues?

    Thanks
     
  20. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    Where did you get the expected echo signal value of 5 to7uV from.?
    The effective RMS power of a 200Vppk transducer drive is 100Vpk * 0.707 = ~ 70Watts.

    Typically to measure depths down to 1500 mtrs, you should be driving at around 1kWatt effective.

    Echo sounder transducers are designed to mount through the hull of the boat or off set using a boom mount.
    The print head transducer you are planning to use would have to be mounted inside the boats hull as the transducer is not water proof, this means a further reduction overall echo signal return strength.

    I have asked a number of times, what is the actual application, in other words how will the system be used.?

    http://www.raymarine.com/view/?id=197

    E
     
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