Impedance Matching for Echosounder

Thread Starter

naumankalia

Joined Feb 19, 2016
43
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
 

ericgibbs

Joined Jan 29, 2010
18,766
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
 

Thread Starter

naumankalia

Joined Feb 19, 2016
43
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
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?
 

ericgibbs

Joined Jan 29, 2010
18,766
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
 

Thread Starter

naumankalia

Joined Feb 19, 2016
43
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
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
 

Rafael B

Joined May 15, 2016
5
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
 

ericgibbs

Joined Jan 29, 2010
18,766
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
 

crutschow

Joined Mar 14, 2008
34,280
Thanks for reply. You mean i do not need matching even power amp output impedance is 5 ohm?
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:

RichardO

Joined May 4, 2013
2,270
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.

hi,
With only 200Vppk into a 80R transducer at 33kHz for 11mSec you will have problems getting decent echo's from 1500 mtr depths.

I guess you know that your receiver will require TX pulse suppression and Timed Gain control.
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.
 

Thread Starter

naumankalia

Joined Feb 19, 2016
43
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.
 

Thread Starter

naumankalia

Joined Feb 19, 2016
43
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
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.
 

ericgibbs

Joined Jan 29, 2010
18,766
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
 

Thread Starter

naumankalia

Joined Feb 19, 2016
43
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
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.
 

Thread Starter

naumankalia

Joined Feb 19, 2016
43
Eric[/QUOTE]
hi n,


The design of the receiver input protection and time swept gain is very important.
Design for a 10microV signal in series with the transducer.
Eric
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
 

ericgibbs

Joined Jan 29, 2010
18,766
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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