Impedance mismatch issue in receivers

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In summary, a user is asking about the potential side effects of an impedance mismatch between the output impedance of an acoustic piezoelectric sensor and the input impedance of a preamplifier specifically designed for the sensor. The user also inquires about the frequency range of operation and the need for a square wave rather than a sine wave. The conversation delves into the importance of cable length and capacitance, and how it can affect signal loss and waveform quality. The user also mentions the potential use of a line driver to maintain sharp edges in the waveform. Ultimately, the key to reducing impedance mismatch is to match the transducer, cable, and load impedances.
  • #1
nauman
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Hi all

I have an acoustic piezoelectric sensor whose output impedance should be ideally 50 ohm and a preamplifier circuit specifically designed for this sensor having input impedance of 50 Ohm. The piezoelectric sensor output voltage range is from uVrms to few mVrm. These low voltage signals are then amplified by preamplifier.

My query is that if there is a mismatch b/w output impedance of sensor and input impedance of preamplifier, what will be the side effects? In case of Transmission chain, it is obvious that such mismatch (e.g. b/w Power Amplifier and Transducer) causes power losses, however, what will happen in case of receiver chain when very low voltage signals are present ?
 
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  • #2
If the preamp is specifically designed for it, why would there be an impedance mismatch? Can you post links to the datasheets for the transducer and preamp?
 
  • #3
berkeman said:
If the preamp is specifically designed for it, why would there be an impedance mismatch? Can you post links to the datasheets for the transducer and preamp?
Thanks for reply. Both transducer and preamplifier are own developed and are not COTs. Controlling the output impedance of acoustic transducer is quite challenging and main impedance deviation is in output impedance of transducer (>67 ohm instead of around 50 ohm).
 
  • #4
Do you have gain/phase plots of the transfer function from an acoustic driver through the (liquid?) medium to the output of the transducer? What is the frequency range of operation? Can you post a PDF or JPEG image of that gain/phase plot?
 
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  • #5
berkeman said:
Do you have gain/phase plots of the transfer function from an acoustic driver through the (liquid?) medium to the output of the transducer? What is the frequency range of operation? Can you post a PDF or JPEG image of that gain/phase plot?
Frequency range of operation is from 90 KHz to 110 KHz.
As transducer is being used only in receiving mode (i.e. as a hydrophone), there is receive sensitivity vs frequency graph available indicating how much voltage will be generated at transducer output for standard input of 1upa sound pressure in water for a typical frequency range. If you are interested in this graph, i can post it.
 
  • #6
Great, that would be helpful. And have you also measured that same transfer function to the output of the preamp? That might start to show you if the impedance mismatch makes much of a difference.
 
  • #7
Since your signal frequency is so low (110kHz), the signal wavelength in coax cable will be roughly 6880ft.

Using a 'rule-of-thumb' that a cable less than 1/10 wavelength long has minimal impact on a signal, I wouldn't worry about it until the cable length approaches 1/10 of a mile.

I would worry more about the capacitance of the cable. For example, using RG-58 coax at 30pF/foot, you can expect the following signal level losses due to the cable capacitance loading the transducer: (the 67Ω with the cable capacitance acts like an RC low-pass filter)

Length Loss
7ft . . . .1%
50ft. .. 10%

If you need a fast risetime waveform (square wave) at the receiver rather than a Sine wave, you need either a short cable or some active electronics (a Line Driver) at the transducer to keep those sharp edges. That's another case of the 'factor-of-ten' rule-of-thumb, if you need a square wave, for many uses you need a bandwidth of around 10x the pulse repetition rate.

Cheers,
Tom
 
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  • #8
berkeman said:
Great, that would be helpful. And have you also measured that same transfer function to the output of the preamp? That might start to show you if the impedance mismatch makes much of a difference.
sensitivity graph.JPG
 
  • #9
nauman said:
Frequency range of operation is from 90 KHz to 110 KHz.
It looks like your plot is centered on 100MHz, not 100kHz. Am I misreading something?
 
  • #10
berkeman said:
It looks like your plot is centered on 100MHz, not 100kHz. Am I misreading something?
Sorry, frequency axis label is misleading (i.e. should be Hz instead of KHz), sensor is certainly centered at 100KHz
 
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  • #11
Tom.G said:
Since your signal frequency is so low (110kHz), the signal wavelength in coax cable will be roughly 6880ft.

Using a 'rule-of-thumb' that a cable less than 1/10 wavelength long has minimal impact on a signal, I wouldn't worry about it until the cable length approaches 1/10 of a mile.

I would worry more about the capacitance of the cable. For example, using RG-58 coax at 30pF/foot, you can expect the following signal level losses due to the cable capacitance loading the transducer: (the 67Ω with the cable capacitance acts like an RC low-pass filter)

Length Loss
7ft . . . .1%
50ft. .. 10%

If you need a fast risetime waveform (square wave) at the receiver rather than a Sine wave, you need either a short cable or some active electronics (a Line Driver) at the transducer to keep those sharp edges. That's another case of the 'factor-of-ten' rule-of-thumb, if you need a square wave, for many uses you need a bandwidth of around 10x the pulse repetition rate.

Cheers,
Tom
If he can make his transducer, cable chas. and load impedances roughly the same there is low or negligible capacitive load effect regardless of cable length; he just gets a gain of ~1/2 with pure delay.

Ideal cable assumed of course).
 
  • #12
Or if you just pick a cable with ## Z_0 ## matching the transducer, then the receiver can be any impedance ## Z_L ## , again with no attenuation or distortion, but with gain = ## Z_L/(Z_L+Z_0) ##.
 

Related to Impedance mismatch issue in receivers

What is an impedance mismatch issue in receivers?

An impedance mismatch issue in receivers occurs when the input impedance of the receiver does not match the output impedance of the source device. This results in a loss of signal and can cause distortion or poor quality audio.

What causes an impedance mismatch issue in receivers?

An impedance mismatch issue can be caused by a variety of factors, including using cables with different impedance ratings, using devices with different impedance levels, or using incorrect settings on the receiver.

How can an impedance mismatch issue be identified?

An impedance mismatch issue can be identified by checking the input and output impedance ratings of the devices being used, as well as checking for any distortion or poor audio quality in the receiver's output.

What are the consequences of an impedance mismatch issue in receivers?

The consequences of an impedance mismatch issue can include loss of signal, distortion or poor audio quality, and potential damage to the devices involved. It can also lead to frustration and inconvenience for the user.

How can an impedance mismatch issue be resolved?

An impedance mismatch issue can be resolved by using cables with matching impedance ratings, adjusting the settings on the receiver, or using impedance matching devices such as transformers or attenuators. It is important to ensure that all devices in the audio system have compatible impedance levels.

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