# Receiver and Transmitter

I am using the following receiver and transmitter for my project to form a ultrasonic communication system. I transmitted 5V Peak to peak voltage, 40 Khz frequency sinusoid from the transmitter and received the signal through the receiver. But the receiver shows the peak-to-peak voltage of about 30V (though I transmitted 5V) when I bring it closer to the transmitter. I was supposed to find out the loss in the received signal and design the Operational amplifier accordingly. But I am stuck due to this amazing amplification in the signal. Please help me out.

MA40S5 Transmitter

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## Answers and Replies

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I take it you have the transducers very close and are operating near resonant frequency.

Just because you see more voltage coming out than going in doesn't imply amplification. Without a load, the receiving end isn't delivering power, it's just performing the equivalent of an impedance transformation, like you would see with a tuned LC network, or a step up transformer.

The higher output voltage may simplify gettting the gain you need if you run into a high impedance aplifier that still has significant gain at your operating frequency ( a lot of op amps have little gain in the tens of kHz ).

vk6kro
It may be useful to measure the receiver output at a distance of 2 meters or so. This is a more realistic distance for normal use.

The output will probably be down enough to need a lot of amplification.

thats really helpful....but what is the crieteria for designing the opamp....for eg: I sent 5V and received 3 V at receiver....how should the op-amp be designed so that the received signal will be suitable for the operation in DSP board?(to bring the level up to a level that can be sampled.)

What are the specs on the ADC? Many samplers accept a range from 0V to the supply voltage that runs the IC (3.3V for many ARM microp's with an ADC). If you can be sure that your signal will be -3V to +3V or slightly less, then, in the case of an ARM chip, you would want to cut it in half (gain = 0.5) and give it a DC offset of 1.65V. That way the signal will range from 0.15V to 3.15V. Then your software can shift and scale it to it's proper value (you may have to calibrate it).

This is just an example of something similar that I did with a microprocessor. Basically, you'll want to create an op amp network that scales the received input to the input range of your ADC. Look at the summing and differencing amplifier configurations for an OP amp. You can use a resistor and a POT as a voltage divider for a DC offset in one of the inputs. That way you can adjust the offset. You can also add a POT to the input from the receiver to adjust the gain.

Your receiver probably has a real low power output. That means that it'll give out on you if you try to draw too much current from it. Make sure you consider the input impedance of your op amp. You'll want it to be as high as possible.

Thanks but it became a bit unclear to me as I am a beginner :(

I need to build an opamp circuit in such a way so that the received signal in DSP board would be few 100mV. I think there should be a general formula for the gain of the opamp based on the distance(radius) from the receiver to transmitter. What are the parameters that I need to design the opamp circuit.

Lets take this example : Transmitter transmitted 5V p-p, receiver received 3V at distance 2m. Now if I vary the distance, the received amplitude goes low for sure. I need few 100mV into the DSP board. How can this be done?

Before getting too excited about the op amp, check it's gain vs frequency plot. You'd be surprised how many lack an open loop gain of even 10 by the time you get up to 40 Kc. This forces you to use a goodly number of stages with a small gain each. Look for op amps with a gain bandwidth over 5MHz at least.

-Mike

Lets take this example : Transmitter transmitted 5V p-p, receiver received 3V at distance 2m. Now if I vary the distance, the received amplitude goes low for sure. I need few 100mV into the DSP board. How can this be done?
I assume that you mean that the DSP input range should be around -300mV to +300mV. Is that correct? It looks like you need just a plain old inverting configuration with no DC offset. You can find it on this http://en.wikipedia.org/wiki/Operational_amplifier#Inverting_amplifier". Of course, it would be best to look it up in your textbook or lab manual.

To go from -3V ~ +3V to -300mV ~ +300mV, simply divide by 10. That's your gain. Simple, yes? Now pick an Rin. For the inverting configuration, the input resistance happens to be exactly equal to Rin. Note that it's not that simple for all op amp configurations but it is in this case. I would suggest trying 10kOhm for Rin. Also, try using a 10kOhm resistor as the load for your receiver. Did the peak voltage drop from 3V to a lower number? If so, recalculate the gain. Now that you know the Gain and the input resistance, solve for Rf (called the feedback resistor) in the equation for an inverting op amp [Gain = Rf/Rin]. It should be 1kOhm for a gain of 1/10 unless you needed to change the gain. Feel free to change anything and experiment. Also, put a potentiometer(POT) in place of Rf and adjust the resistance. See what happens.

From there, you can repeat the process for different distances. It's most likely that you find that the voltage at the receiver falls as an inverse square of the distance (1/d)^2. To keep things simple, pick an input resistance and stick to it (10kOhm is my suggestion). The only thing that will vary over distance will be Rf.

Mike mentioned the gain bandwidth product but for a gain like this you should be ok. At least I think you should. My old 741 from my lab kit had about a 1MHz GBW. I'm sure you have something similar.

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sophiecentaur
Gold Member
I am using the following receiver and transmitter for my project to form a ultrasonic communication system. I transmitted 5V Peak to peak voltage, 40 Khz frequency sinusoid from the transmitter and received the signal through the receiver. But the receiver shows the peak-to-peak voltage of about 30V (though I transmitted 5V) when I bring it closer to the transmitter. I was supposed to find out the loss in the received signal and design the Operational amplifier accordingly. But I am stuck due to this amazing amplification in the signal. Please help me out.

MA40S5 Transmitter
There is no paradox here.
You didn't get more power out than you put in (that's not possible) but, if the receive transducer is resonant, then you can get a higher voltage variation than the voltage applied to the transmit transducer quite easily.
Power = Current times Voltage
You will find a very small current flows in the receive transducer and a much higher current flows in the transmit transducer.

thanks guys it really helps!!! :-)

"
From there, you can repeat the process for different distances. It's most likely that you find that the voltage at the receiver falls as an inverse square of the distance (1/d)^2. To keep things simple, pick an input resistance and stick to it (10kOhm is my suggestion). The only thing that will vary over distance will be Rf.
"

Question:

For my project, I need a Opamp which automatically adjusts the resitance Rf to a constant Gain. So, I wont have to vary the resistance using a potentiometer instead. Can you please name any of such Opamp?

I assume that you mean that the DSP input range should be around -300mV to +300mV. Is that correct? It looks like you need just a plain old inverting configuration with no DC offset. You can find it on this http://en.wikipedia.org/wiki/Operational_amplifier#Inverting_amplifier". Of course, it would be best to look it up in your textbook or lab manual.

To go from -3V ~ +3V to -300mV ~ +300mV, simply divide by 10. That's your gain. Simple, yes? Now pick an Rin. For the inverting configuration, the input resistance happens to be exactly equal to Rin. Note that it's not that simple for all op amp configurations but it is in this case. I would suggest trying 10kOhm for Rin. Also, try using a 10kOhm resistor as the load for your receiver. Did the peak voltage drop from 3V to a lower number? If so, recalculate the gain. Now that you know the Gain and the input resistance, solve for Rf (called the feedback resistor) in the equation for an inverting op amp [Gain = Rf/Rin]. It should be 1kOhm for a gain of 1/10 unless you needed to change the gain. Feel free to change anything and experiment. Also, put a potentiometer(POT) in place of Rf and adjust the resistance. See what happens.

From there, you can repeat the process for different distances. It's most likely that you find that the voltage at the receiver falls as an inverse square of the distance (1/d)^2. To keep things simple, pick an input resistance and stick to it (10kOhm is my suggestion). The only thing that will vary over distance will be Rf.

Mike mentioned the gain bandwidth product but for a gain like this you should be ok. At least I think you should. My old 741 from my lab kit had about a 1MHz GBW. I'm sure you have something similar.
Question:

For my project, I need a Opamp which automatically adjusts the resitance Rf to a constant Gain. So, I wont have to vary the resistance using a potentiometer instead. Can you please name any of such Opamp?

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i need help in designing a 4 button ir transmitter for controlling the direction of a buggy