Ultrasonic transmitter-receiver not stble

[Techman_7]

I am working on ultrasonic barrier project. It has transmitter & receiver. Transmitter transmits 40 KHz FM wave by using two 555 timer ICs. The receiver consist of two LM 567 PLL ICs & a relay that operates when incoming modulating signal is within the capture band of LM567. The transmitter & receiver system works well up to 1 meter only. But the ultrasonic transducer datasheet mention range upto 16 meters. The receiver components for LM 567 are calculated as per datasheet.

But as distance increases the relay chatters. i.e. the modulating signal fluctuates.

Carrier frequency= 40KHz ; Modulating frequency= 220 Hz
Ultrasonic Sensor-Multicomp MCUSD16P40B12RO

Is it because FM modulation index is high?

Can u please help me in this regard......

 

[kripacharya]

post your schematics & prototype pics.

In addition, you have mentioned 220Hz FM of the 40Khz carrier. The LM567 are narrow band pll chips, and would not track the modulation very well. What purpose does the modulation serve anyway ? And why do you need two '567 ?

The range of your system depends heavily on the Tx power and the Rx sensitivity & gain. Post your schematics !

If you have access to an oscilloscope, it would be worthwhile to post here some of the signals in your system.

 

[Audioguru]

The LM567 detects amplitude changes of an AM signal, not frequency deviations of an FM signal.
If the signal frequency increases and decreases past the narrow bandpass of the LM567 then its output will be on only when it is passing near the center of the deviated frequency. The output of the LM567 will be 440Hz when the FM modulation is 220Hz.

 

[Techman_7]

How The output of the LM567 will be 440Hz when the FM modulation is 220Hz?

So to use FM in this application is the modulating frequency to be increased (i.e. modulation index to be reduced) or receiver gain to be increased ?

The transmitter power is 90-100 V peak-peak. The ultrasonic transducer limiting voltage is 120 Vp-p..

Is it have temperature effect? Because am working in lab of temperature around 20 degree Celsius & speed of sound decreases as the temperature is lowered...

The transmitter - receiver has good performance in ambient temperature of 30 degree celsius .

 

[D.A.(Tony)Stewart]

Mixers always create sum and difference frequencies plus harmonics and IMD. Can you determine SNR of the receiver? or noise threshold?

Distance depends on object size and ambient noise.
Relays? show schematic.

Obviously your carrier to noise and signal to noise is poor.

In FM systems the CNR to SNR improvement factor is based on wide band deviation or the modulation index with Fdev/fdata >>1 In your case fdata is 220 Hz. This may more attention to how the system is designed to meet path loss requirements for 16 meters by doing a path loss calculation and receiver noise threshold factors with noise rejection filters possibly required.

 

[kripacharya]

...

Is it have temperature effect? Because am working in lab of temperature around 20 degree Celsius & speed of sound decreases as the temperature is lowered...

The transmitter - receiver has good performance in ambient temperature of 30 degree celsius ...

Since both your 555 and 567 are based on R & C for timing & frequency settings, it might well be a temperature issue. NOT because of any change in speed of sound !!

 

[chuckey]

We need a circuit (or two). Transducers are narrow bandwidth devices, so for FM you need a limiting amplifier. Also make sure your Fdev is low.
Frank

 

[Audioguru]

Why are you modulating with FM? Then the carrier frequency increases and passes the bandpass frequency of the LM567 and it gives an output then the frequency decreases and again passes the bandpass frequency of the LM567 and again it gives an output. So the output is double the modulating frequency.

 

[D.A.(Tony)Stewart]

Imagine how a crude radio picks up noise due to lack of channel filtering and how a great radio uses dual superhet to filter out noise and obtain better sensitivity for distance on reception.

This is one of your problems is the lack of front end filtering going into the PLL.

Can you analyze the noise on a SA? Do a block diagram to define the problem?

- - - Updated - - -

Why are you modulating with FM? Then the carrier frequency increases and passes the bandpass frequency of the LM567 and it gives an output then the frequency decreases and again passes the bandpass frequency of the LM567 and again it gives an output. So the output is double the modulating frequency.

SNR can be improved by using a lower signal frequency and wider deviation ( not simply two 555's but a proper linear VCO) then using a good BPF for the carrier frequency with a high gain low noise preamp, you can limit the signal in the PLL and detect the modulated signal with more path loss variation and then a more sensitive PATH loss BLOCK condition . Keep in mind directionality of your emitter and detector is a tradeoff for GAIN and SNR and dispersion means false reflections off oblique angle objects can introduce self-generated noise, somewhat like RICAIN and RALEIGH fading loss.

However a lower signal frequency increases latency of detection but improves range. THis is how satellites millions of miles away can reach earth at sub <<1Hz modulation rates on microwave carriers.

 

[Techman_7]

Imagine how a crude radio picks up noise due to lack of channel filtering and how a great radio uses dual superhet to filter out noise and obtain better sensitivity for distance on reception.

This is one of your problems is the lack of front end filtering going into the PLL.

Can you analyze the noise on a SA? Do a block diagram to define the problem?

- - - Updated - - -




SNR can be improved by using a lower signal frequency and wider deviation ( not simply two 555's but a proper linear VCO) then using a good BPF for the carrier frequency with a high gain low noise preamp, you can limit the signal in the PLL and detect the modulated signal with more path loss variation and then a more sensitive PATH loss BLOCK condition . Keep in mind directionality of your emitter and detector is a tradeoff for GAIN and SNR and dispersion means false reflections off oblique angle objects can introduce self-generated noise, somewhat like RICAIN and RALEIGH fading loss.

However a lower signal frequency increases latency of detection but improves range. THis is how satellites millions of miles away can reach earth at sub <<1Hz modulation rates on microwave carriers.

Here is the ultrasonic transmitter-receiver schematics. Transmitter has two 555 ICs for FM generation. In receiver the LC tuned circuit is replaced by an single supply inverting opamp amplifier. Currently modulating frequency is 220Hz & carrier is 40KHz. Should the modulating frequency be reduced to stable the chattering relay?

 

[chuckey]

Has your new opamp - not shown on circuit diagram, any gain at 40 KHZ?
Frank

 

[Techman_7]

yesss. The opamp is LM358. I have set the gain of 10 by using 10K resistor in feedback path & 1K resistor connected to inverting terminal with 2.5 V reference given to non-inverting terminal as the supply voltage is 0 to +5V

 

[Audioguru]

The lousy old LM358 is the first low power dual opamp. Then its frequency response, slew rate, noise level and distortion level are poor.
Its datasheet shows that at 40kHz it has not much gain when its supply is 30V, has half that gain when the supply is 10V to 15V and might have no gain when the supply is 5V.
A graph of its slew rate shows that it has trouble swinging its output above only 2kHz when it has a 15V supply and is probably a lower frequency with a 5V supply.

 

[Techman_7]

The lousy old LM358 is the first low power dual opamp. Then its frequency response, slew rate, noise level and distortion level are poor.
Its datasheet shows that at 40kHz it has not much gain when its supply is 30V, has half that gain when the supply is 10V to 15V and might have no gain when the supply is 5V.
A graph of its slew rate shows that it has trouble swinging its output above only 2kHz when it has a 15V supply and is probably a lower frequency with a 5V supply.

But the input signal to first LM 567 is around 1 V peak-peak. It is sufficient for LM567.
But the signal strength varies continuously. Therefore, the modulating frequency input to second LM567 varies so that the variation goes beyond the maximum bandwidth of 14%. And this causes relay chattering..

Can anybody help....

 

[Techman_7]

I also used TL072 opamp but still no improvement