Abdullah2121

Dear friends ...

I want to help me to find ultrasound doppler velocity circuit ...

hint: the transmitter and the receiver works on 2 MHz

dknguyen

2Mhz is a bit too high for the sound to propogate any farther than a cm or so unless you are using RIDICULOUSLY MASSIVE amounts of powers. THat's why 40-200kHz is normally used for air- it's energy is dissipated a lot more slowly in the air so it travels a lot farther because it is lower frequency.

Abdullah2121

ya I know my friend .. but I want to calculate the heart rate by doppler effect so I must use 2 MHz transducer ...

Abdullah2121

and the distance very small about 10 cm only .... so can you help me

dknguyen

I think you can use a heterodyne circuit just like a radio.

Abdullah2121

It's measure the rate ?

Abdullah2121

Is it measure the rate ?

Abdullah2121

And want it by the ultrasound ... the transducer which i have is ultrasound transducer

dknguyen

No, but it lets you pick out the frequency shift which will let you calculate the speed. It mixes two frequencies together to make two new frequencies: one is the sum of the first two frequencies, and one is the difference between the two frequencies. THis lets you turn small frequency differences into large ones which are easier to detect.

http://en.wikipedia.org/wiki/Superheterodyne

They were used in radio circuits, among other things. You'd need to get a book on it or Google, understand how the circuit works, and adapt it to your purposes. 2Mhz is pretty close to the frequency of AM radio so the circuits would probably be fairly similar.

Abdullah2121

after that how can i measure the rate?

dknguyen

You know what the doppler effect is right? If a transmitter is moving relative to a receiver then the frequency transmitted will be a shifted frequency based on speed and direction when it reaches the receiver?

You'd use the equation:
http://en.wikipedia.org/wiki/Doppler_effect

Which means you would probably need some digital circuitry after the superheterodyne receiver to measure the frequency of interest so it can do the calculations. And if you are measuring heartbeat, you'd probably need to do a little more processing based on the when the direction changes (since the heart has to charge direction to beat) and measure the rate at which the direction changes.

YOu could also just use a microphone so a microcontroller could hear the heartbeat too you know.

Abdullah2121

thanx ... but do u know some circuits to subtract two frequencies ... i want it to enter the output signal to F/V converter and to PIC ...

Hero999

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