I am currently working on a microcontroller based project to measure the blood flow velocity using ultrasound doppler.

I have purchased two Spengler 8 MHz continuous mode probes in order to measure the doppler shift.

I have disassembled one of the probes to test it. It is formed of an 8 MHz crystal, which outputs a 2 V ppk 8 MHz sine wave (as viewed on the oscilloscope.) It also has an MC1496 modulator/demodulator chip to remove the carrier frequency followed by an LM324 chip for amplification. The final probe output are two wires, sig1 and sig2.

I have tested the outputs on the oscilloscope and compared them to the RX and TX signals of the sensor after breaking the probe open.

My experience in analog circuit design is very limited, especially in piezoelectric crystal signal conditioning and fetal doppler.

I have exported the signals to Matlab with the hope of detecting the blood velocity using FFT. I performed AM demodulation as well.

I am sure it is based on AM because when I hit the probe head the sig1 amplitude changes and rises, but I am not understanding how the CW doppler probe circuit works.

Based on what I found, this is a common and standard circuit used in probes and devices to detect fetal heartbeat.

How does it work? I do not see any shift in frequency in the heartbeat range using FFT. I have finally found the probe circuit, my outputs of interest are output2 and output3, from the final stage it seems they are using output3: enter image description here (This circuit is taken from https://patents.google.com/patent/CN203970415U/en) I have attached the simulation outputs:

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  • \$\begingroup\$ You would need to draw a reverse engineered schematics of the probe. It's not likely that a differential output would produce 2.7Vpp on one output and 133mV on the other. \$\endgroup\$ Apr 9, 2021 at 21:14
  • \$\begingroup\$ I wasn't getting any much signal on the RX of the crystal eventhough the TX clearly shows a sine wave being sent. I did not place it on my hand just putted it facing an obstacle. but when I hit on it with my finger i can see the pulses in the signals \$\endgroup\$
    – chaosmind
    Apr 9, 2021 at 21:17
  • \$\begingroup\$ The probe had 4 wires out which were the VCC, GND and two others I am supposing they are for data. Is it possible that the two other wires be Signal and its own ground and not differential? there was also an 401 inverter chip inside. \$\endgroup\$
    – chaosmind
    Apr 9, 2021 at 21:20
  • \$\begingroup\$ An interesting challenge. Unless you have a very cooperative volunteer, maybe you should first build what's called a "phantom" to simulate the moving fluid in the body and give you a consistent signal to measure. Then you'll be able to do testing without holding the probe against your own body. \$\endgroup\$ Apr 9, 2021 at 21:36
  • \$\begingroup\$ But first i need to design the analog circuit to have a clean signal for the microcontroller. this is the part I am stuck on \$\endgroup\$
    – chaosmind
    Apr 10, 2021 at 0:01


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