I am designing a receive signal path for Ultrasonic Transducer.
The circuit needs to measure time of flight of ultrasound wave pulses with frequency of 1MHz(about 16 periods - 16us).
Below is the waveform measured. Yellow is Excitation Pulses to transmitting transducer, and Blue is signal received with receiving transducer.
The problem is that the amplitude of the received signal may change due to transducer spacing, or change in transducer mounting position, temperature, and other various reasons i don't know. And due to mechanical property of the transducer, the received signal amplitude naturally ramps up gradually, then ramps down.
I am thinking of applying Automatic Gain Control Circuit after 1MHz bandpass filter to feed to comparator.
I've looked at few options for AGC, but can't decide how to implement.
Unlike RF signals or sound signal , ultrasonic pulses don't have continuous input level. So, the AGC has to respond fast, adjusting within 1 period(1us).
I have to keep the output signal of AGC at constant voltage amplitude(if there is signal), with DC offset of 2.5V. The jitter in Time of flight measurement is critical(1ns jitter may result up to 30% error in some application)