Some more tests with a probe.. this time from kretz technik. Raw acquisition yield interesting signals.. Doesn't it look like a night sky ?
The spectrum seems to contain good ultrasound (with a fair dose of noise) - the signal is between the blue lines, between 2.2MHz and 4.2MHz:
Which yield a classical wirephantom image.
Breaking the wall of sound! un0rick can sample at 64Msps but ... one can offset the start of an acquisition by half a cycle. In practice, it means one can do two acquisitions, with one delayed by half a cycle, and interleave the two series. In practice: getting 128Msps acquisition speed!
In picture, it seems quite nice. The signal is loud and clear!
And notebook at https://github.com/kelu124/echomods/blob/master/matty/20180814a/20180814a-Server.ipynb . Some improvement on the lib too, not perfect yet but someday better!
The objective of this experiment is to check :
Type of signal
Curve of the gain
Impact of gain on noise
FFT - content of such a line
One can also see that the level of noise has improved by a factor of 4.. or more.
But I'd need to be more rigourous to get precise numbers =)
Log as usual is available.
Ultrasounds, traveling in the medium they image, are attenuated by the very same medium and the further they progress, the faintest the echo comes back.
In order to compensate for this attenuation, the echoes are usually amplified by by a variable gain which renormalize these echoes.
The plots below represent in blue, unsimplified signals, and, in green, the amplified signal, with a gain shifting from 0% to 100% in 200us. It seems that makes the echoes great again =)
More in the log.
I just got an interesting mechanical probe, a Bard Site Rite - piezo at 7.5MHz. The plug is quite simple, and digging into the cable yields 2 pairs of cable, one coax, a bigger cable with two "big" cables. I've made sure it's a mechanical probe. The aim is to try and see if I can connect it to my board and get an image.
Good point for this probe is that it unscrews nicely.
and even better, it has changeable heads!
I assume two pairs of cables are for motors / actuators, coax is for the signal coming back (hence the bead). But how can I see that on the pins above?
Going to search for the reference manual and other resources. I'd rather not unscrew the head yet!
Some good news again =)
I got to better control pulses (I need to really read more carefully datasheets), got acquisitions at 64Msps, really looking good, and got some (badly formed at the moment) images!
Acquisitions - going to specs at 64Msps. Thanks David =)
Pulser: getting 200ns wide 75V pulses (blue is the pulse with a piezo on, in yellow is the 'damping' control signal.
Getting some images from a "damaged" probe
More on the log!
That's moving forward! I'm getting some results with the following setup - using a piezo with a fixed distance, and a pulser at 25V.
Getting nice echoes (x-label in us)
The gain control works as well =)
and the FFT looks relatively clean!
I needed to replace some elements, some stuff is erratic, but hey, I've been able to set up all registers, run the analog part, and control the DAC gain. I (just) need to run the ADC now, the fpga still thinks s.he's not working ;)
Some pulse control works, and pulses too! With a 10x probe, on a 25V pulse
Next step.. digitalization. More on the log.