Originally posted by Dennis the Mennis
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Love those great graphs you get with your USB scope! I have a configuration question: can you configure the graphs to have 10 dots per square instead of 12 (for easy estimate of values)? And can you configure it to show the time scale on the horizontal axis? It would be great also if description of plot can be put on graph so it can be understood without surrounding text. Just some ideas. I hope to get a similar tool some day.
Ok, now lets look at your data.
1. I think your sync pulse looks pretty normal and you should be able to discriminate targets. When I compare these new graphs to habitbraker, they quite similar in shift and location, don't you think so?
So my question is: what targets do you use for your discrimination test, and what results do you get again? (My best test target is US Nickel. TGS does not discriminate silver or even copper really).
2. I have seen a similar "spike" on my RX null signal at times. I did not consider it might be due to capacitor C9, but that could make sense due to saturation of the LF353 op amp. (The good news, as you said, is that the spike does not seem to hurt the detection. Anything very periodic at the TX frequency should not hurt.)
However, the "spikes" on the sync pulse can be definitely related to capacitor C9, because when the LF353 op amp saturates (DISC op amp, pins 1,2,3), it causes a "ringing" output at pin 1 which then can make a messy sync pulse. (Surprisingly, the messy sync pulse does not seem to cause much trouble).
So lowering value of C9 until there is no "ringing" in output at pin 1 should make the sync pulse look nicer, I believe. I think a "nicer" sync pulse may give you a little better DISC range. I looked at your video of the "wiper signal" and there is a small ringing "blip" that goes near zero at one end of the DISC pot - it may be making the spike in your DISC sync pulse signal at the very max pot setting -- but probably goes away most of the time, so no big worry.
3. I would like to see some more graphs of your LF353 output at pin 1 (compared to TX signal on other trace) for different values of C9. What value of C9 makes the ringing completely disappear? Maybe go down to 50pf, 40pf, 30pf, etc.
Actually, it looked pretty good at 68pf from your graphs, I would choose that value to keep maybe. It would be interesting to see your sync pulse at max and min DISC pot setting with C9 = 68pf.
4. I think your blob test made the point I wanted to test (even with your genuine Rolex watch involved). I need to think about it some more and maybe do my own tests carefully -- when the blob is really close to the coil maybe that causes some other effect entirely. So requires careful interpretation. But I think it shows that the significant "null signal phase" is not really what we see on our scopes, and so we shouldn't go crazy trying to set a particular value.
Thanks for all the work making the graphs and videos!
Cheers,
-SB
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