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Originally posted by Max
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Hi,
"When you look at circuit, you wonder: why is audio proportional? Comparators U106 make a binary signal, original signal amplitude is forgotten -- right?
."
No, it isn't. In theory any comparator has a binary output (+Vcc or -Vee) but it really isn't on practice: simply doesn't exist a comparator that give output at 2 discrete levels of voltage... not considering also that you have also output stage voltage drop... in real devices.
What happens really: you have an op amp that works in open loop at highest possible gain... that is similar in behaviour to an ideal comparator ...that doesn't mean you have really, always 2 discrete levels at output and no intermediate voltage.
When you consider gain you have the answer: consider +8 and -8V as supply voltages and open loop gain of 200000 (as typ of lm393)... how much differential input voltage is required to saturate device ?
deltaV+ = +8/200000 = +40uV
deltaV- = -8/200000 = -40uV
So... you have full saturation voltages out the interval +40 to -40uV at differential inputs.
But what you'll get inside the interval, what if you have e.g. +10uV there ?
V=10uV --> Vout = +10uV*200000= +2V
+2V , not +8 or -8V... so it's an "intermediate" voltage out of discrete limits.
Why I'm doing all this ? Simple... you actually HAVE 10uV and similar voltages there... expecially when you'll measure submicrovolt influence of a small coin at 30cm from coil bottom! So you'll get not full saturation at comparators... but intermediate values and then (sadly) intermediate sound... cracks.
So, when you consider solid signals... like coins at 15cm... you'll get full sound... and real saturation of device, but on weak signals you have linear amplification and intermediate values, thus resulting in poor sound.
This explains why the audio on TGS is proportional... after a particular depth/size combination... so input level threshold... the device will give you a proportional tone related to weakness of signal or the far/small target.
Kind regards,
Max
"When you look at circuit, you wonder: why is audio proportional? Comparators U106 make a binary signal, original signal amplitude is forgotten -- right?
."No, it isn't. In theory any comparator has a binary output (+Vcc or -Vee) but it really isn't on practice: simply doesn't exist a comparator that give output at 2 discrete levels of voltage... not considering also that you have also output stage voltage drop... in real devices.
What happens really: you have an op amp that works in open loop at highest possible gain... that is similar in behaviour to an ideal comparator ...that doesn't mean you have really, always 2 discrete levels at output and no intermediate voltage.
When you consider gain you have the answer: consider +8 and -8V as supply voltages and open loop gain of 200000 (as typ of lm393)... how much differential input voltage is required to saturate device ?
deltaV+ = +8/200000 = +40uV
deltaV- = -8/200000 = -40uV
So... you have full saturation voltages out the interval +40 to -40uV at differential inputs.
But what you'll get inside the interval, what if you have e.g. +10uV there ?
V=10uV --> Vout = +10uV*200000= +2V
+2V , not +8 or -8V... so it's an "intermediate" voltage out of discrete limits.
Why I'm doing all this ? Simple... you actually HAVE 10uV and similar voltages there... expecially when you'll measure submicrovolt influence of a small coin at 30cm from coil bottom! So you'll get not full saturation at comparators... but intermediate values and then (sadly) intermediate sound... cracks.
So, when you consider solid signals... like coins at 15cm... you'll get full sound... and real saturation of device, but on weak signals you have linear amplification and intermediate values, thus resulting in poor sound.
This explains why the audio on TGS is proportional... after a particular depth/size combination... so input level threshold... the device will give you a proportional tone related to weakness of signal or the far/small target.
Kind regards,
Max
But wait 
-- I would think noise at that stage is greater than linear region anyway. Any signal small enough to be in linear region totally swamped by noise. Yes, we adjust sensitivity control to be above noise, and deep coin could move signal into linear region, but the noise would come with it. I think very low probability it stay in linear region because of noise -- but I'm just guessing!
. But still you are right because it all adds up and linear region is less volts for sure.I will keep it in mind for design experiments
.But pulse width very important too -- I keep watching signal from comparitors on scope, very low sweep. If disconnect output from rest of circuit, pulses very square (binary), even skinny ones.
Reconnect, and pulse leading edge rounded off by capacitor (trailing edge still sharp, as predicted). Very quick pulse never rise too high, less voltage. That is good I think - makes "chatter" pulses quieter. Maybe that is why trailing edge designed to drop fast, so many chatter pulses will not combine into one big false pulse.
This makes me think: maybe improvement if leading edge rises even slower so skinny chatter rejected even more -- increase C25 and increase R40? Just something more to play with, even though Tesoro is always right!
(but they must design for mass production, we can tune
.Regards,
-SB
! I would think it is that little spring connector that makes contact when you pull out earphone plug -- maybe metal-on-metal you made a diode
I study and appreciate layout even more, you kept "Sync Detector" drive signals nicely away from sensitive areas. I'm not good with PCBs or chemicals (or spending money), so my first try will probably be transfer to copper board from printer, then dremel to cut away copper between wires -- more craziness
. But I amuse myself...
!
i envy you on LT1008 !!!
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