Geotech PI

[Chris]

Hi Carl

Why are you not calling your detector "Geotech PI"? Sounds not bad. Or?
I had a closer look at your PI-1 circuit and found that in your next revision you should connect PIN 7 of IC7 as well to digital ground.
This makes in function no difference but from my point of view belongs IC7 (the control) to the clock circuit and should be connected to digital ground (-5D).
Which function has the var. resistor R33?
And what is AutoTrack Speed?

Chris

[FJ]

Re: Geotech PI

I'm curious why you consider -5D "digital ground". This circuit uses a bipolar supply and the -5V is the negative rail while the + side of the battery is the ground connection.

R33 isn't a variable resistor???

AutoTrack is usually called a self adjusting threshold (S.A.T.) circuit and the "AutoTrack" control will vary how fast it recovers or reacts. This circuit is used in a motion machine which means you have to keep the coil moving or it will tune out the target.
Fred

[Chris]

Re: Geotech PI

Hi Fred

I know that -5 is the negative rail in the circuit. I mean all IC's and components of the clock circuit except IC7 are connected to negative -5D. The rest is connected to -5. I assume that -5D is the digital (for clock) and -5 is analogue ground. And from my point of view belongs the IC7 (power PIN's and switch inputs)to the clock circuit as well and should be connected than to -5D. I'm not saying that's wrong and it has the same function as it is connected to -5. Only to be consequent in design.


Thanks for the explanation of the Auto Track function.

Chris

[Chris]

Re: Geotech PI

Hi,

error, I mean what is the function of R23 (variable resistor)?

Chris

[Russ_NY]

Re: Geotech PI R23

R 23 is used to balance the level of the two sample pulses going thru the two 4066 gates into the integrator. the internal resistance of the 4066 is not always consistant from gate to gate within the device, and the 2 input resistors for the integrator may not be perfectly matched.

Russ

[Carl]

Re: Geotech PI

I might go with Geotech PI, was looking for something a little snazzier. Most of the snazzy names are used, Piranha being one of my favorites.

Which supplies to use for the switches is usually determined by how the switches are used. My preference is to put analog switches on the analog supplies, especially when the analog being switched is sensitive to noise. In ADC design, we always use the cleanest analog supplies for the sample-and-hold switches. The switch that is used for the non-VCO audio chopper would be better suited for digital supplies.

Russ is correct about R23. It is used to balance out the differential integrator, due to mismatches in R24/R25 and R26/R27, plus the mismatches in switch resistance, although that tends to be minor. I did this to avoid requiring precision resistors. There is a slick way of adjusting R23: before IC6 & IC8 are installed, ground TP5 and apply +5 to TP4, then adjust R23 until the voltage difference between pins 2 & 3 of IC8 equals zero.

Fred is correct about autotrack. Some detectors call it SAT. I made the retune speed adjustable ('cause it's simple to do) and made it switchable.

Someone has pointed out, correctly, that the JFET-input opamps, like the TL072/082, have enough input leakage current to screw up IC8b when AutoTrack is off. My personal intent for the "off" option is for short-duration pinpointing. Normal hunting is done in AutoTrack mode. HOWEVER... if IC8 is replaced with a CMOS opamp, then you could probably hunt for quite some time with the switch off.

- Carl

[Esteban Cabrera Grinok]

Re: Geotech PI

Carl,

Do you know piranha (piraña) in Guarani indian language (South America) mean "devil's fish"?

"Piraña" (guarani) is contraction of two words. Pirá: fish. Añá: devil.