Surf PI 1.2 Test point scope pictures ???????????????????????

[kenny444]

Hello fellow members and kit builders, would someone please post oscilloscope screen shots of test points 1 and 2 so that those amongst us with little experience can learn and so be useful in the future to others. Also any thoughts on what the Tune pot may do as mine seems to do nothing. Thank you for reading and thank you for you consideration. Kenny

[Martin G7MRV]

I'll second that!

Although experienced in electronics (im a broadcasting engineer) I seem to be having trouble finding enough information about the coils ideal impedance (ive seen everywhere from 200uH up to 350uH mentioned), and exactly what I should be looking for on the 'scope when testing!

Does anyone have the target specs for the coil, and a guide to alignment of the board and selection of damping resistor? As Ken says, a few 'scope screen shots, or even sketches, would be very handy for those of us for who detector circuits are a rather new subject

Cheers
Martin

[kenny444]

Martin, firstly I got a meter to measure the inductance, aiming for 250uH to 300uH, but not all cable/wire types produce the same Inductance for the same size and turns. Multi core with various coatings, single strand of different thickness's all differ. 1 wind your coil, 2 measure your coil, 3 disconnect the the fixed damping resistor, 4 solder in a variable pot combination ((1200R in series with 200R) both in parallel with 1200R), 5 switch on, 6 scope the test pin 6 IC (5534) center the wave form with the offset cermet, 7 adjust the pot for slight ringing on the return from -(minus peak), 8 switch off. 9 measure pot setting (coil removed) fit fixed 1W or 2W resistor of that value. that is one coil damped. Ken

[Silver Dollar]

For pictures of damping look in this thread; http://www.geotech1.com/forums/showt...s-best-Damping

Here is a diagram.

The signal will move in the direction of the red arrow. When it gets to min the little bumps in red will appear.
You want to set it to min just before the bumps appear. It's best done with a scope and variable resistor.
The width of that lower pulse (that the red arrow points at) is the slowest time your coil can see. In this case it
looks like 30 us (20 us per division) so you would miss the small gold that a 10 us coil might see.

For inductance it is a trade off. A larger inductance will have a higher voltage for the RX but less current for the TX.
It will also be slower. Large inductance's are sometimes used for towed coils where you are looking for large deep objects.

The "sweet" spot appears to be right around 300 uh. If you run separate RX and TX the RX might like 450 uh better.

Some run a center tapped coil so the TX sees 80 uh and the RX sees 300 uh. This puts more current into the coil for
a larger field strength.

[kenny444]

Thank you Silver Dollar, I fully appreciate the information and feel sure that other forum members who are building and reading will also benefit for your input. Grateful.