IDX-PRO: is my RX phase correct?

[joop]

I can't get my IDX-PRO to discriminate iron. In order to find the cause I have first looked into the coil configuration. I am using a white's coil, see picture. When connecting the coil to a tone generator (through a 1k resistor) and looking up the maximum amplitude while changing the frequency the TX coil resonates at about 6.57 kHz, the RX coil is at 4.90 kHz.



I have hooked up the TX oscillator to my RIGOL scope (yellow line), the scope measures 6.562 kHz which looks okay to me. The blue line shows the output of U1 (NE5534) the RX amplifier. I am measuring a phase difference of about 200 degrees.



Do these figures look okay? 200 degrees seems a bit to high, I remember the TGSL having much smaller numbers...

[Davor]

200=180+20 ... it is OK

[joop]

Aaaah, of course... actually, it is a bit more: about 220 degrees, 180+40. Okay looking good, need to go into the rest of the electronics.

Thanks Davor!

[golfnut]

Hi Joop, My IDX is ok. It rejects Iron and ferrite well. Here is an image of my Tx volts and preamp out..



Can yours phase out ferrite by adjusting the GEB pot?


Im trying to see if one or both channels are playing up..


If you can get rid of ferrite, then the initial phasing is ok and you may have a build error on the Disc gating phase shifter?

Are you certain your PP switch is the correct way - your not having a senior moment with the PP on ??



S

[joop]

Hi Golfnut,

Yes, I can phase out ferrite with the GEB pot in the mid position. I am looking into the demodulator now. I have compared both outputs of U8 (DISC and GEB channel) to the TX signal with VR1 and VR3 set at lowest and highest level, which gives a phase variation of 210-260 degrees for the DISC channel and 90-105 degrees for the GEB channel.

The sampling capacitors C4,C5 and C6 behind the CD4066 all give a voltage rise when moving a copper coin in front of the coil. When I move an iron nail in front of the coil, C4,C5 voltage rise and C6 voltage drops. So initial discrimination is looking good I guess. The error must be further down the schematic.

Yes, PP is off, nothing connected. Thanks for helping out.

[joop]

oh boy, in the original schematic D3 is a 1N5223 (2.7V zener) while in the parts list I got from the IDX thread D3 is a 1N4148. This might be the cause for my trouble.

[joop]

Yes! Iron rejection is working now! Did not have a 2V7 zener available so I have used 3 1N4148 diodes in series.

I am so happy!

[Davor]

Many people use 1N4148 in other direction, but it slows down discrimination mechanism, and reduces sensitivity. However, comparators by themselves are perfectly doing al the discrimination switching, and all there is to a zener is level shifting. I achieved it in a modified IGSL by resistors, and it has a very nice recovery speed.

[joop]

Thanks Davor, will try the zener tomorrow. Didn't have the right value in stock. Just wanted to find out where the error is.

[Davor]

Zener in this case can't be replaced with diodes in series because there is almost no current running there and voltage drop is below what you expect. In IDX you can't replace it with resistors because the PP channel interferes. So zener there is

[Koala]

I have tried both the zener and 1n4148 reversed not much between them. Disc worked with both. Distance was about the same but the detector just felt better with the zener. If a put a load of different types of targets on the ground both work the same but somehow the zener felt easier to pinpoint.

I think Davor "slows down discrimination mechanism" could well be the reason.

[joop]

I can confirm that the diodes work as well. Will try and use the zener anyway.

[galaksi]

oh boy, in the original schematic D3 is a 1N5223 (2.7V zener) while in the parts list I got from the IDX thread D3 is a 1N4148. This might be the cause for my trouble.

D3= 155223 or D3=1N4148+1N4148 serial and reverse.

[golfnut]

I get a similar thing with mine. It is as if a glitch coming thru can lock up the comparator for a short time. - As it feeds back to the comparator output to the input.

In use a glitch can leave the unit deaf to targets and deaf to the user for fractions of a second which means I can miss things in a sweep. Maybe try a 2v7.

Here is my phases while its working good.



S

[joop]

Thanks Golfnut, will do some field testing first before messing around with my GEB and DISC phasing. Made a casing today and hope to assembly the detector this weekend. Maybe I can take the IDX for a walk on tuesday...