Hammerhead noise/Carl

[gene L.]

Hello, After studying the hammerhead schematic I was wondering about IC3 and IC4,why the inputs of both are connected to IC1 without Isolation . Could that be a noise starting point? Just a thought! Let us know when the new hammerhead boards are ready?! Thank-You Gene L.

[bbsailor]

Re: Hammerhead noise/Carl

Are you suggesting a series resistor and bypass capacitor to better isolate IC3 and IC4?

These connection points of IC3 and 4 are to the -VB supply to derive separate -5V supplies for the digital and analog ICs. The purpose of this was to try to reduce noise from the analog ICs from crossing over into the digital supply and vica versa.

Also, I think C1 should be 1000uf possibly in parallel with a .01uf to improve filtering. I have a 1000uF at 25 volts that seems to fit perfectly in the space allocated for C1 on the Hammerhead circuit board. Just add the .01uF to the board underside on the same C1 solder pads.

I am starting to populate the board and any hints you have discovered would be helpful.

It would be certainly helpful if we could have all those who have built the Hammerhead circuit share what they learned.

I am anxious for Carl to get the next version of the Hammerhead writeup ready for posting which incorporates the most recent builder input.

bbsailor

[Ray Huffman]

Re: Hammerhead noise/Carl

I finished the second HH this weekend. 1 addition to protect it from me was to add a diode to the -Vb input so I will not blow all the power supply ICs if I accidently hook power up backwards. Which I did once. Since I use a Car battery for power it tends to smoke it. It is mounted in a plastic radio shack project box With all controls mounted on top of the box. The box is mounted to a Bounty hunter rod and arm cuff. It was easy to do this. But I will have to get a plastic rod for the bottom half for the coil.
I built the 8" coil and use ethernet cable RG 59 or is it rg 58 anyway I hang the coil from the ceiling on some fishing line. The only problem is it sees the metal parts cabinets nearby. So for testing and calibration I use a 1.5" coil 100 turns of 22awg enamel wire. It is not faraday sheilded. I used a 680 ohm resistor for damping. Thats enough for this Post. My next one has Questions for Carl and others.
Ray

[Ray Huffman]

Re: Hammerhead noise/Carl

I have a question or two on coils. If you get the target say a 22 gr gold nugget
to close to the coil does it change the inductance? My guess is yes.
When I move a target accross the coil I get a change in the output of IC6 pin 6
which changes the slope of the signal which in turn causes the ouput of the VCO
to change indicating a target. I do not get any other indication of a target
other than that. I was looking at Eric Fosters post
http://www.findmall.com/metal/school1/config.pl?noframes;read=8134
and looking at the signal he gets from an aluminium target he gets a definate signal
shortly after the pulse decays. Is that the decay signal I should see?
If so nothing is there. What am I doing wrong. I set the main gating pulse to look
at the signal at that time but no ouput. Am I expecting more than I can see at that
Point which is TP4. My point in all of this is my coil the problem. I made a 1.5"
coil to cut interference and it does not give any better or worse target indication than the 8". If I
should see a decay signal when the target is stationary in relation to the
coil when should I see it. Thanks for your help. I keep learning as I go.
Ray

[Carl-NC]

Re: Hammerhead noise/Carl

I agree that the more isolation and filtering you can do on power supplies -- especially the switching types -- the better. Joe (bbsailor) is also correct, that a smaller high-frequency cap in parallel with the electrolytics is a "good thing" to do.

However, I don't think that's my problem. One of the mods I did on this board was to sync the 7660 off the main clock, so that power noise is synchronous. What I am seeing, is an interferer that is some high multiple of 60Hz (don't recall), because I can sync it on the scope with the line trigger.

It also doesn't change at all if I remove the sync pulse to the 7660, and let it free-run. Strangely, bending my coil to a figure-8 also didn't help, and I'm running twisted pair from the coil to the PCB.

All this started about the time I bought 2 new computers and a laser printer, and networked everything. I've tried unplugging all that mess, but no dice.

- Carl

[bbsailor]

Re: Hammerhead noise/Carl

I am planning on using an separate audio circuit board using a low voltage op amp. I will drive this op amp circuit with a 8 ohm to 1000 ohm matching transformer (Radio Shack). I will just put the 8 ohm side of the transformer in place of where the speaker is located on the schematic. I will initially try a gain of 20 on the audio op amp, although I can go up to a gain of 200 which may be too much. The op amp has an input pot (10K) that I can adjust for the optimum S/N ratio. The transformer provides a voltage gain of about 10, so having an adjustable input gain control for the op amp circuit is very helpful.

This separate audio op amp with (9 Vdc) power supply may prevent audio interactions that were reported on earlier posts. I just need to add a DPST on/off switch to turn on both batteries; the main PI battery supply and the op amp audio supply. Once I get the PI circuit working, I can the switch the op amp audio power supply over to the main 12V PI supply and see if I get any interaction.

I am planning to use two Radio Shack 23-338A 6- volt, Ni-MH 1500 ma, R/C Packs wired in series and planning to use an external charge plug.

I hope this provides some useful input for those building the Hammerhead.

bbsailor

[Robby_H]

Re: Hammerhead noise/Carl

Carl,
I haven't built your circuit but I pinched the power supply around the 7660. Your option 2 which triggers the fet from the collector requires a collector resistor that low in value in order to turn the fet on properly that it can load down the supply sufficiently causing ripple on the output of the pre-amp as this transistor is on and drawing relatively high current during the sampling period.
The other source of noise (hf ringing) I have had here was from ferrite beads in the wrong spot and chased this one for days. In the meantime I also decoupled the negative rail to the 7660 and -5v reg but I can't say whether this decoupling was needed in the end as I left it in.
There is nothing worse than noise you can't account for. I have to turn my soldering iron off to stop some noise and at one stage here noise was from a plugged in, coiled up extension lead on a shelf directly under the job and the other was from a noisy tv switch mode p/s in the guys shed next door. My shed tv is noisy and has an inline amp and I have to remember to unplug the amp also. One thing that seems certain is if the noise is always there and dead constant then it is likely from the circuit itself.
Rob.

[Ray Huffman]

Re: Hammerhead noise/Carl

Ok I answered my own questions after rereading several articles and looking at the waveforms. Also on another note as far as carls noise problem I found that the wires going to the controls can also create problems so I twisted each control pair together. Not sure if it was the solution but the noise problem is gone. You guys I hope have the patence of Job when us newbies get on the forum.
Ray