TP4 waveform wrong?

[exciter]

Thx NC-Carl!
Is there a way to modify the Threshold to get TP9 more negative, so that the speaker makes no noise?
Or to get the voltage on Tp9 more stable?
I´ve controlled all values, but maybe some resistors tolerance is to big?

[Reg]

Hi Exciter,

I suspect that Carl is right and the noise you hear is external noise caused by some interference nearby. PI type detectors such as the Hammerhead are affected very easily by external noise.

You should try Carl's suggestion of reducing the sample width. This is R 46. By reducing this pot, you reduce the width of the time the signal is sampled. This could reduce the noise some.

Now, you should also try adjusting the pulse frequency a little. The pulse frequency adjustment is R2. Adjust this control slowly and listen carefully to the noise to see what setting is best and has the least noise. Sometimes, this adjustment is critical, meaning just a little adjustment will make a big difference. This takes practice to find the right adjustment.

One other thing you can try is what I mentioned before and that is to make a figure 8 coil.

Cheers,

Reg

[1843]

Sometimes, this adjustment is critical, meaning just a little adjustment will make a big difference.
This takes practice to find the right adjustment.

Hi Reg,

You are right! Why frequency is such critical??? Some PIs become stable only when they are set to a critical frequency...

Another question is that how much sample pulse width is important in detection depth?
In low frequency PIs (65-100Hz), sample pulse width is high (45us) and if we reduce it, Their reaction to the targets will be slow...

What is your idea?

Best regards

[Carl-NC]

Another question is that how much sample pulse width is important in detection depth?
In low frequency PIs (65-100Hz), sample pulse width is high (45us) and if we reduce it, Their reaction to the targets will be slow...

The integrator has 2 time constants... one when the switch is "on" for charging the cap, and one when the switch is "off" for discharging the cap.

Cap charging is determined by the charge current (voltage applied to the feedforward resistor), the cap value, and the amount of time the switch is on. Discharging is determined by the cap & feedback resistor, and the amount of time the switch is off, which is roughly the pulse period. So the slower you run the pulse rate, the more integrator discharge you get. Reducing sample width weakens the cap charge, so you end up with poor integrator performance.

- Carl

[1843]

Thank you Carl for fast reply.

[exciter]

Hi,
thx for reply.
today i was on a small lake with a little beach.
before i went, i turned R46 to a small value and HH worked very well
found much trash and some coins
the difficult thing is to get a feeling for how deep the target lies and so on.
i will go an build a new case and coil, because it doesnt look any good:
for the body i will take this :

here is a picture of our HH

[gef12]

short one

Hi exciter
It might pay to wrap your coil lead around the shaft firmly . 3 reasons
Walking thru the bush / scrub your lead as it is may get caught on shrubry and pull the cable ... Secondly The cable being loose and waving about might cause some instability in your machine. Thirdly if you more than enough cable to spare you can shorten it thus lesson the total capacitance on coil and cable.
For info.
cheers
gef

[exciter]

hi gef12.
ok thanks. i will do this. im new to this, so its good someone gives help
I check this out in a few hours.

[Reg]

Hi 1843,

Your question about the frequency being critical is a little difficult to answer. Probably the best way I can think of is there are a multitude of different frequencies that can beat with the frequency of the Hammerhead and end up with an audio response. So, the signals can be coming from a wide range of places. To minimize the audio interference, you have to find that right frequency that makes the audio from the external source seem invisible. To do this, you can sync to the other audio or make the beat frequency to far off it just sounds like an intermittent noise or not at all, depending upon just how the beating occurs.

Because of the differential integrators, only the higher frequencies are a problem. Low frequencies such as the power line noise are generally canceled to a point. However, if the gain is left at the very high gain, and the delay is less than about 15 usec, then the low frequencies can start to cause problems. The reason is because the gain at the early sample and that of the later sample are not the same.

Eric Foster mentioned this in his post about preamps that I linked to on the first page of this thread.

So, the noise problem can be difficult to deal with for a lot of reasons and in some cases, there are no easy answers.

As for sample width and what is going on, Carl did a good job of explaining what is happening. If you look with a scope at the output of the differential integrator, you will see the signals droop and decay.

I recommend a person pick a weak target signal and adjust the sample width to see just what happens. Remember, the sample width is like a window. The wider it is, the more noise it can pick up. On the down side, a very narrow sample width does not allow for maximum signal gain either.

I would recommend you use a small piece of lead as a test target. Since lead has a short response, it will show what happens to short signals. Now, you want to bury that target deep enough that the signal is weak, or use something as a spacer between the target and the coil if you do an air test, so the signal is weak enough that any slight difference is noticed. Now, by reducing the sample width, you will be able to hear any target change. What you should hear is if you adjust the sample short enough, you will get some depth or signal loss. However, because of the short sample you should reduce some of the noise problems also. So, there is always some form of trade off.

Your ultra low pulse rate really could use a different form of sample hold than that used by the HH. It would benefit from a form of sample hold like that used in the Stuart microcontroller PI. No, you don't need all the complexity used there, but do need the cap C6 and the analog switch just before it. By using this type of sample hold, the signal doesn't decay like it does in the HH. Each sample would need its own opamp. Then the outputs of those amps could feed the differential integrator.

I would recommend you use a faster pulse rate if you are using a differential integrator. On the HH IC 8a is the differential integrator.

Cheers,

Reg

[Reg]

Hi Exciter,

I am glad to hear your detector is working better. I think you will find it will work better when you are farther away from the city and industrialized areas. The farther you are away from any of the noise generating sources, the less noise you will experience. Normally, regular power lines do not cause a problem, but if you have some of the large high voltage ones nearby, then they can cause problems with any detector, especially PI type detectors such as the HH.

Also, you need to make sure your frequency control is an external adjustment so you can easily make changes to see if it helps. This is one that is important when noise is present.

It looks like you guys were having fun with the detector. The more you use it and work with the controls, the more they will make sense. Also, you might want to read the HH article again and look carefully at the pictures to see if things make more sense now. It will all become clearer with time and practice.


Cheers,

Reg

[exciter]

Reg you are right
its fun to use it.
i will go an make freq. an external control. only got width, threshold, sens, volume, master delay external for now.
will go and buy some plastic screws to easy change the coils.

[1843]

Reg

Thank you for reply,
Cheers

[exciter]

hi,
have one question about coil connection.
Im using PL-259 connector parts.
is this good or should i use some other connectors?
can i use Aluminium case and connect is to Ground/V+ for HH. Must it be isolated, because i heard, that Detector makes noise, when he touches Ground???

[Carl-NC]

PL-259 is fine. Connect the shell case to HH ground (which, confusingly, is the + side of the battery).

[exciter]

Is it normal, that when i knock slightly against the coil, HH beeps??
i firmly wind the coil.
or does this always happen, when using a coil without mounting it to the shaft, and loose lead?