Preamp

[6666]

Somewhere on a broken hard drive. Will have to redo it from memory if I find the time. Very simple circuit.

Thanks.

[Davor]

In the presence of a target, the Rx signal is the difference of two exponentials: the Rx coil own decay minus the decay of the target. A low damping R means the Rx coil decays slowly and sampling will need to be done later, reducing S/N.

Actually ... no. In case Rx coil is balanced and you don't get high voltage spikes, so that you don't have non-linearities at front end, the Rx coil's response is a cyclostationary one, on top of which is piggybacking a target signal. If you subtract the cyclostationary component (the one that is non changing from one cycle to another), you'll see a target signal in perfect exponential form, perfectly linear in lin/log scale, starting much sooner than you expect.

Mono sucks at this department due to the preamp non-linearity at high voltages.

[Old cart]

As I remember, the spiral wrap was to give a distance from the coil bundle to the shield. The coils themselves were bundle wound, not spiral wound.

Spider web or basket wind coils have indeed lower capacitance by themselves, but most of that advantage is lost when shielding.

I thought that the basket weave coil was self shielding so that no external shield is required.

[Teleno]

Actually ... no. In case Rx coil is balanced and you don't get high voltage spikes, so that you don't have non-linearities at front end, the Rx coil's response is a cyclostationary one, on top of which is piggybacking a target signal. If you subtract the cyclostationary component (the one that is non changing from one cycle to another), you'll see a target signal in perfect exponential form, perfectly linear in lin/log scale, starting much sooner than you expect.

Mono sucks at this department due to the preamp non-linearity at high voltages.

A target invariably unbalances the coil causing a spike ranging up to a few volts. This spke decays at the tau of the LRC around the coil. The target signal induces a voltage in the opposite direction.

[Monolith]

Do you still have the circuit ?

Here is the link to the thread, discussing the initial circuit.

http://www.geotech1.com/forums/showt...5228#post45228

Basically it is a soft clipping circuit.

When we use a hard clipping circuit, the reverse recovery of the diodes messes up the wave form.
Using specific very soft recovery Schottky diodes would help in this respect.

Using the transistor diode, again, choosing a specific type of transistor, with soft recovery is about the same. However, a further development of the circuit, I have not found it yet, uses biasing on the transistor base, applying feedback from the opamp output.
Using feedback, when the output goes near the rail, the transistor starts conducting strongly, so the higher voltage is not amplified, and then clipped softly so that the opamp does never saturate.

With PI, we are only interested in the part of the signal that is near 0V, therefore we can amplify the part of the signal that we are interested in, very much and discard the unimportant part.

[6666]

Here is the link to the thread, discussing the initial circuit.

Thanks.

[Sean_Goddard]

That is what I wanted to have clamping for, so we can avoid saturation of the front end and have a nice low noise amplifier. Most Pi's I see, the RX signal bounces a lot at the sample point. What I would like to try is to eliminate as much of this as we can so we can use high amplification and filtering to get a smooth threshold but fast response. I have some ideas but am still researching some ideas.

If we can characterise the system noise, we can work out how best to reduce it. ��

[daverave]

i tried a small 10 pf capacitor across the fed back resistor of the first amp...made the threshold more stable but i had to change the sampling position to get any signal...so i just now removed the 10 pf capacitor and put it back to normal...the noise from the first amp seems to be the major problem of instability of the threshold...hope you find some way around this problem.

[Monolith]

That is what I wanted to have clamping for, so we can avoid saturation of the front end and have a nice low noise amplifier. Most Pi's I see, the RX signal bounces a lot at the sample point. What I would like to try is to eliminate as much of this as we can so we can use high amplification and filtering to get a smooth threshold but fast response. I have some ideas but am still researching some ideas.

If we can characterise the system noise, we can work out how best to reduce it. ��

Finding the source of the noise is obviously the first step. What exactly causes the noise.
A lot has been spoken about thermal noise. But, is this really the problem? If the RX signal bounces at the sample point, it seems this is not thermal noise.
It is a well known fact that pushing the sample very early makes the detector unstable. Why? This is one question we should find the answer to.

When we take a wide sample, say 20us wide, we integrate/average, the whole signal, including the high frequency noise during that time.
If we take a very short first sample, let's say 1us wide, we get a much higher amplitude of very short target TC response, but, any high frequency noise, makes the signal amplitude jump up and down.
Now, if we integrate/average the output, we reduce that noise again, but eliminating the noise before amplification is always better.

So, what is that input noise. Where does it come from? Would white noise/thermal noise bounce he signal? What is the frequency of the "bouncing"? Can we correlate this frequency to anything emanating from the detector circuit?
Is the noise totally random?

Let's start with this.

[Monolith]

i tried a small 10 pf capacitor across the fed back resistor of the first amp...made the threshold more stable but i had to change the sampling position to get any signal...so i just now removed the 10 pf capacitor and put it back to normal...the noise from the first amp seems to be the major problem of instability of the threshold...hope you find some way around this problem.

That is good and useful information. What is the value of the feedback resistor? Then we can calculate the frequency of this low pass filter.

[daverave]

i just used the normal 1 mega ohm feedback resistor....if i used a 15pf cap across the feed back resistor the threshold was really nice and stable but again i had to move the sample to different position to around 30 uS delay and the detection depth was fair but not as good as without the capacitor.

[Monolith]

i just used the normal 1 mega ohm feedback resistor....if i used a 15pf cap across the feed back resistor the threshold was really nice and stable but again i had to move the sample to different position to around 30 uS delay and the detection depth was fair but not as good as without the capacitor.

The cutoff frequency of the 1M-15p LP filter is about 10kHz. At this frequency it also integrates short TC targets.
What kind of test target do you use?

You say the threshold was stable. This means that there are several additional stages of signal processing up to your "stable signal". But the noise is less with the filter on the pre-amp, ergo, the noise is present at the pre-amp stage.
So lets see if we can define the noise frequency.

Do you have anything running at a frequency of above 10kHz in your circuit?
Is there anything that could produce harmonic frequency noise in your circuit?

Could the noise be from outside? like fluorescent lights?

Could you try 2p, 4p 8p 10p to see at what frequency the noise really starts?

[daverave]

my test items are US nickle...9ct gold ring....and UK old half crown coin...if i went down to say 5pf cap the noise on the wave form was still there but at 10pf a lot better....i have no frequency in my circuit above 4 khz.

[Monolith]

my test items are US nickle...9ct gold ring....and UK old half crown coin...if i went down to say 5pf cap the noise on the wave form was still there but at 10pf a lot better....i have no frequency in my circuit above 4 khz.

The Nickel has a short TC, but it is a relatively large target, good for testing.

Do you see the waveform on the Scope?

[daverave]

yes i can see the waveform...maybe once ive put circuit in metal box the threshold stability should be better.