Pickup induction coil limiter

Thread Starter

Karenzee

Joined Mar 6, 2015
44
Q: it says that you have adapted the coil pick ups - what does this mean ?
A: basically, this technology has limiters applied in order to prevent them being used for certain surveillance practices. I buy them from the factory and remove the limiters that inhibit certain signals. This means that they are able to capture a wider and more fascinating range of signals.

Anyone have any informations on this?
 

MisterBill2

Joined Jan 23, 2018
8,745
In normal communications with signals of widely different amplitudes it is useful to limit the maximum amplitude of a signal, such as static bursts, in order to prevent overloading the following stages. In audio monitoring applications it is often used to protect ears of the listener. The gain needed to hear a conversation a hundred yards away will produce a painful blast from the noise of a door slamming 50 feet away.Thus amplitude limiters are valuable accessories sometimes.
A limiter on a coil pickup would be used to prevent over voltage damage to an amplifier if the connected geophone were dropped. Geophones are very sensitive vibration pickups used to detect small earth movements in geological researching. In a typical house they can easily detect footsteps. But a bump can produce a much larger output signal that will damage an amplifier stage, thus the need for voltage limiters.
 

Alec_t

Joined Sep 17, 2013
12,065
Apart from voltage limiting, perhaps bandwidth limiting is involved, e.g. by putting a capacitor in parallel with the pickup?
 

MisterBill2

Joined Jan 23, 2018
8,745
I followed that link and now I also am puzzled as to what sort of limiter would prevent a coil pickup from being used for snooping purposes. The only coil snooping that I am aware of is picking up from telephone transformers and ringer coils in the older style telephones. And how one could have such a limiter on a pickup coil and not make it useless for any purpose is not clear either. Of course, there are also guitar pickup coils, but there also, music frequencies are quite similar to speech frequencies. But guitar pickups do often have hum-bucking arrangements that would prevent recording phone signals.
But I would not call a hum bucking coil a limiter by any means. But that could be what the person means. It would limit the picking up of wider magnetic fields.
 

Thread Starter

Karenzee

Joined Mar 6, 2015
44
I followed that link and now I also am puzzled as to what sort of limiter would prevent a coil pickup from being used for snooping purposes.
Just found this link

There was no further information about the nature of the limiting circuitry, or the means of boosting the signal level, but perhaps the coils I bought were not the type with limitations, I couldn’t tell.
This page also mention induction receivers and recommend telephone pickup coil or 100mh air core.
 

MisterBill2

Joined Jan 23, 2018
8,745
My guess is that the guitar pickup caused problems with the laptop because it has a fairly strong magnet, while the other inductors do not have one. And the telephone pickup ha by far the largest effective aperture, so of course it will be more sensitive. Also, it has a lot more turns. Some pickup coils may have a lot more turns that others, and since the output is based on flux passing through conductors, the more turns the more output. At the very low currents needed for an amlifier input resistance is not likely to have much effect.
So now it would be interesting to know what your project is.
At one time our military folks discovered that "listening" to computer noise could allow knowing what was being entered, which resulted in a whole lot of shielding efforts and a program named "TEMPEST", which I have not heard mentioned recently. So picking up near field transmissions for whatever purposes is certainly one possibility. But I think that the lack of security from using a smart phone instead of a credit card is certainly not worth the risk. But for many folks being cool is far more important than anything else.
 

Thread Starter

Karenzee

Joined Mar 6, 2015
44
From what i've read so far it would be interesting to build a receiver more sensitive than the telephone pickup coil.

I found no details on the "wider range of signals" mentioned in the first post.

It woud be helpful to know what is the inductance of the telephone pickup coil and what makes a good sensitive inductor receiver.
 

MisterBill2

Joined Jan 23, 2018
8,745
To determine the inductance without an inductance meter is quite simple. You need a capacitor of an appropriate and stable value that is known. Then put that capacitor across the coil and use an oscillator and voltmeter to determine the resonant frequency. Knowing the capacitance and the frequency then the inductance just takes some math. The applied voltage needs to be low enough so that the magnetic saturation is not reached and high enough to be above the ambient noise level.That will give a close approximation of the inductance of the coil.
 

MisterBill2

Joined Jan 23, 2018
8,745
any info on amorphous magnetic core inductor?
I am not familiar with anything going by that name. I have seen filter chokes for automotive applications that use magnetized cores so that they can carry more current without saturation. But that may be different.
My method for determining inductance would work just as well with an inductor with an iron core, or a powdered iron core, a ferrite core, or an air core. The required capacitance would be different as would the frequency used, but the principle and process would not change. But if you have opened one of those telephone pickups and if you can measure the wire size, you have another option available. You can estimate the circumference of an average turn, measure the total coil resistance, and then use a wire resistance table to calculate the length of wire. Then, using the length per turn you can calculate the number of turns. Of course, going from the number of turns to the actual inductance, using a look-up table may not be super accurate.

AND, now I am wondering if the "limiter" that the supplier removes is simply a series resistor, or possily a two-resistor voltage divider. That would limit the amplitude of the signal and probably put it below the noise level of a typical cheap amplifier module.
 

Thread Starter

Karenzee

Joined Mar 6, 2015
44
AND, now I am wondering if the "limiter" that the supplier removes is simply a series resistor, or possily a two-resistor voltage divider. That would limit the amplitude of the signal and probably put it below the noise level of a typical cheap amplifier module.
The author of the blog above also searched for a "limiter" but found nothing

Datasheet for regular telephone coil

About amorphous magnetic inductor

Because of the absence of crystalline structure amorphous alloys are magnetically soft (lower coercivity, lower core loss, higher permeability). High resistivity gives lower loss at higher frequencies. The losses are among the lowest of any known magnetic materials
 

MisterBill2

Joined Jan 23, 2018
8,745
OK, I am familiar with soft-iron versus hard iron. Often referenced as High frequency versus low frequency iron, and transformer grade versus magnet grade. Yes, that stuff would have a better frequency response, no doubt. But how much difference it would make in an application is a different question. That relates to the question of what you want to do with the signals that you intercept. For listening it probably does not matter much, for intercepting private data it may matter quite a bit. Like the guy sitting near the cashier in Starbux, capturing credit device information. That guy needs the ultimate response and sensitivity.
 

Thread Starter

Karenzee

Joined Mar 6, 2015
44
Telephone pickup coil are good for general listening, but for some devices (clocks) not sensitive and precise enough.

Not having the informations concerning the mentioned "limiter" and the "wider range of signals", the amorphous core seems to be an interesting (and cheaper) choice.
 

MisterBill2

Joined Jan 23, 2018
8,745
Telephone pickup coil are good for general listening, but for some devices (clocks) not sensitive and precise enough.

Not having the informations concerning the mentioned "limiter" and the "wider range of signals", the amorphous core seems to be an interesting (and cheaper) choice.
My guess is that the telephone coils already have a relatively softer core because it would cost less and be easier to shape into the desired form. So you may already be close to what you are looking for. You may be able to experiment by making a coupling loop out of a few turns of wire and then usin a signal generator to drive it, while noting the relative response as the frequency is varied.Even if it is not perfect it could give you a good insight into what your pickup can do.
 
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