Mod TX of Coinmaster, TM808, Eagle

[mikebg]

To WM6

WM6, look into your English dictionary to see that "modulate" means change. If we use the term "dumping" of TX tank circuit, then the real process is CHANGE OF DUMPING due motion of TX coil relative to a halfspace core (ground) and change electromagnetic properties of the core (because the soils are different). A thorough visual explanation will be made with pieces that we have now begun to plot.

[pebe]

I don’t see any likeness to your first circuit and I don’t agree with many of your terms

If you connect an AC voltmeter across TX coil, you'll notice that when TX coil is close to the conductive ground, the amplitude of oscillation decreased. This amplitude modulation is parasitic, because ground absorbs energy from LC tank circuit.

The amplitude simply ‘varies’ – it is not ‘modulation’.
It is not caused by anything ‘parasitic’ it is due to the loading on the tuned circuit – the same way any supply is affected by loading.

If TX signal is modulated by ground, RX coil receives two ground signals:
1. AIR signal, modulated by change of height. The AIR signal is due to imperfect induction balance.
2. GND signal, modulated by change of height. The GND signal is due to soil properties.

The Rx coil will receive a single sine wave whose amplitude and phase are the vector sums of the two signals you mention.

In the modification, current pump carges tank circuit with a large portion of energy. Thus the LC tank overflowing. Indeed, excess energy is returned to the supply rails. When the TX coil is close to the conductive ground, the excess energy becomes less and supply rails receive less energy returned.
Return of energy to the supply rails is not the best solution to the problem of "amplitude dismodulation".

This is not so. The circuit is that of an oscillator whose amplitude is controlled by negative feedback – as in normal practice. When the amplitude reaches a predetermined level, feedback reduces the forward bias of the cross-coupled pair (and hence the gain of the oscillator) to prevent any further increase. Excess energy is not returned to the supply rails because there was never any excess to start with.

You have introduced a new word ‘dismodulation’. I have never heard of it!

[WM6]

If you connect an AC voltmeter across TX coil, you'll notice that when TX coil is close to the conductive ground, the amplitude of oscillation decreased. This amplitude modulation is parasitic,

.

Hi mikebg,

I mean here we can talk about dumping not about modulating signal. Maybe we have problem with terminology.

[mikebg]

Parasitic amplitude modulation of TX

If you connect an AC voltmeter across TX coil, you'll notice that when TX coil is close to the conductive ground, the amplitude of oscillation decreased. This amplitude modulation is parasitic, because ground absorbs energy from LC tank circuit.
The TX transistor operates as current pump, charging LC tank with energy. White's uses the worst circuit diagram for TX, because each current pulse contains stabile portion energy. When the height of sensing head above conductive ground changes, the amplitude of oscillation in TX tank modulates.
To avoid this are used amplitude stabilizers for TX (see attached TX circuit of Fisher 440). Garrett uses P-I controller for amplitude stabilisation. I posted several TX circuit diagrams of Garrett in this section.
If TX signal is modulated by ground, RX coil receives two ground signals:
1. AIR signal, modulated by change of height. The AIR signal is due to imperfect induction balance.
2. GND signal, modulated by change of height. The GND signal is due to soil properties.
Damage from parasitic amplitude modulation for detection of deep targets, will be described in detail in other postings.
In the modification, current pump carges tank circuit with a large portion of energy. Thus the LC tank overflowing. Indeed, excess energy is returned to the supply rails. When the TX coil is close to the conductive ground, the excess energy becomes less and supply rails receive less energy returned.
Return of energy to the supply rails is not the best solution to the problem of "amplitude dismodulation". I tried to find the best solution, and since experimented circuit works very well prepared for it publication of the Forum.
The REMI group warned me that if I publish this imperfect circuit diagram, they will nominate me for the prize "carbon paper", which gives designers incompetents reckless copying of foreign structures.
The bad thing is that they refuse to improve my circuit, I've posted in the forum. Improvement was my task for the implementation of which should have read the literature on P-I-D controllers and their methods of adjustment. Moreover, should make mathematical analysis for antenna matching and analysis for efficiency of TX coil with limited weight. I have no desire to do so, because my TX circuit is working well. I hope that someday they will do and this job because their hobby is to create superior designs.

[pebe]

Mod TX of Coinmaster, TM808, Eagle

The purpose of the modification is to suppress parasitic AM of TX when the height of sensing head above conductive ground changes.
The new element is a Schottky diode SD. Resistor R3 is only required during setup.
For setup, TX coil is placed above salt water. The resistance of R2 is reduced untill SD start to conduct short current pulses. This is energy that returns to the supply rails, but not to overdo because it appeared harmonic frequencies in the TX spectrum.

Component values and waveform amplitudes would have helped to understand the circuit. The circuit on its own is quite meaningless, as is the unqualified term 'parasitic AM of TX'.

[WM6]

Mod TX of Coinmaster, TM808, Eagle

The purpose of the modification is to suppress parasitic AM of TX when the height of sensing head above conductive ground changes.
The new element is a Schottky diode SD. Resistor R3 is only required during setup.
For setup, TX coil is placed above salt water. The resistance of R2 is reduced untill SD start to conduct short current pulses. This is energy that returns to the supply rails, but not to overdo because it appeared harmonic frequencies in the TX spectrum.

Hi mikebg

What mean here parasitic AM (parasitic amplitude modulation)?

[mikebg]

Mod TX of Coinmaster, TM808, Eagle

The purpose of the modification is to suppress parasitic AM of TX when the height of sensing head above conductive ground changes.
The new element is a Schottky diode SD. Resistor R3 is only required during setup.
For setup, TX coil is placed above salt water. The resistance of R2 is reduced untill SD start to conduct short current pulses. This is energy that returns to the supply rails, but not to overdo because it appeared harmonic frequencies in the TX spectrum.