GPX Series Operating Frequency

Prospecting Australia

Help Support Prospecting Australia:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Joined
Mar 4, 2018
Messages
661
Reaction score
1,263
G'day

Just wondering if anyone can confirm the operating frequency for the GPX series of detectors please?

Also, does it vary between the timings?

Cheers
 
BigL said:
G'day
Just wondering if anyone can confirm the operating frequency for the GPX series of detectors please?
Also, does it vary between the timings?

Presumably you're referring to the frequency of transmission of the pulse induction pulses?
The GPX series use MPS technology to vary the rate at which pulses are transmitted, so if I understand the technology correctly, I don't think there is any particular 'operating frequency' as such:
1557970806_mps.jpg
 
BigL said:
G'day

Just wondering if anyone can confirm the operating frequency for the GPX series of detectors please?

Also, does it vary between the timings?

Cheers

2.4mhz to 2.5mhz and it varies when you hit autotune or manual tune.
The detector clock frequency doens't change between the timings, but the timing cycle time does.

If you are asking in terms of a transmit waveform, the cycle time being different between different timings, then technically the frequency is different between timings, but there is a heck of a lot more to it than that.

Cheers Mick
 
Adding to Grubstake above:
From what I can tell with Pulse Induction detectors frequency of the pulses can vary between different brands/types of PI detectors & isn't as critical to know as it is with Induction Balance (VLF) type detectors where lower frequencies (3khz) are better on large, deep targets & higher frequencies (48khz) are better on smaller, shallow targets.
Minelab MPS (Multi Period Sensing) type metal detectors are sensitive & deep seeking simultaneously.
Minelab said:
MPS technologies transmit multiple frequencies and are therefore simultaneously sensitive to small and deep large targets.
I can't find anywhere that Minelab quote this frequency range? Maybe for good reason but no doubt measurable with the right equipment.
I have seen somewhere that the SDC (Multi Period Fast) operates at 3khz (3000 signals per second) which is much faster than conventional PI detectors?

IMO the Pulse length/Pulse delay used in the different MPS detectors timings & in the SDC would be more important in understanding why certain timings or detectors are better on certain types of gold.
 
Hi Grubstake,

Yes I was referring to its transmission frequency. I'm inclind to think that it's TX frequency wouldn't vary much otherwise it would have issues with its TX output stage heating up from standing waves.

Has anyone tested its output on a spectrum analyser or occiliscope?

Minelab interprets MPS as "Pulses of different time periods". I don't interrpret this as various TX frequencies at all... as in MULTI mode on a Equinox 800. I interpret this as pulses of a given frequency with varied timing between the pulses. Two completely differrrent things!

Anyway, the reason I ask is I want to understand the relationship against my self resonance test's conducted on various GPX mono coils.
 
BigL said:
Minelab interprets MPS as "Pulses of different time periods". I don't interrpret this as various TX frequencies at all... as in MULTI mode on a Equinox 800. I interpret this as pulses of a given frequency with varied timing between the pulses. Two completely differrrent things!

Anyway, the reason I ask is I want to understand the relationship against my self resonance test's conducted on various GPX mono coils.
The quote I put in above is from Minelab & their interpretation not mine. They also say:
Minelab said:
Minelabs revolutionary BBS, FBS and MPS technologies transmit multiple frequencies and are therefore simultaneously sensitive to small and deep large targets.
https://www.minelab.com/anz/knowledge-base/getting-started/glossary-of-terms#255029
So they put MPS in with BBS & FBS as multi frequency transmission.

Maybe someone with the correct measuring equipment can help with your coil tests? I doubt Minelab will reveal anything except something broad like between 0-100khz.
 
I did here a small bit of talk once it is around the 100khz area.

Though I don't believe any thing until I see it
in real life. :)

Poses another interesting question.
I wonder which Air Craft Radar interferes with a GPX. ?
Altimeter or weather. ?
Most commonly caused by large passenger aircraft.
Think Boeing 747 and such.
 
Thanks gents for your responses above.. things are starting to make a little more sense now.

I'll have a few more questions to come soon, but in the meantime how do you know it's 2.4 to 2.5 MHz ??? Have you or someone else measured it or witnessed it under test? The reason I ask is because this isn't correlating with my self resonance tests on ML and NF mono coils
 
BigL said:
Thanks gents for your responses above.. things are starting to make a little more sense now.
I'll have a few more questions to come soon, but in the meantime how do you know it's 2.4 to 2.5 MHz ??? Have you or someone else measured it? The reason I ask is because this isn't correlating with my self resonance tests on ML and NF mono coils

AuMan is a technician who manufactures electrical gear, me I'm just a bloke with a (very) basic understanding of such matters, but in this case I believe he is referring to what he called "the detector clock frequency" and not to the transmitted signal frequency that you're seeking. I'd assume that the detector clock frequency more or less corresponds to processor speed in computing terms.
 
Basically, the frequency I speak of is the engine that controls the timings. Speed it up and sample delays, transmits and cycle time are shorter.
When you adjust the manual tune, you adjust this frequency.

Pulse induction doesn't work like vlf (Continuous wave) detectors.
Vlf detectors do have a frequency, eg 20khz and the coil will have a 20khz sine wave applied, the higher frequency ones are normally more sensitive, but more susceptible to ground conditions.

In Pulse Induction detectors, there will be a transmit pulse where the coil is energised and quickly turned off to induce eddy currents in metal targets.
Then at a small period of time after the transmit pulse ends samples of the received signal will be taken and summed(a calculation is made) to cancel the ground effects.
Sample delay plays a big part in sensitivity as well time between samples and how long they sample for.

To compare one type of pi detector to another based on frequency just won't work. They are a very complicated detector!

Cheers Mick
 
Clock speed is process related, ie the speed of number crunching or how many times you perform a task over a given timeframe.

I'm only interested in knowing the frequency at which the GPX detector is transmitting on, regardless if it's pulsing a sine wave, square wave or whatever. The type of emission is irrelevant, the frequency on which, not at which it's transmitted is my question., ie RF. Has anyone measured this please?

I agree comparing one type of PI against another won't work, particularly if the operating frequencies and reception algorithms differ. But if they are the same, like all GPX's are which is why the coils are interchangeable then.....

And so that's my question, has anyone measured a GPX operating frequency?
 
... Bingo!

great thanks Ded Driver, will check it out once I get home from work tomorrow. An occiliscope is one tool you can use to measure the frequency so hopefully his vids sheds some light. cheers!
 
Yes it may vary but that's fine, I'm only interested in the frequency of the signal itself which is transmitted to the coil, this is not the same as the frequency in time or delay in time between transmissions. To energise the coil with current your detector is transmitting an RF signal to the coil and this is what I want to understand.

This RF frequency can be any combination of signals, short or long.. different types of signals, whatever.... Minelabs own pattern if you like. If you know what you transmit, and know what you get back, and can decipher the difference, you are effectlvely analysing the return signal = computer algorithms.

The coil must be tuned for the same operating frequency of the detector, otherwise you will not saturate the coil with current = shit performance.

I think I mentioned already I have measured the self resonance of some GPX mono coils, made by Minelab and Nugget Finder.
 
BigL said:
Yes it may vary but that's fine, I'm only interested in the frequency of the signal itself which is transmitted to the coil, this is not the same as the frequency in time or delay in time between transmissions. To energise the coil with current your detector is transmitting an RF signal to the coil and this is what I want to understand.
This RF frequency can be any combination of signals, short or long.. different types of signals, whatever.... Minelabs own pattern if you like. If you know what you transmit, and know what you get back, and can decipher the difference, you are effectlvely analysing the return signal = computer algorithms.
The coil must be tuned for the same operating frequency of the detector, otherwise you will not saturate the coil with current = shite performance.
I think I mentioned already I have measured the self resonance of some GPX mono coils, made by Minelab and Nugget Finder.

You're thinking in terms of analog AC circuits, whereas PI is closer to DC digital: current is either on or off. Have a look at this article by Reg Sniff - he explicitly deals with your query close to the beginning.

UNDERSTANDING THE PI METAL DETECTOR (Reg Sniff):
http://chemelec.com/Projects/Metal-1a/Understanding-the-PI-Detector.htm

Reg Sniff said:
One question that is often asked is what is the operating frequency of a PI. This question is often asked by someone who is trying to relate their knowledge of VLF's to the PI. Unfortunately, because of the nature or differences between types of detectors, comparing a PI to a VLF is sort of like comparing an apple to a potato, so trying to relate the operating frequency of a PI to a VLF or sensitivity to small gold is of little value. The differences between the two types of detectors or the affects of their operating frequencies are quite dramatic so it is best to not try to use the same standards when trying to determine certain things about a PI.

As for a PI, the pulse rate or pulses per second (pps) refers to the number of high current pulses that occur over the time specified. Rates vary from a few hundred to several thousand per second: Generally, more pulses allow for a little better averaging and thus a little better signal to noise ratio. However, a detector will have a tendency to consume more current with a higher pulse rate. A faster pulse rate doesn't mean a detector will detect small gold better. In fact, it is quite easy to build a PI that has a very low pulse repetition rate (PPS) that is very sensitive to very small gold while designing a PI with a high PPS that is not sensitive to small nuggets.

This PDF includes a diagram showing some PI coil waveforms:
Pulse Induction Metal Detectors - The Principle
 
I think you've missed the point BigL.. any Techs on here please correct me if Im miss-stating this.
A PI machine does not transmit an RF signal like a VLF (that transmits a continuous sine wave of a certain frequency).
In a PI machine the Tx pulses energise the coil. The machine then samples & analyses the rise & decay rates of the induced Rx signal . Hence the term "Pulse Induction'.
 
DD,
Pretty close.
It is important to understand that a detector potentially transmits a pulse train of both varying period? and mark/space ratio.
A true square wave has a fundamental frequency and an infinite number of odd harmonics.
As I understand it, (never measured it personally) the pulse train has a varying period so there is a different fundamental frequency for each varying period.
I haven't done the sums to calculate each fundamental frequency.

This signal is then applied to "the coil" that effectively attenuates the odd harmonics falling outside the bandwidth of the coil leaving only the varying fundamental frequencies.

"the coil" is effectively a tuned LC circuit with its center frequency determined by the inductance and capacitance.

I would expect that all the fundamental frequencies fall within the bandwidth of the coil and are transmitted relatively un-attenuated into the soil.

Hopefully others will correct/clarify my comments if I'm on the wrong track in trying to clear up the OP's question :)
 
Unlike VLF, PI systems may use a single coil as both transmitter and receiver, or they may have two or even three coils working together. This technology sends powerful, short bursts (pulses) of current through a coil of wire. Each pulse generates a brief magnetic field. When the pulse ends, the magnetic field reverses polarity and collapses very suddenly, resulting in a sharp electrical spike. This spike lasts a few microseconds (millionths of a second) and causes another current to run through the coil. This current is called the reflected pulse and is extremely short, lasting only about 30 microseconds. Another pulse is then sent and the process repeats. A typical PI-based metal detector sends about 100 pulses per second, but the number can vary greatly based on the manufacturer and model, ranging from a couple of dozen pulses per second to over a thousand.

If the metal detector is over a metal object, the pulse creates an opposite magnetic field in the object. When the pulse's magnetic field collapses, causing the reflected pulse, the magnetic field of the object makes it take longer for the reflected pulse to completely disappear. This process works something like echoes: If you yell in a room with only a few hard surfaces, you probably hear only a very brief echo, or you may not hear one at all; but if you yell in a room with a lot of hard surfaces, the echo lasts .

In a PI metal detector, the magnetic fields from target objects add their "echo" to the reflected pulse, making it last a fraction longer than it would without them.
A sampling circuit in the metal detector is set to monitor the length of the reflected pulse. By comparing it to the expected length, the circuit can determine if another magnetic field has caused the reflected pulse to take longer to decay. If the decay of the reflected pulse takes more than a few microseconds longer than normal, there is probably a metal object interfering with it.

The sampling circuit sends the tiny, weak signals that it monitors to a device call an integrator. The integrator reads the signals from the sampling circuit, amplifying and converting them to direct current (DC). The direct current's voltage is connected to an audio circuit, where it is changed into a tone that the metal detector uses to indicate that a target object has been found.

PI-based detectors are not very good at discrimination because the reflected pulse length of various metals are not easily separated. However, they are useful in many situations in which VLF-based metal detectors would have difficulty, such as in areas that have highly conductive material in the soil or general environment. A good example of such a situation is salt-water exploration. Also, PI-based systems can often detect metal much deeper in the ground than other systems.
 
Ded Driver said:
I think you've missed the point BigL.. any Techs on here please correct me if Im miss-stating this.
A PI machine does not transmit an RF signal like a VLF (that transmits a continuous sine wave of a certain frequency).
In a PI machine the Tx pulses energise the coil. The machine then samples & analyses the rise & decay rates of the induced Rx signal . Hence the term "Pulse Induction'.

Let me put things this way... if you take a VLF and modify its TX signal so its pulsing as apposed to being a continuos wave, and of course you set the reception alogorithms up to interpret the returned pulses, you now have a PI. So, your still energising the coil with current except your now doing it in an intermitant fashion = Pulsing. So from an electronics standpoint the parameters of the coil don't change at all unless of course you change the operating frequency of the detector
 

Latest posts

Top