BarnacleBill
New member
I have seen several questions and some misinformation about the multi-freq issue.
The commonly used VLF detector operates on a single frequency unless it is like an X-terra that changes when the coils are swapped out. Below are the waveforms from an X-terra X-30, Fisher ID Excel, and Fisher CZ-20 measured on one of my digital O-scopes.
[attachment 232819 ComboWForms.gif]
The first two(X30 & Excel) are sine waves, with the CZ-20 being a square wave. Now the CZ-20 the Scope has locked onto the Fundamental frequency of 5KHz and yet the CZ-20 is a dual frequency detector. In order to see both frequencies we turn to a Spectrum Analyzer where we can see the Fundamental and Harmonic at 15KHz.
[attachment 232820 cz20wvF.jpg]
Now if you are going to measure the transmit output of a FBS detector with a Frequency Counter or O-Scope you have to be careful because it will trigger on what it finds most convenient. What's being transmitted is a complex waveform and your counter depending on how it is adjusted is locking onto what it finds most convenient. Below is an example of the waveform with various rising & falling edges which your counter could be locking on to. Also notice the small horizontal blue markers(lines), the signal is processed in the time domain and the algorithms(software) of the computing engine is where the magic happens in comparison to frequency domain processing used in non-BBS/FBS detectors.
[attachment 232821 Bruce.gif]
OK so where are the multiple frequencies coming from? Well time and frequency are the inverse and rectangular waves are a complex variant of the square waves like used in the CZ-20.
Below is a little graphic I made showing how you can derive multiple frequencies by sampling the waveform at various time periods.
[attachment 232822 scope.gif]
By varying the length of the pulses and spacings you can create a wide variety of frequencies. Measuring the FBS transmit output on a bench doesn't really give the whole picture, because once the detector begins to measure the ground it can vary the transmit waveform.
The FBS detectors listen in on what Minelab calls three Channels. The purpose of the three Channels is to have optimum frequencies for low conductive targets, medium conductive targets, and high conductive targets being processed all at once.
Oddly enough some of the above general concept is related to the X-terra platform. On the X-terra you have a choice of 3KHz, 7.5KHz, plus 18.75KHz coils and those frequencies are NOT harmonically related. Now let's design a new X-terra model called the Variable X-terra. It will transmit on all three frequencies at once, but it will have the ability to automatically vary it's three core frequencies widely within a certain Band(or as Minelab calls it on FBS a Channel). Let's predefine our band(Channel)limits; our 3KHz can automatically vary from 2KHz to 5Khz, our 7.5KHz from 6KHz to 12Khz, and our 18.75KHz from 15KHz to 23KHz. So we are standing in a field and turn on the Variable X-terra.....the internal computer takes a look at the conditions and says wait a minute we have some problems here. The ground is showing a strong response at 18.75KHz so off the computer goes and chooses 17KHz as our Low Conductivity Channel. Likewise 3KHz is showing EMI problems so the computer searches around and finds 4KHz as quiet and that is our High Conductivity Channel. It turns out 7.5KHz is just ducky so the computer leaves that alone. None of the chosen frequencies are harmonically related because of the use of "multi-period rectangular waves".
Why is this superior to a Fundamental plus two harmonics? Because in order to shift the Harmonics I "have to" shift the Fundamental. And if I end up in a situation where I "have to" shift the Fundamental then the two harmonics are going to shift also. Now consider where I have an EMI issue on the Fundamental frequency but when I shift it I run into an EMI issue on one of my Harmonics, you're stuck between a rock and a hard place. Using the FBS method it can arbitrarily shift the frequencies to optimize the detector. Therefore because of the "multi-period rectangular waves" Minelab can transmit 28 frequencies and listen in on the best frequency within each Channel(Band) for the three different conductivity levels.
Therefore, and I hope this is clear, the three Frequencies being listened to today may not be the same as being listened to the next day depending on local conditions. The computer is designed to pick the best three and can arrive at that decision in seconds with what could take you half an hour of twiddling to do.
HH
BB
The commonly used VLF detector operates on a single frequency unless it is like an X-terra that changes when the coils are swapped out. Below are the waveforms from an X-terra X-30, Fisher ID Excel, and Fisher CZ-20 measured on one of my digital O-scopes.
[attachment 232819 ComboWForms.gif]
The first two(X30 & Excel) are sine waves, with the CZ-20 being a square wave. Now the CZ-20 the Scope has locked onto the Fundamental frequency of 5KHz and yet the CZ-20 is a dual frequency detector. In order to see both frequencies we turn to a Spectrum Analyzer where we can see the Fundamental and Harmonic at 15KHz.
[attachment 232820 cz20wvF.jpg]
Now if you are going to measure the transmit output of a FBS detector with a Frequency Counter or O-Scope you have to be careful because it will trigger on what it finds most convenient. What's being transmitted is a complex waveform and your counter depending on how it is adjusted is locking onto what it finds most convenient. Below is an example of the waveform with various rising & falling edges which your counter could be locking on to. Also notice the small horizontal blue markers(lines), the signal is processed in the time domain and the algorithms(software) of the computing engine is where the magic happens in comparison to frequency domain processing used in non-BBS/FBS detectors.
[attachment 232821 Bruce.gif]
OK so where are the multiple frequencies coming from? Well time and frequency are the inverse and rectangular waves are a complex variant of the square waves like used in the CZ-20.
Below is a little graphic I made showing how you can derive multiple frequencies by sampling the waveform at various time periods.
[attachment 232822 scope.gif]
By varying the length of the pulses and spacings you can create a wide variety of frequencies. Measuring the FBS transmit output on a bench doesn't really give the whole picture, because once the detector begins to measure the ground it can vary the transmit waveform.
The FBS detectors listen in on what Minelab calls three Channels. The purpose of the three Channels is to have optimum frequencies for low conductive targets, medium conductive targets, and high conductive targets being processed all at once.
Oddly enough some of the above general concept is related to the X-terra platform. On the X-terra you have a choice of 3KHz, 7.5KHz, plus 18.75KHz coils and those frequencies are NOT harmonically related. Now let's design a new X-terra model called the Variable X-terra. It will transmit on all three frequencies at once, but it will have the ability to automatically vary it's three core frequencies widely within a certain Band(or as Minelab calls it on FBS a Channel). Let's predefine our band(Channel)limits; our 3KHz can automatically vary from 2KHz to 5Khz, our 7.5KHz from 6KHz to 12Khz, and our 18.75KHz from 15KHz to 23KHz. So we are standing in a field and turn on the Variable X-terra.....the internal computer takes a look at the conditions and says wait a minute we have some problems here. The ground is showing a strong response at 18.75KHz so off the computer goes and chooses 17KHz as our Low Conductivity Channel. Likewise 3KHz is showing EMI problems so the computer searches around and finds 4KHz as quiet and that is our High Conductivity Channel. It turns out 7.5KHz is just ducky so the computer leaves that alone. None of the chosen frequencies are harmonically related because of the use of "multi-period rectangular waves".
Why is this superior to a Fundamental plus two harmonics? Because in order to shift the Harmonics I "have to" shift the Fundamental. And if I end up in a situation where I "have to" shift the Fundamental then the two harmonics are going to shift also. Now consider where I have an EMI issue on the Fundamental frequency but when I shift it I run into an EMI issue on one of my Harmonics, you're stuck between a rock and a hard place. Using the FBS method it can arbitrarily shift the frequencies to optimize the detector. Therefore because of the "multi-period rectangular waves" Minelab can transmit 28 frequencies and listen in on the best frequency within each Channel(Band) for the three different conductivity levels.
Therefore, and I hope this is clear, the three Frequencies being listened to today may not be the same as being listened to the next day depending on local conditions. The computer is designed to pick the best three and can arrive at that decision in seconds with what could take you half an hour of twiddling to do.
HH
BB
Thanks again for an EXCELLENT post!
