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channels and frequencies

A

Anonymous

Guest
One of you engineer types.....the explorer is touted for scanning 28 frequencies (FBS), and yet the manual/auto channel select is for 11? or 16 I think.
So, if you are in EITHER manual or auto mode, is the machine scanning the 28 freqs, and responding on the most clear channel?
What is the difference between a channel and a frequency?
Thanks
Dennis, channeling the answers.....
 
The frequency's are going to stay at 28 all the time. Its just that they will be different ones at different channels. The lowest and highest will stay the same, but the other ones will change with the different channels, to match what is needed in the area you are detecting in. So it makes 11 different settings of the frequency's, to match the ground and minerals of the area. When you Noise Cancel, it sets it then, that is why you should always Noise Cancel every time you change a location.
 
Here's what to do. Put on your head phones and hold the explorer's coil close to one ear. As you get the coil close to the phones you will hear a high pitched noise. This is your head phone's speaker coil acting as an antenae and picking up the signal from the explorer's transmit loop.
Now hit noise cancel, you will hear the pitch change at even intervals as the explorer cycles through the channels.
What it is doing is shifting the transmitted frequencies by a certain amount. The explorer's electonics are looking for a channel where it picks up the least amount of electrical noise from power lines and other sources. That is why you need to hold the coil still when doing noise cancel, otherwise the explorer will think the targets are electrical noise and perhaps not choose the best channel.
Some sources of noise are fairly narrow band, meaning they are very strong at one frequency and not at others. It's like when you dial in a radio station, you can go a little ways off the station and still get reception, but very far and nada. Noise cancel works well with these noise sources.(Tuning out the single transmit frequency from someone else's nearby metal detector is a good example).
Other noise sources are more broad band or are powerful enough to have many harmonics that can also be picked up by the explorer. Noise cancel doesn't do much for these. Under power lines is a good example, I can sometimes gain 1 or 2 clicks of sensitivity by noise canceling but seldom much more.
So to answer your question the explorer(supposedly) is using all 28 channels, but when you noise cancel it will try increasing these frequencies a slight amount in ten steps, looking for a step that has the least noise. For example if the lowest frequecny is 2.5 kilohertz it may bump it up to 2.6,2.7,2.8 etc.,I'm not sure what the step size is.
Hey Kirk and other geeks, have some theories on how they do this if anyone wants to hear. I think you said you and Cody had emailed each other on this. I'd love to hear them.
Chris
 
Chris,
I think that your explanations are quite good and there is not much that I could add to that.
In my email communications with Cody we discussed the subject of frequencies used by the explorer. It is my belief that explaining the operation of the explorer from a frequency standpoint is like trying to "force a square peg in a round hole". Trying to explain the operation in terms of frequency does nothing but confuse the issue. I believe that Minelab only uses the frequency terminology in order to illustrate the real increased performance over one or two frequency operation. They can accomplish their purpose without confusing people with too much technical talk.
In the 5,537,041 patent it shows the transmitter coil driving voltage as a square wave around zero.
* The fundamental of the signal sets the minimum frequency and the harmonic frequency components.
* The rise and fall time of the transmitted signal sets the upper frequency limits.
By changing the period of square wave (through the noise cancel function), then all of the frequency harmonics are changed at the same time. The received signal is analyzed in the time domain using sample data techniques in such a manner that frequency components that are not present in the transmitted signal can be rejected. I further believe that the 11 different channels used in the noise cancel function are simply 11 different steps in which the period is changed.
I think that your statement about the effects of broadband noise is correct in that this type of noise is much more difficult to eliminate using the noise cancel function (or any other technique for that matter).
Keep in mind that all of this is just my opinion and may not be correct.
HH,
Glenn
 
Charles,
Yes, you are correct. The lower (1.5 kHz) and upper (100 kHz) are fixed and the remaining 26 frequencies vary between those two end points depending on what conditions are present when you run Noise Cancel.
Andy
 
Thanks Andy, I was pretty sure I was right. But it seems some others didn't think so, but I wasn't going to argue.
Charles
 
What I've found is that all the frequencies shift with noise cancel. It's a complex square wave and they are changing the base clock in the microprocessor/FPGA. I don't have the numbers with me, but I do know all the frequencies change. I've verified this with a crude spectrum analyzer (O-scope w/ FFT function). You can also see it by looking at just the output of the coil drive circuit.
I keep seeing people saying that the top and bottom freqs don't change, but they actually do.
HH- Rhoderman
 
Rhoderman,
I've also seen that stated as gospel, along with sensitivity in Auto not going over manual setpoint. I'm sure that both are wrong.
I suspect that there is one pulse train derived from the Digital signal processor/micro processor clock that is used by a number of divide by n circuits to produce the different frequencies. The top and bottom frequencies (and middle ones for that matter) would stay close to their original frequencies, just shifted a small percentage as the noise cancel functions cycles through the channels(each a small change in the master frequency derived from the clock.)
Any idea if this (shifting pulse train freq or divide by n functions) could be done with software or have to be done with hardware?
Also believe that noise cancel is simply an event counter that looks for signals on the recieve circuit. If you are holding the coil still the only signals seen by the explorer should be caused by EMI. I suspect it listens for a time period T as it shifts through the frequencies and which ever channel had the least signals is selected as the least noisy signal. I'm not sure if it uses a default sensitivity when doing the noise cancel or if the user sensitivity is used, which would raise some questions on reliablity- Too low and wouldn't pick up any noise on any channels, too high may lead to erratic operation. My findings with noise cancel is that it seldom makes much of a difference, maybe a click or two in sensitivity.
Also have read somewhere that on certain channels there are more or less frequencies monitored. I don't think any of them use all 28, seem to remember channels xyz used 11 freqs, ab used 13, and etc. Not at all sure about this but am wondering if the freqs that land on or get shifted in the noise cancel process to frequencies close to fundamental harmonics of 50HZ(Europe and other countries, including Australia methinks) or 60HZ(US) power lines are automatically excluded.
Chris
 
what is your opinion on noise cance.
should i reset each trip of just leave it where i cut it off................
max
 
... unless you're having noise problems on one of those trips.
Noise cancel, like Chris in Wisconsin says, is primarily to get rid of interference from electrical AC line sources, such as motors. Most other interference will be way above anything the Explorer looks at. Radios typically work at higher frequencies and shouldn't pose a problem. One other thing that may cause you interference is another detector that happens to match your frequency. The Explorer can be set to ignore that other machine by using noise cancel.
One interesting thing I have found with noise cancel is that lower numbers work best in my yard. Now, if you had to pick the worst possible place to plant a coin garden, you would end up in my yard, in my coin garden! There is a wire for the sprinkler system that runs across part of it. My dirt has a very high concentration of iron particles and dragging a magnet through it collects a lot of metal fuzz. If I get the sens too high, I suffer lots of high-right falses. I found that if I hit noise cancel while swinging the coil over an area without any targets, it always picked a low number. If I did it motionless, it would pick random numbers, sometimes high or mid, and it would false like the dickens. To me, this says lower noise numbers are better for soil with a ton of fine iron in it. You might find that a higher number works somewhere else. That's what I really like about this machine - it's adjustability!
If you really want to find the best possible noise number manually (and the manual doesn't tell you this), kick up the sens to max 32 manual and go to the noise menu. Walk through the numbers slowly as it takes about 3 to 5 seconds to settle in. You'll find some numbers are really noisy and others aren't quite so bad. Pick the best one. It may not be completely silent, but you want the one that is the least noisy. Then back off the sens to where it's tolerable and go huntin'.
Something else that may not be an issue, but I want to share it - I think the noise numbers may change the way the machine responds to gold. I swear I could get a better air test with my gold ring at certain noise numbers. I thought it sounded better on some numbers than on others. I was having a beer in the shop at the time, so it may have been easier to convince myself, but you might want to experiment.
Sorry for such a long post...
HH&HH (Happy Huntin' and Happy Holidays!)
- Rhoderman
 
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