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Freq Shift on the F75 LTD? help:confused:

J

jamt225

New member
Can anyone help me understand the Freq shifting on the LTD and how, why, when, etc. I should use it?


Electrical interference In many (probably most) machines, elimination of electrical interference is best achieved by setting the discrimination level to the top of the iron rejection range, then reducing the threshold setting (if threshold control is available; it may be labeled sensitivity). Even if the machine chatters in air, while actually in motion searching over the ground it will usually quiet down, except for occasional random pops which dont sound like targets. (Frequency shifters found on some machines are beyond the scope of this essay.)

http://detectorstuff.com/2009/03/02/discrimination-mode-sensitivity-and-depth-in-single-frequency-vlf-metal-detectors/
 
Directly from the manual:
"To shift frequencies:
1. Hold the MENU button down. Do not release the button.
2. Push the trigger forward to decrease the frequency
Or pull the trigger back to increase the frequency.
Each frequency change requires a push of the MENU button.
Each actuation will shift the frequency by one value.
The LCD will display the frequency, from F1 to F7. F1 is the lowest frequency. The default frequency is F4. The F75 will retain any change in the frequency setting, even after powered off.
See the Search Techniques section of this manual for more information on electrical interference."

If you are getting interference, you can try to reduce or eliminate it by adding discrimination, changing the frequency setting, using a different mode, reducing sensitivity or a combination or those things. The couple of places I've had interference issues with the LTD, reducing the sensitivity helped the most; but combined with other setting changes, I could add just a little back to the sensitivity.
tvr
 
Thanks TVR, I've read that and understand how to change the freq. I'm trying to get a better understanding of why F1 is better than F7 or F5. Apparently the machine always resets to F5 when turned off. I recently tried the machine on the beach and apparently F7 is the freq of choice? Why? I didn't know that before I went to the beach and what tells me to change freq when trying for the best setup? When do I want to experiment with Freq? What do I gain other than less noise if anything? What's the science or tech behind the freq changes and how do I benefit, example: finding more?

Is there a step by step procedure when setting the machine and processes?

ie: 1st sensitivity, 2nd discrim, 3rd notch, 4th threshold, 5th freq, 6th gb etc.


If so, what is that, and how do I maximize the machine to the fullest when setting it up?

From what I've been reading there is a lot more to this machine, that the average owner is not aware of.

Quote from Dave Johnson:

"In the F70 and F5 which are more recent designs, the interactions between control settings and signals are even more complex, but we did a better job of hiding the details thereby giving the user an improved sense of predictability."

http://detectorstuff.com/2009/03/02/discrimination-mode-sensitivity-and-depth-in-single-frequency-vlf-metal-detectors/


Then you have this review talking about more depth with the F70

http://detectorstuff.com/2009/03/09/fisher-f70-review/

"Fisher retained the most useful functions and made them very accessible to the user. There
 
Now that I've read those links ... probably need to know something about the processing algorithms used as well as the actual frequency information for F1 through F7 other than lowest through highest. Been a few years since I've actually done digital signal processing; but with multiple filtering, windowing and transform operations, there is a lot you can do as long as you can process fast enough to provide timely responses. Really need to be inside the algorithms and see the responses from the modeling to be able to give a good answer. That said, interference can be splattered in strange ways so the best transmit / receive frequency may not be very predictable.

So; good questions. I really can't add too much to the why.

What I've tried when it's noisy is to set sensitivity to where it is not totally overwhelming to hear a beep, but too noisy to hunt comfortably. Then I'd step through the frequencies one at a time and sweep and see if one made it better. Then I'd take discrimination up a little, then try the frequencies again. Tried DE and BP modes. I haven't stepped through all modes in the interference situations. I probably should. BP has been the quietest over all. If I found a little discrimination or a frequency helped, leave it there and drop sensitivity until there were only a few pops and zips. Then dig the good targets it finds.
Hope I haven't muddled things.
Cheers,
tvr
 
No that's exactly the path I was trying to get on. I don't speak the electronic engineering/detectoring language very well so it's hard for me to use the right words.

The Fisher is a lot more technical than any of the other machines I've used the last 40 years so I'm trying to retrain this old tired brain of mine. The frustration is I know this machine has detecting powers I am as yet unable to unleash, I can smell and taste them. I'm pretty stubborn, hard headed, and obsessed when I get this type of issue so eventually I'll figure it out somehow in my own crazy method.

Thank you very much your reply is of great assistance. I know there is a method to the madness.
 
If your machie is being affected by outside interference (as opposed to ground mineralization), you should hold your coil in the air while changing the frequency. You may find that one setting quiets the machine down better than the others. If it chatters more once on the ground or sweeping, then its due to ground conditions and switching frequencies probably wont help at all. They are all close enough in frequency that I really dont think there is any advantage in using one over the other. Many users reported that they seem to do better with the F7 setting, but I was never able to determine by my finds, that it ever made any difference.
 
The F7 setting was recommended for salt water beach noise along with -1 on the threshold.

Otherwise, I'm not sure what the freq shift does. I've played with it a little but not enough to form an opinion on it's effect. But I'm sure it does something to hurt and/or improve the detecting abilities of the machine. Just don't understand enough yet.
 
I think I forgot to say ground balance it first before working on the other things.

And I had to laugh as I read:

Scully said:
If your machine is being affected by outside interference (as opposed to ground mineralization), you should hold your coil in the air while changing the frequency.
Click to expand...

while remembering walking around a couple of locations with the coil in the air as I used the detector as a direction finder to isolate where the noise sources were ... one of four transformers in a cluster at one place and one of two power distribution panels at another. Scully is absolutely right.

And I can hear our 17 year old, as if she had been watching the walk with the coil in the air saying "Mom, why do you let him do that, I hope no one sees him. He looks like such a goober"!!!!
tvr
 
Thanks TVR!

This also was some help:

http://www.dankowskidetectors.com/content/Thomas_Dankowski_Fisher_Intelligence_5thed.pdf
 
and its making your machine chatter, you might be able to adjust the frequency and find an alternate frequency that is quiet or quieter. Toggle through the 7 different frequencies (F1, F2, etc.) and see if you can find one that quiets the machine down. This slight frequency adjsutment can minimiize outside noise in most cases.
 
Thank you "Scully" the problem is not knowing how to change frequency, it is why change freq? Does it get better depth in some ground? Does it help find certain types of targets? Or is it strictly for emi and ground chatter? etc.

Still working to understand that.
 
I don't ever recall when a frequency shift totally eliminated really bad interference, but one number often will be just a bit more quiet then the others and a little more sen. could be used verses the other freq. numbers. Which, is good, as every little bit helps. HH jim tn
 
Nice findmall thread ... and still pertinent. The pattern control seems even better than the size scaling would indicate with the small coils. The little coils on the LTD amaze me at how close I can get to things like poles and fences.
tvr
 
Most be a problem linking to his site?

Thomas Dankowski (NASA Tom)

dankowski (one word) detectors dot com

In his forum about 3 pages back this thread

Rcpt Ack of F75 Ltd prototype


I will probably be blacked balled or banned since I went around the built in blocks to get NASA Tom's link on here as any way I tried to post it admin had it setup to block NASA Toms web link? Don't know what the problem or bad blood is (and don't care), but the info is good at NASA Toms.
 
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