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Offline GoldDigger1950
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« Reply #60 on: March 30, 2010, 08:16:59 pm »
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Xavier, that's almost correct but the swing is less making it markedly less sensitive. Since BFO machines aren't know for their sensitivity to start, making it less sensitive is really not good. The greatest loss is in one direction or the other depending on what the phase relationship is at the moment you detect metal. As the shift and phase angle increase or decrease between the two oscillators, the swing is only one way rather than potentially both ways.

A wobble in both oscillators is like when you tow a trailer behind a car. A slight wiggle is amplified in the overall stability of the vehicle. Add an anti-sway system and the wiggle is much smaller.

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Offline xavier
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« Reply #61 on: March 31, 2010, 12:47:55 am »
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Hi GD

Well what you say makes sense but then the reference oscillator is not a reference oscillator or it would be like a ruler it?s one centimetre or it?s not if it?s over then it?s to big and the frequency goes up and if it?s under then the frequency goes down but what you are saying is that the first destabilizes the second then the second destabilizes the first even more and you end up with a big difference of the two oscillation?s so now if I understand you right the more unstable the BFO the better the sensitivity will be well to a point of course. But then would it not be wise to have a third oscillator? A real reference oscillator and two more to do the frequency shift?

Regards Xavier   


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Offline treasuremomo
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« Reply #62 on: March 31, 2010, 01:05:06 am »
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BFO=Best Forgotten Old--technology!!! Don't waste your time on BFO Beat Frequency Oscillator technology...a TR or VLF can whip the crap out of that BFO junk. And the PI detectors excel in salt water.
BOO TO BFO!!!!!!!

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Offline GoldDigger1950
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« Reply #63 on: March 31, 2010, 01:05:22 am »
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That'd be overkill, Xavier. Initially, the BFO designers tried their best to make the reference oscillator as stable as they could. They even went to what is considered the extreme of adding the crystal to control it. Then, the issue of sensitivity came up. Adding audio amplifiers and other means added nothing to the detection depth of the machines so it was the brave and bold who went back to the old way and used the rather unstable but venerable old method.

Newer models now use a technique that is used in superhet radios where a phase locked loop is added but detuned with an unstable component to keep it stable during front panel tuning but allow it to drift instantaneously for brief periods of time. It brings back the sensitivity to the circuit while allowing it to return to a tuned state afterwards.

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« Reply #64 on: March 31, 2010, 01:26:33 am »
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Treasuremom let me start of by telling you that I have build VLF, PI and BFO also that BFO is a far cry from as you call it CRAP
Would you consider an old locomotive as CRAP because it does not go as fast as a diesel engine?

Xavier


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« Reply #65 on: April 03, 2010, 10:04:50 am »
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Hi,

O.K., i have swapped the SRC coil's terminals, the "ground" is now closer to the ground and "hot" is on upper part of the coil. This did not curred the "ground effect" but had diminished its magnitude quite some. The detector now reports shallow coins and small items without a problem. Great. I am learning slowly. I have also get rid of coil's faraday shild. This doesn't sound like a good idea but the unit performs much better without it, the sensitivity on small items increased. Since the coil is approx 1 cm inside the plastic "dish" tuching the grass now cuses a small audiable sound (high peach) but nothing critical.

regards,
d.


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« Reply #66 on: April 04, 2010, 06:42:59 am »
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Hi DK

The shield is there to protect the coil from interferes caused by ground effect but at the cost of loosing sensitivity on small objects I have found this to be the case on the VLF also the coil connections on the PCB should be at opposite ends of the PCB I also used the audio device to separate the RX and TX circuitry I am also going to shield the RX circuit with a faraday shield and copper pour on the board I think that this will help. I also have a 4mm ground track all around the board this reduces the length of the tracks going to the neg with and a 3mm pos track going through the middle by doing it this way I have less tracks that could result in some sort of coil effect. I?m sure that there are other things that one can do to reduce all those nasty interferences but I have not found them yet.

Regards Xavier 


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Offline kjamesgraham
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« Reply #67 on: April 10, 2010, 09:38:48 pm »
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Well HomeSlice...
I've added it to my (project list ).  I will build it, test it and try to respect it with a quality package. I plan to have fun with it who knows.  Thanx for posting it.

regards

Kenny

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« Reply #68 on: April 19, 2010, 12:43:09 am »
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Quote:Posted by treasuremomo
BFO=Best Forgotten Old--technology!!! Don't waste your time on BFO Beat Frequency Oscillator technology...a TR or VLF can whip the crap out of that BFO junk. And the PI detectors excel in salt water.
BOO TO BFO!!!!!!!

There was a day I would have agreed with this. But not so much any more fore the reasons that a BFO is easy to build, easy to understand, easy to wind the coil, requires no special tools and can be built in a weekend by most anybody. They then can take it in the yard and have a blast with their kids digging up small treasures.

Sure a VLF has discrimination. But the coil can be very hard to wind and get right, and the detector can take a lot of parts and time to build.

So if you are going into a park you'd want to have a discriminating detector. But something to goof around the kids with, a simple BFO can't be beat. And there are a couple of good designs on the internet.

I would also like to not that the recent huge Anglo-Saxon find in England (see the National Geographic special) could have been found with a BFO it was that shallow. Some still use them for bottle hunting.

My single complaint is that a BFO should not be advertised as a discriminator!  Cheesy

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Offline GoldDigger1950
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« Reply #69 on: April 19, 2010, 01:24:34 am »
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Quote:Posted by technos
My single complaint is that a BFO should not be advertised as a discriminator!  Cheesy


Why not? If it discriminates, there's no problem. Or are you one of those guys who thinks there's no such thing?

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