BFO frequency ??

[wyndham]

I have been reading up on BFO & IB style detectors and downloaded several projects that may teach me a great deal that I need to learn.
I understand the principle of a BFO & IB and my question, since the designs are 20- 30 yrs old, can newer faster clocks be used to possible gain better response. How far can this be carriered. Can it go into the GHz range and would there be any benifits. Wyndham

[bbsailor]

Wyndham,

Lower VLF and inductance balance (IB) frequencies in the 3 to 6 KHz range tend to penetrate the soil for better detection depth. Higher frequency VLF and IB machines (25 to 100kHZ) favor small gold targets like nuggets.

BFOs depend on the coupling of the oscillator coil which is also the search coil to be slightly shifted (this the beat frequency compared to a fixed frequency oscillator) in the presence of metal.

VLF and inductance balance machine do allow discrimination of metal type and even allow some form of ground balancing which improves potential target depth.

Pulse induction machines depend on high energy (or at least higher energy than a VLF or IB) pulses and a time delayed sampling to receive the eddy currents flowing the the target after the main pulse dies down. Only recently has some form of discrimination been introduced into PI designs.

BFO machines are interesting from a historical perspective but have been replaced by the improved performance of VLFs, IBs and PIs.

Looking for the best machine in the abstract is not recommended. There are best machines to suite general purpose hunting or specialized machines to suite a particular environment or target type. No decision about the suitibility of a particular metal detector should be made in the absence of two key questions: (1) where you want to hunt and (2) what are you looking for?

bbsailor

[wyndham]

bbsailor
Just trying to learn right now. As you've seen from some of my other post on Eric's board, I'm starting from the beginning but have also read about some new ideas in the Ghz range where a harmonic return signal is recieved instead of an induced eddy current it could be an adaption of either IB or BFO, that was where I was going with my question. Wyndham

[Sid]

frequency

Hello, from my experince the optimal frequency for BFO is somewhere below 40kHz. Of course, sensitivity grows with rising frequency, and the sensitivity to soil as well. High frequency is a real adventure, just consider that the attenuation in some soils can reach 200dB on 100MHz, also the skin effect reduces the chance to discriminate metals. Good luck, Sid.

[Unregistered]

The Arado VFA90 offered high sensitivity to gold and non-ferrous metals and low sensitivity to silver paper and coke and the advantage of no response to salt water.
Almost foolproof rejection of iron and exceptional depth for a B.F.O. type machine, (U.K. old penny ten inches) with the stock eight inch coil. There were also 5 and 10 inch coils. Can't remember the frequency but used variable frequency acoustics with the audio tone of 300 Hz rising over non ferrous, dropping over small ferrous and giving a null, high note, null over large can size iron.
I to often wonder if some of the older ideas could be reworked to take advantage of modern componant developments.

Brian

[digger barns]

VFA 90 searchcoil ran at 9khz with a ref freq of about 9mhz the designer of this detector is reluctant to give up it's secrets even today. This detector sold for an incredible £180 back in 1969.

[Unregistered]

Mine cost £159 in the U.K. which compared with the A.H.Pro £250 and Fisher 553 D £274 didn't seem to bad. Coils were £20 and offered the opportunity to increase depth or sensitivity to certain metals ie gold by swapping to a coil of the same size but different colour. Wonder how that worked ?

[diggerbarns]

it may have been £159 it was a long while ago. Seem to remember reading in an old treasure hunting magazine a detailed description of how it worked. But i no longer have the mags.

[Unregistered]

The Arado VFA90 offered high sensitivity to gold and non-ferrous metals and low sensitivity to silver paper and coke and the
I to often wonder if some of the older ideas could be reworked to take advantage of modern componant developments.

Brian

Just wondering Brian, how does the Arado do in trashy iron? Does it have any "see thru" in nails?
Steve(in MS)

[Unregistered]

It was very good in heavy iron, especially with the small black 5 inch coil but I don't think you could claim it had the see through ability of say the Compass 77b.

Brian