ETI561 BFO

Well then the reference oscillator must be at the same frequency and that is quite low for a BFO that's perhaps the reason why it's so stable I have had schematics where it was running at over 450Khz I never did figure out why they run at such high frequencies why not try to make one that runs under or at 30Khz? 

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Xavier

Yes and when you say 10cm you must add the distance above the ground normally about 5cm not to shabby I'd say if you had a bigger coil you would find that you will pickup a fridge at 100cm but then you must decide if you are looking for large or small objects.
The Beat-frequency oscillator (BFO) is the simplest (and oldest) type of metal detector technology and is a good starting point for learning how metal detectors work. The basic beat-frequency metal detector employs two radio frequency oscillators which are tuned near the same frequency. The first is called the search oscillator and the other is called the reference oscillator.

The outputs of the two oscillators are fed into a mixer which produces a signal that contains the sum and difference frequency components. This signal is feed to a low-pass filter removing the harmonics. As long as the two oscillators are tuned to the same frequency, the output will have no signal.

When a metallic object disturbs the magnetic field of the search coil, the frequency of the search oscillator shifts slightly and the detector will produce a signal in the audio frequency range.

Although once popular, BFO's are no longer being made by professional metal detector manufacturers. They are simple and inexpensive, but do not offer the accuracy and control of modern PI or VLF detectors. Attempts have been made to add new features such as discrimination and more advanced models were produced in the 1970s, but they were soon replaced by recent, more sophisticated technology.

BFO designs are still used in cheap hand-held devices and in low quality, toy type detectors. The vintage BFO detector is more of a curiosity and collector's item than a usable piece of equipment.


PS I got this from the net Cut and paste.

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Xavier

I totally agree with you on that but remember that the high frequency has difficulty of ground penetration that is why VLF was so good compared to a BFO running at 110Khz a VLF will run at 4.5Khz giving it the advantage that lower frequency's penetrate better look at your PI it runs at what 20Hz? 

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Xavier

If using the colorburst crystal oscillator makes for instability, maybe you could use a TCXO. Temperature compensated crystal oscillator. The xtal oscillator with compensation is built into one module. All you need is VCC and 50ohm output. Another idea might be a phase lock loop and some divide by n. Bet there might be a PLL with built in division in one package. I've been away from the latest goodies available, so can't suggest anything though.

Jim

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Hi,

assembled modules available.

d.


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