Is BFO dead?

[Wessex warrior]

Hi,

Recently came across this BFO design: http://www.bobsdata.com/metal_detector/index.html

It is quite advanced as BFO detectors go, using JFETs for the oscs for a frequency of 494kHz, and the designer says that: capable of detecting very small objects at a depth of at least 8 inches.


This seems quite impressive for a BFO detector. So, that got me wondering, is BFO really dead like some people say it is? Maybe they are right, but it would seem to me that BFOs discriminate quite well, with the exception of large pieces of iron.

Regarding ground balance, could this be overcome using two search coils, one for each of the two oscillators, so that ground mineralization is cancelled out?

[Carl-NC]

The "Bob" schematic reminds me of a 1980 Popular Electronics article showing a similar JFET BFO design, written by Robert Kreiger. Then I saw the "RGK" initials on the Bob schematic, might be the same guy.

In any case, BFOs typically tend to drift, can't cope with bad ground, and have relatively poor depth. Yes, you could put both oscillator coils in the search head, but they will need to have good induction balance to prevent frequency locking. This will help with drift and bad ground but will give targets a weird high-low/low-high response, making it tough to discriminate. I've built one of these.

[Gwil]

Hi WW

You might like to look at Thomas Scarborough's "Beat Balance" designs. These use two oscillators and two search coils that are loosely coupled. A target affects the coupling and therefore the beat frequency.
Haven't built one so can't comment on how good they are.

[homefire]

I still use a Ol BFO to Locate Black Sands deposit to pan for gold. My GMT has sorta relinquished it to Closet Queen Status..

[Wessex warrior]

Thank you for the replies,

Well, one advantage of BFOs is the are simple and easy to build with different search coils, so for that reason I have built a BFO detector with a 4inch coil for mudlarking the Thames in London, reason being that there are a lot of targets there, and the good finds are often on surface on just below. Hopefully it will be useful.

[ZOUjiarui]

Hello everyone, I have a question, can I locate the underground treasure through the way that the bfo locates the black sand, and let the bfo indicate the place that has been excavated.
Thank you for your reply.

[Carl-NC]

The change of soil will be very subtle, and with BFO just raising & lowering the coil may have a bigger effect as moving over a change of soil.