Discriminating BFO detector

Hi,
Xavier, i am using an old version of PaintShop pro (ver 5) i have bought aprox. 10 yares ago. It is ok  for makeing few PCBs on your own, or prototypes, but if you have larger series in mind, you'll need something with CAM support (i.e. "gerber" files). There is a bunch of free software availabel, but most is running under Linux, i am using one of those under SUSE 10. Local company makes 100 euro format PCBs a bit under EUR300, and for almost the same price up to 500 PCBs, but under 100, it doesn't pay.

The discriminating BFO circut caught my eye since it looked very well designet, maybe too well. I wanted to use it with low impedance ear phones so i added a 100nF capacitor and 2.2K resistor between earphones and transistor's collector to prevent the second LED from getting constant ground (beeing "ON" all the time) and diminish the "antenna" effect of phone wires to the circuit. The reference oscilator is remarcably stabil (only few Hz up and down over prolonged period), even though i used 180 uH molded chocke instead of properly winded HF coil. So the first think it occured to me was to add 33p capacitor in series with turning capacitor and 56p in parallel to "expand" the band and make fine adjustment possibel. It worked like a mirical, i could shut off the middle diodes at any time. But than i went out on my back yard, it was a cold late evening and some 10 degrees diference caused a constant down drift, until i ran out of turning capacitor. I blamed the molded chocke first, but the scope revealed the reference oscilator O.K., it was the turning capacitor/ search coil to blame. So the "expanding band" capacitors had to go. I tried again, this time i left the detector outside for some 15 minutes, again everithing worked extremly stabil, the setting of turning capacitor was now a bit coarse, but not a problem at all. Testing it inside, ewerything worked superb, a stabil detector with extremly good detecting range (for a BFO) and wery, wery sensitive mixer circuit makeing visible and aoudiable every minute change in frequency, causing the detector sensitive on small items (i.e. coins). But takeing it outside on a wet soil it revealled another problem. Due to its sensitivity the "ground" effect (bringing the coil to diamagnetic soil) became evident. Since the real outside flor is never flat and while walking, one can never hold the coil at the same distance from ground a constant "waawaaa" is heard, makeing extremly difficult to distinguish small targets. Changeing the coils and pushing all frequencies up approx 4 times this problem disappears, but the detecting range and sensitivity suffers fairly and the change of discriminator diodes happens too fast to be oserved. So i don't really know how to deal with this yet. I think this detector needs an authomatic ground balancing circuit.
Any ideas from you guys?

regards,
d.

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http://www.thunting.com/smf/index.php/topic,9197.msg81587.html#msg81587

Only the Reference Osc is Crystal controlled.

The search coil Osc is still Inductively Variable.   

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http://www.thunting.com/smf/index.php/topic,9197.msg81648.html#msg81648

I would like a copy of the schematic for this unit...I have the majority of the parts on-hand from what I see, and would like to further explore the chance of building this one...

Thanks

herbert982

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http://www.thunting.com/smf/index.php/topic,9197.msg81715.html#msg81715

 Golddigger . That is what Homefire said !!!!!!!!   Cornelius

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http://www.thunting.com/smf/index.php/topic,9197.msg81864.html#msg81864

Thank you...now that Ive looked closer I have the schematic.

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http://www.thunting.com/smf/index.php/topic,9197.msg81870.html#msg81870