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Offline giuhalftrackTopic starter
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« on: August 08, 2016, 09:45:38 am »
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I'm working with a BFO detector (cheap version from a Italian magazine of '80). I'm trying to improve this machine to learn more about BFO detectors.

I wonder if there are "tricks" to avoid frequency lock.

In the design of this machine, you need to set a variabile inductor at "motorboating point", I vas looking at the scope and had in CH1 the coil frequency (300KHz) and in CH2 the reference freqeuncy (ca.300KHz).
I have read the "BFO theory.pdf" from a famous website, and what i'm trying to do is set the detector like this:

coil 300KHz
reference 300.5KHz

but when setting the variabile inductor i approch the 299KHz sound stop and I  experience frequency lock.

think the reference pick-up frequency from coil not by air but from power lines... I wonder if cut traces and use separate batteries for the oscillators can solve problem.. later i post schematics ok ?

thank you and sorry for bad english ( Wink still learning)

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« Reply #1 on: August 08, 2016, 10:21:36 am »
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   You need to post the schematic.  Welcome to the Forum.

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Offline giuhalftrackTopic starter
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« Reply #2 on: August 08, 2016, 10:35:29 am »
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here the schematics, pay attention to power supply "direction" (physics notation).
I'm using the original PCB fully assembled, just made some cut of trace to separate power lines, the detectors works properly but give an high picth tone and poor sensivity.
trying to set a better frequency (less freqeuncy difference between ref. and search) gives frequency lock.

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« Last Edit: August 08, 2016, 10:39:24 am by giuhalftrack »
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« Reply #3 on: August 08, 2016, 11:06:06 am »
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  I would leave as is but loose the feed back from Reference Osc.

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« Last Edit: August 08, 2016, 11:07:55 am by homefire »
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Offline giuhalftrackTopic starter
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« Reply #4 on: August 08, 2016, 12:37:07 pm »
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really ?? and how i get "frequency mix" without this connection  ??

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« Reply #5 on: August 08, 2016, 01:32:39 pm »
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OOOOPS.  Your right.  Your still getting too much feed back causing your osc to lock in to each other.  Need to figure a way to tame down one of the osc. ?  Shield one of them ?

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« Reply #6 on: August 08, 2016, 01:47:48 pm »
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I have discard the old inductor (broken and not shielded) and fixed this one (see the perfboard adapter??).
Right now I'm fitting an RC filter on search oscillator to see if this solve the problem.

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« Reply #7 on: August 08, 2016, 02:12:57 pm »
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In any BFO design, your component tolerances should not be very tight. You need to have circuit drift so using 1% components should be avoided.

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Offline giuhalftrackTopic starter
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« Reply #8 on: August 08, 2016, 02:31:20 pm »
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Quote:Posted by GoldDigger1950
In any BFO design, your component tolerances should not be very tight. You need to have circuit drift so using 1% components should be avoided.


there are no 1% components, the only "valuable" component in this PCB are the polistyrene capacitor (old ones, or for thermal stability i think).

I was trying right now. the frequency lock appear when the reference oscillator is 7Khz down the search one... I think this is a big difference for generating a sensible beat frequency.

This is what I'm planning... (in the picture)

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« Reply #9 on: August 08, 2016, 04:00:57 pm »
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Can you determine which Osc is locking up ?  Attenuate the other.

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