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Offline giuhalftrackTopic starter
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« Reply #10 on: August 09, 2016, 01:08:49 am »
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Quote:Posted by homefire
Can you determine which Osc is locking up ?  Attenuate the other.


ohhh interesting !!! never tought about that, ths evening I' try...

yesterday I found another mistake, the reference oscillatore was running very bad with changes in amplitude e frequency, I disconnect the search oscillator and then set up the reference oscillator alone, It result in a perfect sinewave
at 311.5 KHz ... drifting in frequency is still present but with values of 10Hz (I think this is a good result).

Then I noticed that the search oscillator (powered alone) does not oscillate at all  Shocked probabli I need to modify the coil inductance for let the oscillation run smoothly.

some picture just for fun  Cheesy


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Offline GoldDigger1950
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« Reply #11 on: August 09, 2016, 05:53:35 am »
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Quote:Posted by giuhalftrack
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 never said YOU did but this web site is for teaching and sharing so I mentioned it. Also, I did see your capacitors which brought the notion to mind. Sloppy components are the best for BFO, which is considered by many to be a dead technology.

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Offline giuhalftrackTopic starter
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« Reply #12 on: August 10, 2016, 03:21:15 am »
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Quote:Posted by homefire
Can you determine which Osc is locking up ?  Attenuate the other.


I don't know how to do this.. there is a method ?

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Offline GoldDigger1950
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« Reply #13 on: August 10, 2016, 06:13:43 am »
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I recall some early designs used a spring, stretched taut but not tight as an inductor so that slight movement would change the FO (Frequency of Oscillation) by any movement at all. This gave the required instability to the oscillator(s). Usually only one was needed, just the reference oscillator. You just have to remember that you do NOT want oscillator stability in a BFO design. Doing so makes the circuit "lock up," as you call it. What is happening is that the beat frequency oscillator and the reference oscillator are doing something called sympathetic oscillation. They synchronize their frequency of oscillation by proximity to each other. Not good when you need a difference frequency to detect a field change. Look it up. Stop ignoring my suggestions. I am indeed giving you the answer but for some reason you are simply ignoring me. Your loss.

FYI, I am a semi-retired electronics engineer with dozens of metal detector designs in my notebooks and a few patents in that area.

Posted on: August 10, 2016, 02:11:08 pm
Quote:Posted by homefire
Can you determine which Osc is locking up ?  Attenuate the other.

When the condition exists, they are BOTH locked and you cannot tell which is causing it. Isolate them and see if you can see which reaches the frequency of the "locked" circuit. That is most likely the reason for sympathetic oscillations.

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« Last Edit: August 10, 2016, 06:33:53 am by GoldDigger1950 »
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« Reply #14 on: August 10, 2016, 11:31:20 am »
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  Have you tried what was suggested on Geotech ?

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Offline giuhalftrackTopic starter
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« Reply #15 on: August 10, 2016, 03:52:31 pm »
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Quote:Posted by GoldDigger1950
I recall some early designs used a spring, stretched taut but not tight as an inductor so that slight movement would change the FO (Frequency of Oscillation) by any movement at all. This gave the required instability to the oscillator(s). Usually only one was needed, just the reference oscillator. You just have to remember that you do NOT want oscillator stability in a BFO design. Doing so makes the circuit "lock up," as you call it. What is happening is that the beat frequency oscillator and the reference oscillator are doing something called sympathetic oscillation. They synchronize their frequency of oscillation by proximity to each other. Not good when you need a difference frequency to detect a field change. Look it up. Stop ignoring my suggestions. I am indeed giving you the answer but for some reason you are simply ignoring me. Your loss.

FYI, I am a semi-retired electronics engineer with dozens of metal detector designs in my notebooks and a few patents in that area.

Posted on: August 10, 2016, 02:11:08 pm
When the condition exists, they are BOTH locked and you cannot tell which is causing it. Isolate them and see if you can see which reaches the frequency of the "locked" circuit. That is most likely the reason for sympathetic oscillations.


GoldDigger I don't want disrespect you, if you think i did, then i ask you to forgive me.
I was working on an assembled unit taking for good the components and the values placed on the PCB (I have double check the values against the original design).
So if I have understand your suggestions I need to have a "non ideal" oscillator that can have some fluctuations, I don't know the tollerance of the capacitors but I can change them with 20% ones.
The thing about the spring is amazing I want to try this solution anyway.

Posted on: August 10, 2016, 03:40:26 pm
Quote:Posted by homefire
  Have you tried what was suggested on Geotech ?

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not yet, are you the same person on the other website ? Smiley
my oscillator stops oscillate when i lower the frequency by repositioning the core of the inductor, i think i need to recalculate the tank circuit for 100KHz

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« Reply #16 on: August 10, 2016, 04:04:22 pm »
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I use the same name on Geotech.  

I would think the tuning cap would allow you to go that low.  You can just add a Cap in series.

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« Last Edit: August 10, 2016, 04:09:01 pm by homefire »
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Offline giuhalftrackTopic starter
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« Reply #17 on: August 11, 2016, 01:21:24 am »
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Quote:Posted by homefire
I use the same name on Geotech.  

I would think the tuning cap would allow you to go that low.  You can just add a Cap in series.


ah ok I'll try this.

If i remember correctly my tuning cap let the frequency go up and down of 1-2KHz (not sure now i have written down on a piece of paper).


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