frequency lock BFO (need of design tip)

[giuhalftrack]

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", and what i'm trying to do is set the detector like this:

search osc. 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.. or using an RC filter with cut-off say about 400KHz will help...

thank you and sorry for bad english ( still learning)

[WM6]

Take for reference oscillator 3-th harmonic (of 100 kHz) instead of same basic frequency as search oscillator.

[giuhalftrack]

Take for reference oscillator 3-th harmonic (of 100 kHz) instead of same basic frequency as search oscillator.

ok, it looks difficult to me how can i do it ?

I set the reference to 100Khz and filter the output to take the 3-th harmonic ???

is this a valid solution ?

Thank you

[homefire]

No filter needed. Just set the Ref Osc to 100Khz.

[kt315]

or using an RC filter
--
yes, split the supply, use RC filter on every generator separated. R=200 Ohm, electrolitic C=47...100uF.
---
I experience frequency lock.
---
thats very general trouble for all types of BFOs.

[giuhalftrack]

or using an RC filter
--
yes, split the supply, use RC filter on every generator separated. R=200 Ohm, electrolitic C=47...100uF.
---
I experience frequency lock.
---
thats very general trouble for all types of BFOs.

I have tryed the RC method but only on one oscillator.. but I have used different values calculated like a low pass... mmhhh writing this I realize to have used a low pass with cut off of 300KHz... think I wrong.. need to have a cut off below ... i check my previous calculations and give a try

[giuhalftrack]

No filter needed. Just set the Ref Osc to 100Khz.

ok i need to re-calculate tank circuit in oscillator, because cannot reach 100KHz, the oscillator turn off...