ETI 566 2-box Metal Detector Project _ Oscilloscope Readings _ Directed to Karl-NC

[ZuMi77]

Dear Karl - NC,

I have constructed the ETI 566 project.
The attached images show the Oscilloscope readings of the Tx.
I have tuned the Tx to 20 Khz and the Rx to the 3rd harmonic 60 Khz. I can't figure out if the 3rd harmonic is strong enough.

Kindly give me your feedback for the transmitted fundamental signal 20 Khz, is it strong enough? and for the 3rd harmonic.

The Rx response is very weak. The two antennas should be placed around 50 cm away from each other for the signal to be detected.

I don't know if there is something wrong with the 3rd harmonic or with the receiver. I have built the two circuits very carefully.

[Carl-NC]

I've never built this so I don't know much about it. But it's very annoying when a construction article contains no waveform plots to help guide you. Your TX appears to be squegging properly and the amplitude doesn't look awful. It might be possible to bump it up a little higher but I doubt it will make much difference.

Why are you tuning the RX to the 3rd harmonic? The fundamental should give you the strongest signal.

[daluka]

https://www.worldradiohistory.com/UK...-Detectors.pdf

Page 51. "After the two units are assembled the oscillator must be adjusted for correct operation and the transmitter and receiver set to the same frequency ..."

[ZuMi77]

Dear Carl-NC,

I am tuning the receiver to the 3rd harmonic as stated and recommended in the article edition on Oct.10, 1980 and the article I've found on this website:

"The transmitter puts out a pulsed signal at 20 kHz. The pulsing is principally to provide a modulated signal for the receiver that may be demodulated and put through a speaker or headphones."

"The receiver is tuned to the 3rd harmonic of the transmitter, i.e: 60 kHz. This provides better sensitivity than if it were tuned to the transmitter at 20 kHz. It seems to work like this: the transmitter is deliberately designed to have a considerable harmonic content in its output. The inductive coupling between the antenna loops is greater at the third harmonic than it is at the fundamental frequency, but ground penetration is better at 20 kHz than 60 kHz as the ground resistivity increases with frequency."

However, in the same editions:
"After the two units are assembled the oscillator must be adjusted for correct operation and the transmitter and receiver set to the same frequency".

I have attached an image from the Apr. 4, 1980 edition which states that the transmitter is operating at 100 kHz.

I would think we have incorrect texts.

I will tune both units to 20 kHz as per your recommendations.