Off-resonance probe, strange behavior

[Mullihaka]

Hi!

I made one. The envelope follower part works, but the probe as whole doesn't. Whatever metal I take close to the coil, the level decreases and that way also the faster envelope follower. The oscillator frequency is 28 kHz. I have tried to increase and decrease the frequency, but the problem stays.

The issues in the "Probe PCB from Silverdog" thread have been noted.

All hints appreciated.

[WM6]

It could be good to have more tap's on your coil and try with different proportion in both parts of coil windings.

[Qiaozhi]

Not sure what's going on with your build of the off-resonance detector, but you can rest assured that the design does actually work.
Can we see some photos?

[Mullihaka]

It was constructed on Vero board, so the photos are of little use. It's in pieces already and waiting for a new trial with new parts. I tried commercial inductors and home made ones, wound on a ferrite rod from an old AM radio. Before a new trial, it seems that I need to understand two issues.

First: in the middle of page 35 (component value selection for the transistor base circuitry) it is stated, that "the peak is set to 33.9 kHz", meaning the the combined impedance of R2 and C2 in parallel (I assume). However, the capacitive reactance of the C2 gets smaller with increased frequency infinitely. Therefore, I don't understand the word "peak" in this context.

Second: as the output level of the oscillator increases only, when the frequency increases, the probe detects only non-ferrous items. Is that correct?

[Qiaozhi]

It was constructed on Vero board, so the photos are of little use.

The original design was also constructed on Veroboard, but unfortunately this board appears to have gone AWOL.

It's in pieces already and waiting for a new trial with new parts. I tried commercial inductors and home made ones, wound on a ferrite rod from an old AM radio.

When I rebuilt the probe into the same enclosure (using one of Silverdog's boards) I found that the probe failed to work, but the only component I had to change was R1, which was lowered from 3k3 to 1k2. This appears to be a tolerancing issue, and it would be advisable to replace R1 with a 5k trimmer to enable finding the best operating point.

Before a new trial, it seems that I need to understand two issues.

First: in the middle of page 35 (component value selection for the transistor base circuitry) it is stated, that "the peak is set to 33.9 kHz", meaning the the combined impedance of R2 and C2 in parallel (I assume). However, the capacitive reactance of the C2 gets smaller with increased frequency infinitely. Therefore, I don't understand the word "peak" in this context.

You are correct. We shouldn't have used the word "peak". The purpose of R2 and C2 is to cause the bias voltage of Q1 to be dependent on the oscillator frequency. The cutoff frequency should be near to 28kHz, but I seem to remember that it was actually slightly higher (33.9kHz), which was probably due to the combined effects of other components, layout, and component tolerancing. Although BFO and off-resonance designs are fundamentally simple in their operation, in practice they do tend to be fiddly to get working properly. Luckily, with this design, all the fiddly bits are associated with the TX circuit. Once it starts working, the circuit remains stable.

Second: as the output level of the oscillator increases only, when the frequency increases, the probe detects only non-ferrous items. Is that correct?

I'm sure the original prototype reacted in this way, but the one built on the Silverdog PCB does not discriminate between ferrous and non-ferrous targets. It might be worth replacing R2 with a 100k trimmer to allow adjustment of the transistor bias.
Personally, I've found the probe circuit to work reliably using the components specified, but you may have to experiment somewhat if you're using other coil types than the ones listed in the book.