Does changing the Sensitivity have any effect or affect on the frequency of a VLF metal detector ?

[ToddB66]

Hi All !

Prefacing my title question, I've read that within the VLF frequency range (3kHz to 30kHz) as a general rule, lower frequency metal detectors go deeper and higher frequency detectors go shallower. However, I suppose that target depth might possibly alter this general rule.

Again prefacing, I've read that lower frequencies in the VLF range are not as effective at reporting tiny gold, but higher frequencies are more effective at reporting tiny gold.

Being a student of electronics with limited knowledge, I'll venture a presumption that factors such as ground with heavy ferrous mineralization and/or trash items will also have a negative effect on this "general rule" of frequency depth. As we all know, surprises do pop-up depending on the particular metal detector model, settings and operating technique being employed and of course field and ground conditions.

OK, back to my title question, please advise, does changing the Sensitivity control have any effect or affect on the frequency of a VLF metal detector ? If not, then how does the Sensitivity control circuit work to increase or decrease the sensitivity to certain targets in the ground without also altering the frequency ?

Thanks in advance for replies !

ToddB66

[Koala]

You can read all the theory you like. But in the real world in the ground I see no difference between old analogue detectors running 4 or 14khz on normal target sizes. Discrimination does seem to be slightly different or is that difference in the machines I don't know. Small thin lead and foil is closer to iron on the 4khz machine.

However a smaller coils does like the smaller targets like cut halfs and 22 cases with only a slight loss of depth on pre decimal size coins. Only downside is the coverage.

Sensitivity is normally the amount of gain/amplification of signal on most detectors. Normally affects all targets by the same percentage.

Again in the real fields maximum sensitivity does not always give you maximum depth. Too much chatter and small signals just get swamped and lost in all the noise. Just below the chatter is best.

[ToddB66]

Does changing the Sensitivity have any effect or affect on the frequency of a VLF metal detector ?

You can read all the theory you like. But in the real world in the ground I see no difference between old analogue detectors running 4 or 14khz on normal target sizes. Discrimination does seem to be slightly different or is that difference in the machines I don't know. Small thin lead and foil is closer to iron on the 4khz machine.

However a smaller coils does like the smaller targets like cut halfs and 22 cases with only a slight loss of depth on pre decimal size coins. Only downside is the coverage.

Sensitivity is normally the amount of gain/amplification of signal on most detectors. Normally affects all targets by the same percentage.

Again in the real fields maximum sensitivity does not always give you maximum depth. Too much chatter and small signals just get swamped and lost in all the noise. Just below the chatter is best.

Hi Koala !

Thanks for your reply.

In reference to the above sentence I colored blue, am I correct to deduce that changing the Sensitivity setting up or down on most VLF metal detectors has no effect/affect on the frequency of the signal, just the power of the signal ?

For example, if the frequency was 7.8 kHz and I change the Sensitivity setting, the peak-to-peak distance of the frequency sine wave would not change.....correct ?

ToddB66

[Skippy]

It's just a threshold-based system. If the "target" signal is above a certain threshold, you get a beep. Vary the threshold, you get more / less sensitivity. In practice, the threshold is often fixed, and the "target signal" gain is varied before it reaches the threshold comparator, with the same end result.
Almost all detectors have a fixed operating frequency, and those that offer adjustment only vary it by a few percent, primarily to avoid interference from other detectors operating nearby.
And just about 100% of commercial machines DO NOT vary the transmitted "power", any gain changes are performed on the receive side. A notable exception is the XP Deus, which, as well as the normal RX gain controls, has 3 TX power levels, in order to maximise battery life. But as TX level is increased, RX gain is proportionally decreased, and vice-versa.

[Koala]

Correct peak to peak stay the same

Wrong transmit power stay the same.

Sensitivity is how the receive signal processed. Bit like turning up the volume.

This is general there are exceptions it would be better to know which detector we are talking about.

[homefire]

Most detectors the Sensitivity in only a function of Receive Gain.

[ToddB66]

homefire,

Yes, that's what Skippy said in his post #4.

ToddB66

[Skippy]

"how does the Sensitivity control circuit work to increase/decrease the sensitivity to certain targets"
This is an interesting question. If you read the blurb for the Fisher F75, for example, you will see that JE (jewelry) mode offers higher sensitivity to lower-conductivity (higher corner-frequency) targets, relative to the regular DE (default) mode.
This difference is determined by the details of how you process the RX coil's signal before you do the 'threshold comparison' process. Detectors commonly split the receive signal into vector components, often 90 degrees apart, though not always. Mathematically speaking, Real and Imaginary. It is these two signals that get processed (differentiator/integrator, low/high pass filters, for example), and then recombined in some fashion for 'threshold comparison'. I don't know the details, but for example if you compared ( R + I) to a threshold, you would get different results than if you compared ( 0.5 R+ 1.5 I )
And as the Fisher F75 uses DSP maths to do this processing, it can presumably do more tricks, like complicated maths, and user-selectable profiles (the DE / JE modes).
But what remains absolutely constant is operating frequency and amplitude.