Just a little more help toward understanding the RX Gain and Discrimination sensitivity controls.

[Mike Hillis]

It helped me and since I have a little free time at the moment I thought I'd post it.

Just some bench testing results to help show the interaction between the two controls.
In the bench testing I used the 12" concentric coil just so I'd have a large sweet spot. The coil is not V rated. The program was coin and jewelry, Best Data, 3 Frequency, and the test object was a clad dime.

The object of the test was to see the interaction between the two controls, not to ascertain depth capabilities. I just want to see the differences in settings.

First I looked at RX gain settings, with minimum Discrimination sensitivity setting of 1 which is expressed as RXGain/Disc Sens = distance from coil.

2/1 = 3"
4/1 = 4.25"
6/1 = 5.25"
8/1 = 6"
10/1 = 6.5"
12/1 = 6.5"
15/1 = 6.75"

Then I looked at Discrimination sensitivity settings at low, medium, and high RXGain settings. Still expressed as RXGain/Disc Sens = distance from coil.

1/10 = 3"
1/20 = 3"
1/30 = 3"
1/40 = 4"
1/50 = 4"
1/60 = 4"
1/70 = 5"
1/80 = 5"
1/90 = 6"
1/100 = 9.5"
_________________________
8/10 = 6"
8/20 = 6"
8/30 = 6"
8/40 = 7"
8/50 = 7"
8/60 = 7"
8/70 = 8"
8/80 = 8"
8/90 = 9"
8/100 = 10"
________________________
15/10 = 7"
15/20 = 7"
15/30 = 7"
15/40 = 8"
15/50 = 8"
15/60 = 8"
15/70 = 9"
15/80 = 9"
15/90 = 9"
15/100 = 9.5"
_________________________

I drew a few conclusions from the bench testing. The first conclusion was that at a RX Gain of 8 I had pretty much multiplied the signal as much as I was going to. Any higher RX Gain settings only marginally improved depth of detecting. Thereafter, at higher Rx Gain settings, something else was taking place. A little more bench testing needed here which I haven't done yet but I'm pretty sure I know what is going on. Just need to prove it.

Second conclusion was that the Discrimination sensitivity settings give one much more reliable performance in dealing with EMI than one might think. Basic performance ranges are clearly defined and are wide enough to handle large changes for stability as might be required by site conditions. If I was operating in the medium range of 8/40 through 8/60, if EMI forced me to lower my Disc Sens setting to a setting of 48, for example, I haven't lost any basic performance for that range. I would be able to stay within that basic performance range and still obtain a stable detector.

Third conclusion was that there are multiple options to obtain the same results. Using the data above, if I needed to be able to detect a 7" dime, I could get it with a maxed out RX gain with very low Disc sens setting, or I could reach the same 7" with minimum amount of RX gain and a very high Disc Sens, or I could reach the same 7" with medium RxGain and medium Disc Sens, or I could reach the 7" with High RX gain and minimum Disc Sens. All which means that site conditions that prohibit one particular setting due to ground minerals and EMI can be approached with a different setting which would give the same results.

Maybe this will help you as much as it did me. I'm much more confident in how I set these controls now.

HH
Mike

 

[yazoo]

Mike ,That is some interesting info. Thanks Yazoo

 

[hang7575]

Great info, Thanks

 

[Larry (IL)]

Thanks for sharing Mike, that is some good information to know.

 

[Neil in West Jersey]

Great info. In my field testing I found that EMI has a larger impact on Discrimination (and All Metal) Sensitivity controls, while ground conditions (mineralization) has a larger impact on the RX Gain. I am curious to see what your future results look like.

 

[nw1886]

Nice work! Just goes to show that there is more than one way to skin a cat and this info (organized...like you've done) will help many understand the flexibility of it all. Have a feeling this post will be tagged a lot Mike.

 

[jbow]

I tried it and it seemed to do OK to me too. I am not afraid to lower my disc sens when I need to now.

I had heard, when the V3 first came out, that the RX did nothing except amplify whatever you were already hearing but it seems to be more than that... you may remember at first it was said that the RX did not increase sensitivity or depth but in conjunction with other settings it does make the machine better or worse. I just don't think it is as simple as "it is merely an audio amplifier". Maybe it is but if it is... it is amplifying the inaudible to a level where it can be heard and that makes it essentially a sensitivity control (practically). So I guess one could say that the V3 has essentially dual sensitivity controls foor disc and AM. May as well say that because the RX certainly has more affect than simply volume... gain is another word for power or sometimes volume. I don't know if there is any parallel between the V3 and an audio or guitar amp but on most amps there is a volume control that controls the poweramp and a gain that controls the output of the preamp. Both increase volume and both increase power but gain sends one into overdrive. There may be no parallel between this circuit and an audio circuit though... it may just be the same words with different meanings... it will help to better understand exactly what everything does and how everything interacts... I get easily lost in the V3 because I sometimes don't know what did what and why...

Posts like yours are very helpful.

Julien

J

 

[TheMarshall]

[attachment 173732 V3senscntrols.jpg]

Jbow.....here is a simple representative diagram of the typical arrangement of the Whites gain controls.

The Rx sensitivity increases the amplification of the raw, unadulterated search-head's receiver loop signal.

The amplified signal is then fed as a common signal to both the Disc channel and the All Metal channel.

Now each of those two channels are conditioned and amplitude controlled, by their own gain controls.

The art of adjusting these is based on the local noise conditions, be that from the soil mineralisation and/or the EMI.

There is no pre defined formula, for every situation is unique. Only you and your experience can decide the optimum settings.

Keep the 'noise' factor under control with the Rx gain, and choice of filters, then maximise your Disc sens.

I applaud Mike's effort, but is it really as revealing as people's responses appear to imply?????

Tell me what you deduce from Mike's figures, and how you will then apply that, when setting up your detector.

Maybe others will also express their understanding of the data, and how they will utilise that understanding.

I'm not criticising the exercise for what it is, but honestly trying to be objective in its interpretation.

The basic idea is OK, but the method employed is not good enough in terms of reliable data acquisition.

It lacks definition.........

Thanks Mike for the stimulating post..........................TheMarshall

 

[Bradley137]

Very good observations Mike. I've noticed the same things. I generally hunt with 85-95 on the discriminate sensitivity with Gain at 5 or lower with no fear of loss. The V3i is really amazing in this aspect. I like to think of it as having $100 bucks in my pocket when I only need $50.

 

[nw1886]

Mikes work is great in giving a picture for understanding how it all plays out. Accuracy? Expect nothing to be 100% accurate/only consistent, and that is where learning begins.

 

[jbow]

This is all very helpful for people like me who both have a hard time learning and tend to forget in the field... laff.

Seriously, thanks!

Julien

 

[leojack]

Marshall::thanks for that pic.in this thread.I'm a visual guy and it will help with my learning curve, V3i is a complicated md that needs hands-on work.My ques.is this: a noisy,bouncy screen can be emi or mineraliz.If I understand your post it can be addressed with offset changes,however I was under the impression ground mineraliz. would be dealt with by ground balancing often during a hunt.Which I do every five minutes in my trashy around the house searches.I am anxious to get afield with the md but am reluctant to go out without a full magazine of understanding.It Right now I'm not there yet.So to my ques.I assume ground filter adj could help,is it part of the senstiv.screen when I zoom it up,or somewhere nested in one of these myriad of menus...............Thanks for your patience,I could be a mile off base here..............Jack

 

[Rob (IL)]

When the V3 is ground-balanced properly, the V3 sniffs out the grounds VDI number and sets the circuitry to lose sensitivity to targets with that VDI number.

What you want to achieve is the highest Signal to Noise ratio possible. That is to say, you want to transmit the highest signal possible, and you want to incur as little noise as possible. One quick check for EMI is to hold the coil waist high and pull the trigger. If you get a "motor boating sound", it

 

[jbow]

Good post Rob... For me, the more times I hear something explained and the more simply, the sooner it "clicks" with me and then I begin to understand. Just being told what to do, tells me nothing until I understand why I am doing it. Your explaination here is really clear... I was starting to understand but this really helps me.

I have been confusing frequency offset with GB offset because the words are partly the same and I wasn't paying close attention. Realizing my mistake has really helped me. It is simple once I understand it but confused everything when I didn't.

Thanks!
Julien

When the V3 is ground-balanced properly, the V3 sniffs out the grounds VDI number and sets the circuitry to lose sensitivity to targets with that VDI number.

What you want to achieve is the highest Signal to Noise ratio possible. That is to say, you want to transmit the highest signal possible, and you want to incur as little noise as possible. One quick check for EMI is to hold the coil waist high and pull the trigger. If you get a "motor boating sound", it

 

[Rob (IL)]

Use Carl's shortcut for setting the frequency offset at the top of the forum.

 

[nw1886]

That frequency offset is a godsend. I've went back in to tough spots with the upgrade and using offset.....just clarity.

 

[Mike Hillis]

I find your response interesting.

This one bench test I have made public seems to indicate a couple of things:

1. The RxGain has optimum high settings of somewhere around 8 -10. At higher settings the RxGain is having a different effect. This is not unusual. Take the Fisher CZ70 as an example. Though it has 10 sensitivity settings, on many units the optimum setting is a 4. The higher settings don't add any more real performance depth wise, however, they do have an effect. They widen the search loop's footprint. My RxGain bench test results seem to indicate that this is also true of the V3i. Is this true? You do the test yourself and then we can compare notes.

Practical application: Maximum penetration performance with the correct search loop foot print for the site conditions. In trashy environments, you would want a tighter footprint while maintaining the best depth penetration. In less trashy environments, you can get away with a larger foot print.

2. The Discrimination Sensitivity control consists of four op amps. Their ranges are clearly defined. Very much like the M6. On the M6 you could actually hear when you switched op amps and when setting up you tried to stay as close to that top op amp range of settings as possible. You wont hear the changes here but you can memorize the ranges and achieve the same results.

Practical application: Intelligent operator control of the Disc sens settings. If you know the op amp ranges, you can make intelligent decisions of when to use other feature settings to keep you in the peak range you need for the targets you are after.

I'm very familiar with other detectors that uses this style of sensitivity adjustments. My F5 has this same arrangement, with different labels (gain and threshold). The F5's gain control is totally linear throughout it's entire 20 point range and the threshold has more effect within it's single op amp (-9 to 0 settings) I believe its a single op amp because I don't see clearly defined patterns like I have with the V3i though the changes are not linear, per say. The CoinStrike and Gold Strikes use this same type of arrangement, only with different labels (sens and threshold).

The more you know about how the controls relate with each other, the more intelligent you are in making adjustments. These bench test results show this stuff very clearly.

The site conditions always dictate the optimum settings for a given site. The metal detector operator's job is to find them. It is much easier to find them when you know stuff like this.

All that and a bag of chips.
HH
Mike

 

[hang7575]

Mike again thanks for this info ,Was curious if you tried it with the TX boost on? I never use this and wondering if it has any effect on depth?

 

[TheMarshall]

[attachment 174111 V3-MikesData.jpg][attachment 174112 V3graphGains.jpg]

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~​

Hi there Mike.

I have attempted to plot your data in a graphical format, after interpolating it to produce a smoother representation.

For the benefit of other readers....We have 3 curves of Disc sens. adjustments, and 1 for Rx.

(1) With the Rx is set at level 1, the Disc is varied from 10 to 100, in 10 steps.
(2) With the Rx is set at level 8, the Disc is varied from 10 to 100, in 10 steps.
(3) With the Rx is set at level 15, the Disc is varied from 10 to 100. in 10 steps.

(4) Finally, the Disc Sens is set to 1, and the Rx varied from 1 to 15, in 15 steps,

For plotting purposes, I have concatenated the 15 steps into 10, and adjusted the air range figures accordingly, thereby retaining relatively the original values.


Observations:-
Note the curve