Let's made a PC-base metal detector with usb interface !!!

[Aziz]

How about this for an VCA: http://www.edn.com/contents/images/91699di.pdf

Scroll down to an "analog multiplier..." part and enjoy. It is not yet THE thing that you asked for but has a potential to become one. It can also be made with discrete components, and it is incredibly fast.

I think this can become a platform for applying various weighting functions against PI signals and see what comes out of it.

Hi Davor,

thanks, indeed a very interesting concept to know about. But the presented two quadrant multiplier (the AC VCA) has more benefits:
- AC coupled (high-pass filter)
- very low noise design
- blanking of RX (gain=0)

Particularly the blanking of RX is destined to a (mini ground loop) quasi mono coil configuration (larger TX, smaller RX, concentric co-planar coil configuration, not induction balanced). It is also destined to reduce the reactive response in certain detector configurations.

Cheers,
Aziz

[Davor]

Well, yes! However, a bifilar center tapped Rx coil is also a high pass device, and it works best when supplied to a matching impedance. Thus you lose the input opamps in EDN design.

The EDN device may become even better if 1k resistors in a middle of the rings are replaced by some photocoupled voltage source ... something ... to make it float, and in such configuration it could handle a full wrath of a quarter kV PI kick - without any limiters and slow recoverers.

Instead of just blanking the Rx, I thought of applying some weighting function to Rx input gain, and thus extending the usable sampling period, and also exaggerating discrimination properties. Even a simple ramp weighting function (gain) would do, and why not making it behave for bipolar pulsing as well? In such case control voltage may alternate polarity.

My (kind of) goal is making a step voltage device pulsed at ~1kHz or a bit more with receiver capable of earliest possible sampling (thus compensating for a lack of PI kick) and reasonable weighting/discrimination. It is perhaps too much to chew, but my early observations say it should work.

[Aziz]

VCA is working.

Hi all,

finally, I have finished the VCA. Have a look at the output (see below). It's working very nice and quiet. I can go a step forward towards the pseudo mono coils (low coupled TX/RX coils = mini ground loop coils) now.

Cheers,
Aziz

[Aziz]

Interesting Sound Card Mod

Hi all,

I have found an interesting sound card mod:
http://www.rfsystem.it/shop/download...Fi_Pro_USB.pdf
(I'm currently using this sound card.)

BTW, interesting RF application.

Cheers,
Aziz

[bklein]

Try searching on SSM2164 for integrated VCA chip used all over the place for electronic music apps. Analog Devices I think just did a last buy offer but THAT makes a part that can be used functionally in its place (2162).

[Aziz]

Try searching on SSM2164 for integrated VCA chip used all over the place for electronic music apps. Analog Devices I think just did a last buy offer but THAT makes a part that can be used functionally in its place (2162).

Thanks bklein,

indeed interesting noisy chips (a lot noisier than mine). And they have the same temperature dependency as the simple design above. Unfortunately, all VCA's critical issue is the temp. dependency.

Well, I'm not satisfied with the temp. dependency, which is causing drift of course. I haven't tested the VCA circuit in the MD yet and will see, whether it can still be used for it.

The fixed gain pre-amps (or let's say better, the stable control feed-back loop amplifiers) doing much much better of course. I'll probably use two fixed gain pre-amps, which should be switchable (controlable on/off).

I'll see it, when I have more time.
Cheers,
Aziz

[Davor]

Well, I'm not satisfied with the temp. dependency, which is causing drift of course.

Against what

It is a variable gain stage for crying out loud - not a precision instrument - it is supposed to go all over the place, not just drift a bit

[Aziz]

Against what

It is a variable gain stage for crying out loud - not a precision instrument - it is supposed to go all over the place, not just drift a bit

I've found, that VCA is really very critical in low signal detection & processing. Unfortunately, my sound card test measurements reveal it's almost useless application. The temperature compensation is mandatory and is required. I do not need to test it in a MD application therefore. Although, it is a very quiet & nice circuit.

Regarding the log-amp: Same problem with temp. gradient.
(Not required to test it too as the pn-junction (diode, transistor) voltage vs. current is temp. dependent too.)

Best solution is:
- fixed gain (=stable feedback-loop),
- gate-able signal source,
- two different gain dual channel pre-amps.

Cheers,
Aziz

[Davor]

OK, but in case of a blue LED the absolute temperature variation is -1 to -3 mV/K, and gradient change is also a few percent over more than 50 degree change. I'd say a way of swinging a coil does much more than that. Also, regardless of the gradient in air, it will surely be affected by ground proximity, and more than obviously MDs can cope with that.

Anyway, you may always revert to an analog multiplier as suggested earlier

[bklein]

The SSM2164 is used by David Dixon in several commercial modular synth modules including VCO's (where temp. drift is a very critical parameter). Here is a link that describes its use in more detail: http://www.sdiy.org/philgallo/mgbvca.html
I'm not saying it is better than your design but am putting the link out there for members that need something good, simple, and may like to mess with both linear and exp. control.

[Aziz]

The SSM2164 is used by David Dixon in several commercial modular synth modules including VCO's (where temp. drift is a very critical parameter). Here is a link that describes its use in more detail: http://www.sdiy.org/philgallo/mgbvca.html
I'm not saying it is better than your design but am putting the link out there for members that need something good, simple, and may like to mess with both linear and exp. control.

Hi bklein,

thanks, I have seen this circuit. However, what seems to be good in audio applications, it's a totally different ball of game in instrumentation applications.
If I would invest more effort, I would invest in a temperature controller and put the whole VCA into a small box (module).
But that's not the point. All the effort isn't really worth just to make a few experiments.

Regarding the SSM2164 (data sheet facts):
Noise: -94 dBu for 20 kHz bandwidth, which results in 15.5 µV (Rms) noise.
Gain Constant Temperature Coefficient: -3300 ppm/°C (-3,300/1,000,000 = -0.33 %/°C)

No, no way.

Input signal gating can be done much simpler and cheaper. A two stage amplifier with each fixed gain and outputs would be enough to make the experiments.

BTW, the temperature dependency in my circuit comes from the differential stage. So this kind of VCA isn't convenient. I'm not following it anymore.

Cheers,
Aziz

[bklein]

Hi Aziz,
I understand you will not be requiring a VCA now but for kicks here is how the temp. factor can be somewhat negated (I promise I'm done discussing it ) :

[Aziz]

Hi Aziz,
I understand you will not be requiring a VCA now but for kicks here is how the temp. factor can be somewhat negated (I promise I'm done discussing it )

Thanks, indeed a very interesting concept to compensate the temperature behaviour.

It would be a K.I.S.S.-principle killer due to higher complexity. Fortunately, I don't need it anymore. I just was interested on the temperature drift and I have the answer now.

I'm working on the K.I.S.S.-masterstroke now!
World's simplest dual frequency VLF MD (strong K.I.S.S.-principle, Oh man!, I really love K.I.S.S.-systems.). It will be a modularized system.
- Basic version: passive detector controller (no battery required , plug into the sound card). I wonder, whether it will suffice due to very low transmit power (60 mW - 80 mW). But I'm keen on testing this. I'm very very curious.
- Advanced version: optional active booster module between sound card and passive detector controller (requires a battery but is very power efficient).

I hope, I can find some low-loss inductor cores (quite critical).

Cheers,
Aziz

[bklein]

After you accomplish these goals I vote for you to work on a discriminating pinpointer.

[Aziz]

Passive Dual-Frequency VLF

*LOL*

The passive dual-frequency VLF detector is really working nice!
(simultaneous 10.5 kHz & 32.3 kHz, yup!, no battery required ).

I'm surprised about the performance of it despite the fact of lossy components (lossy inductor cores, inductors, capacitors) and very low transmit power (using line-out stereo channels, 1 V (rms), approx. 30 Ohms impedance).

I need a passive RX amplifier:
- either more RX coil windings or
- dual frequency resonant filter (passing & amplifying of the two TX frequencies).

The latter one is complex to tune to the TX frequencies (adding more complexity). I wonder, whether high Q inductors & capacitors would give more improvements.

It's easier to plug an amplifier instead and use the wide band RX coil. BTW, I should plug a small amplifier to the TX side as well. Interesting application and really worth to follow it due to it's simplicity.

Cheers,
Aziz