NEW !!! Full differential PI front end.

[moodz]

Thanks, Moodz. I would like to suggest that it would make a whole lot of sense if you also posted the corrected circuit with the balun, on the off-chance that some of us uneducated dummies might get an inkling. Please?

hiya P ...
the balun idea in version 0 was to reduce EMI ( RF noise ) entering the amp. Baluns dont work at DC so the biasing arrangements that 'float' the coil are to some extent unaffected by the presence of the balun. So I will proceed without the balun for now as I really need to get this front end going .... promised the guys at work

PS I am sure you're not all dummies.

[Carl-NC]

Paul,

This looks a bit better, but I'm not sure that it will offer any real benefits. Would need to give it a try. A while back Aziz posted a coil method that resembles an autotransformer, and it has the potential for true differential operation.

In your "No target" pic the opamp output has a single oscillation, suggesting that the damping needs adjustment.

- Carl

[moodz]

Thanks Carl .... yeh the damping is a bit out by the looks of that dip ... I will post a shot of the scope showing the voltage on each side of the coil ... there is a high flyback voltage on the mosfet side as shown already but there is also a pulse of lower voltage on the other input ... but kind of delayed or something else.

I have noticed that with the addition of the so called blocking diode my flyback voltage is much increased ... my "traditional" single ended version of this circuit using the same coil, mosfet, damping and 5 volt TX supply peaks at 140 volts. The sample point is about 20uS. ( with schottky diode clamping ). With this "balanced" version the clamping time has decreased below 10uS and the peak flyback voltage is approx 260 volts ... though I suppose if I increase the damping this will fall.

moodz.

[Aziz]

Hi moodz,

don't use the schottky diodes. They have a quite long recovery time. The 1N4148 has a much lower capacitance and they recovery from clamping very fast.

This can be seen on the circuit simulation.

Aziz

[moodz]

Thanks Aziz .... I had the impression the voltage drop across the schottky diodes would be lower ... I will give the 1n4148 a go. ( they will be cheaper too. )

I have been tweaking the circuit some and have got the decay rate twice as fast with no ringing that I can see. Peak voltage is now 450 volts ... I feel I could go higher but the front end of my CRO is dear to me.

Will post pics when I rebuild the circuit.

cheers moodz.

[Unregistered]

well...

you need to get your glasses checked ... the scope is set for 5 uS / div ... 20 us is nearly off the display.

Boy, have a look at the small hf artefact amplitude - it is back "normal" at 20us...

[Rov]

U ,unregistered guests ,need to buy yourselfs a cro ,an get your act together!Ha!an do some work of yur own! nitpicks!then post it here! so we can pick!Ha!Trolls!
We enjoy moodz test an experiment,I do,so register an share your knowledge,I want to learn more.

[moodz]

U ,unregistered guests ,need to buy yourselfs a cro ,an get your act together!Ha!an do some work of yur own! nitpicks!then post it here! so we can pick!Ha!Trolls!
We enjoy moodz test an experiment,I do,so register an share your knowledge,I want to learn more.

thanks Rov ...

Progress continues ....

Here is a scope pic showing the end of the tx pulse WHITE ( mosfet gate drive ) and the ouput of the amp GREEN ( no target ) ... the inverting leg of the amp is fed with 4.58 volts for balance. Note this is not as per schematic.

The Tx pulse is set for 100 uS in this case.
Note the fly pulse has decayed in 10 uS this is better than my previous efforts.

NO TARGET



BIG STEEL TARGET ( my what big steel targets you have grandma ! )



Note this is the SINGLE ENDED performance ..... tacked a pot to the inverting side. The differential is significantly different. More in my next post.

I have made further improvements to noise without dropping gain.

cheers from moodz.

[moodz]

Oh .... substituted 1N4148 for above test instead of schottky ( thanks Aziz ) Also the blocking diode is the 1N4004 only. ( no shottky ).
The 4.7 pF cap removed across the 1M resistor. 30 pF cap added across pins 1 & 8.

Moodz.

[Unregistered]

Well...

Well... Your result is much worse now. Opamp stays saturated for ca 30us and is still unbalanced...

[Unregistered]

Well...

U ,unregistered guests ,need to buy yourselfs a cro ,an get your act together!Ha!an do some work of yur own! nitpicks!then post it here! so we can pick!Ha!Trolls!
We enjoy moodz test an experiment,I do,so register an share your knowledge,I want to learn more.

Well... Are you sure you don't need to see a doctor?
Just read and LEARN...

[Unregistered]

Well...

Hi moodz,

don't use the schottky diodes. They have a quite long recovery time. The 1N4148 has a much lower capacitance and they recovery from clamping very fast.

This can be seen on the circuit simulation.

Aziz

BAT81..3 seem faster then 1N4148 - they have a three times lower capacitace! And they ARE "shottkies"...

[moodz]

saturated amp ?

The first 20 us is the trailing edge of the tx pulse , the next 10 is under the back EMF ... what exactly do you want the amp to do during this time ?

[moodz]

SIMPLE PI front end ....
The circuit below is the simplest front end using a comparator at the output the output pulse will get wider as a target is brought near the coil. Ideal for simple CPU processing.
This might be a new way of processing PI front end signals. woohoo :-)
Tx pulse set to 100 uS



NO target



With 50 cent coin in center of coil



The back end processing would have to resolve increase in pulse width to determine presence of target.

This circuit can detect a large metal sheet at 1 meter.

cheerz from moodz.


This idea could do with more development.

[moodz]

here is a version with reduced jitter on o/p pulse ... lost some sensitivity though.