NEW !!! Full differential PI front end.

[moodz]

I was in another thread here where I posed a question as to why PI frontends seem to be unbalanced hence all the problems with noise and front end amplifier design. One side of the coil is basically connected to signal ground and the other to a high gain amp and worse it is a DC amp requiring careful adjustment of an offset voltage.

Well I will have to eat my words ... because I have just knocked up the following circuit and hoohah ... it works like a beauty .

Note in the schem below there is no DC offset adjustment. This circuit worked straight up with the component values listed.

I only thought of this and knocked it up in the last hour or so ... ie Version 1.00

If you look at points A to ground then B to ground with a scope you will see the high voltage spike indicating that the coil is no longer unbalanced ( ie grounded on one side. ) thanks to the balun.

No need to thank me ... beer is fine

With out further ado ... the circuit.



... and the BALUN ... the rest of the components you've seen already.

[Aziz]

Hi Paul,

this idea is looking really very interesting. If I look into the schematics, I am missing a resistor from the non-inverting op-amp input (pin 3) to ground (1MO). Then a fully difference amplifier is implemented.
The difference amplifier has a quite low input impedance. So other low noise types of op-amps could be used anyway.
Correct me, when I am wrong.
Regards,
Aziz

[moodz]

Hi Aziz,
thanks for your comment ... you could be right ... I have only really shown the amp as an example ... the key part is the differential signal feeding the amp.

Note I have also not shown the tank capacitor on the V+ supply ( 4700 uF )

Also V+ for the amp would not be the same as the balanced Tx circuit.

On my lashup the Tx supply is +5 volts and the amp is +16 volts.

There is probably alot that could be done to optimise this circuit but I am confident that this may have some impact on the thinking of designers of PI frontends for future designs.

Cheers ...

[moodz]

Additional component values ...

Oh yeah ...

the diodes are 1N5822 schottky

The MOSFET is IRFP460

Cheers again ...

moodz

[moodz]

Updated schem.

Minor changes ... ( for Aziz )

+Diff amp resistor to gnd.

+Tank cap.

Have not tested circuit with added resistor.

[aziz abi]

merhaba aziz abi

bu işe yarar mı yoksa sallamışmı

[moodz]

Balun core specifications

• MATERIAL 26 Permeability 75
• DC to 1 MHz (Low 'Q')
• Color - Yellow & White
  
  Amidon type no T106-26
  outer diameter = 1.060
  inner diameter = .570
  height = .437
  uH / 100 turns = 900

[Unregistered]

bu işe yarar mı yoksa sallamışmı

Eğer bunu anlamak ... çok faydalı olduğunu

[moodz]

Decay curve measured at point B on schematic ... pulse set to 100 uS

[Unregistered]

language difficulty

bu işe yarar mı yoksa sallamışmı

Eğer bunu anlamak ... çok faydalı olduğunu

This was intended as an English Language website. There are some inconsiderate, self important people who disrupt or hijack the flow of ideas by using Arabic or other obscure languages to carry on back-channel conversations that 99% of readers on this forum will never understand.

If you would at least tell the others here what language you are using we might be able to figure out would you are saying.

If we would all try to stick to English then there would be a single language for everyone to learn instead of myriad of languages.

If your messages are private, then why don't you use PM instead of intruding on the others. Take it somewhere else.

[Carl-NC]

moodz,

I'm traveling this week and cannot do much more than look at your circuit, but I'm failing to see how this is any different than the traditional front-end method. Can you post the scope signals at A and B (at the same scale)?

- Carl

[moodz]

balun revised to floating coil

moodz,

I'm traveling this week and cannot do much more than look at your circuit, but I'm failing to see how this is any different than the traditional front-end method. Can you post the scope signals at A and B (at the same scale)?

- Carl

Ah yes .... actually I fail to see it also as I have left a blocking diode out of the +V feed .... so I have remodded the circuit again to try to demonstrate the benefit of a differential coil over a single ended / grounded coil.
With no balun this time as it probably confuses things unless you are pulse engineer.
Here it is .... explanation below picture.




NO TARGET



10 cm copper target at 25 cm.

Note that this is the output directly from the amp in the schematic above.

Circuit operation : what I am trying to do here is to present the amplifier with a differential input from the coil. In order to do this we must disconnect the coil from signal ground at the time when we want to amplify the signal from the coil ( ie straight after the tx pulse + back emf). On one side this is achieved when Q1 turns off.
On the other side D3 .. the blocking diode .. is reverse biased by the coupled EMF from the flyback pulse. Note that in this circuit there is a voltage on both sides of the coil ... ( A B ) however they are not equal. Note also that in this configuration that even using less than 5 volts to drive the coil we are getting a 250 volt plus peak with a decay rate that lets the sample begin at less than 10 uS.
As you can see from the scope shots above the decay is very good ( well it is for me :-) )
My noise problems with this ampifier have now gone ( fingers crossed ) I could not really get this to work with the 'traditional' input as Carl mentioned above.

The scope shots above were taken directly from the sig out of the amplifier from a bread boarded version of the schematic above. No additional components or tweaking (variable pots, caps etc ) were used ... and the breadboarding is really rough .... believe me.

I am hopefull of more performance tweaks when I can layout a PCB for the front end ( hopefullly this weekend. )

Anyways thanks for reading ...

moodz.

[porkluvr]

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?

[Unregistered]

well...

Well... all I can see from the scope is a poorly balanced opamp output and blocking (or at least decreased ampli) for ca 20us...

[Unregistered]

Well... all I can see from the scope is a poorly balanced opamp output and blocking (or at least decreased ampli) for ca 20us...

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