How do these "chopper" PI's work compared to traditional switch PI's ?

[6666]

How do these "chopper" PI's work compared to traditional switch PI's ?

Recently I have noticed several different designs of PI detector that seem to use a "chopper" rather than a normal 4066 switch between the preamp and integrator.
Or at least to me it looks like the designer's are useing a type of chopper circuit to process the target signal, someone please correct me if I'm wrong and barking up the wrong tree.

There seems a couple of different ways to chop the signal, on the "simple"circuit the transistor is getting turned on at the same time as the TX pulse, other circuits use a delayed pulse to turn on the chopper transistor,
I've attached some of these circuits.

Does anybody know how this chopper circuit works and what is happening to the target signal ?

I have asked on some other forums useing google translate but got no reply, Thanks.

[bernte_one]

Hello 6666,

i Need more Infos for simple chopper 1

[Qiaozhi]

Many detector audio output circuits use this technique, whereby an audio oscillator is progressively turned on as the target signal gets stronger. At first sight this seems to be something similar. If the "chopping signal" is in the audio range, then it appears that this is being enabled by the preamp output.

[6666]

Hello bernte_one
all I have is the schematic for simple chopper 1, no other information, I found the schematic some time ago, probably on Turkish forum, maybe somebody will recognise it and post a link to it ?

[6666]

Many detector audio output circuits use this technique, whereby an audio oscillator is progressively turned on as the target signal gets stronger. At first sight this seems to be something similar. If the "chopping signal" is in the audio range, then it appears that this is being enabled by the preamp output.

Hi yes it seems to be similar in technique to chopping the audio in surf pi or barracuda, but slightly different
in the surf pi they turn the target signal to DC with a diode then chop it
but with the "chopper" circuit they chop it straight out of the preamp then turn to DC with diode, all very interesting and a curiosity

[6666]

Hello 6666,

i Need more Infos for simple chopper 1

Just by luck, I have just found the original article for you

http://4hv.org/print.php?plugin:forum.148849

[bernte_one]

hi 6666,

big thx, as soon as i am back at home i will copy the layout from the Pictures to sprint layout
seems easy to rebuild

[bernte_one]

Hello,

here is my re-work of german simple Pi
watch out for errors i´m only human like you ;-)

GerPi2017.zip

[bernte_one]

corrected file attached
some missing connection and reduced pcb size

GerPi2.zip

[6666]

Hello bernte_one, good work thanks, would it be possible to have a picture of just the copper tracks, I cannot print from my demo copy of layout 6, thanks.

[bernte_one]

tracks.pdf

simple trick for sprinlayout viewer
just make a screenshot strg+print key
put it into paint and print from there you can test print and scale format if you try to put an ic socket on the print

[Wahyu]

hello 6666 any update about this?
i try use LTSpice, its decrease decay curve from preamp, cmiiw

Wahyu.

[6666]

[ATTACH]39269[/ATTACH]

simple trick for sprinlayout viewer
just make a screenshot strg+print key
put it into paint and print from there you can test print and scale format if you try to put an ic socket on the print

Thanks I will try it.

[6666]

hello 6666 any update about this?
i try use LTSpice, its decrease decay curve from preamp, cmiiw

Wahyu.

Hi, I have not had time to experiment to try and find out how this chopper works yet, anything you find in spice would be interesting to look at, thanks.

[Davor]

Shunt switch should not be called a chopper. These builds use a transistor as a switch, which is fine, but in heydays of transistors as switches they were used in reverse, as the reversed BJT saturation voltage is down to about 0V. The reversed transistors work well up to about 6V, because BJTs Veb reversed avalanches at about 7V.