D8 D9 on Minipulse

[Xtrem]

Hello.
For what are the two Diodes (d8,d9)on the circuit??,what are they cutting?
i think they are in the wrong position connected, because
only the negative signal have to pass through the Switches,(Q3,Q4)
the positive signal contains only the transmitting pulse.
I have tryed a 1N60 (200mV) in the opposite position and its working
good.
If im wrong ,please correct me.

[homefire]

They are being used to Limit the Negative to the combined forward voltage drop of the D-8 and D-9. Think Zener. This is a proven working Detector with many successful builds. Build as in the Document or it will most likely not work as intended.

HF

[Qiaozhi]

As Homefire said.
Plus it may be still be working in your case because of the built-in protection diodes in the preamp. However, those particular diodes are not designed to handle the high flyback voltages generated by the coil, and the circuit will eventually fail due to stress.

[Xtrem]

They are being used to Limit the Negative to the combined forward voltage drop of the D-8 and D-9. Think Zener. This is a proven working Detector with many successful builds. Build as in the Document or it will most likely not work as intended.

HF

Only the Negative is usefull, why cut it at -1.4V ??

[Xtrem]

As Homefire said.
Plus it may be still be working in your case because of the built-in protection diodes in the preamp. However, those particular diodes are not designed to handle the high flyback voltages generated by the coil, and the circuit will eventually fail due to stress.

You mean D5,D6 . I ask for D8, D9, at this point flyback dont exist.

[homefire]

Yes, D-8 and 9 are being used to Limit the Negative. Only that portion is of use. They prevent overloading the Integrator. Why overload the Integrator ? If the Integrator is in overload it will not serve it's purpose. HF

[Qiaozhi]

You mean D5,D6 . I ask for D8, D9, at this point flyback dont exist.

My humble apologies. I should have checked the schematic first before replying. When you mentioned two diodes, I naturally assumed you were referring to the back-to-back diodes at the preamp input. It slipped my mind that there are also two in series at the sampling integrator input.

Actually that's a good question, and to be honest I'm not sure of the answer. The reason they are there in the MPP is simply because they were present in the original Minipulse, and I remember Eric stating that this sampling integrator topology was very good..
However, I do have a variation of this circuit running without R18, R19, C12, D8 and D9. So who knows?

[Xtrem]

Thanks for the answers,
Im asking because Minipulse are earlier then Superscan and Goldscan , and the same sampling integrator are in all designs,
in the last 2 (ss , gs4) the diodes are not present.

[homefire]

Maybe that's why Eric said it was a good detector ?