for this design of envelope detector ,
1- what's the function of D2.
2- why the output signal is inverted.
see attached pic.

 

The D2 is used as positive peak clamper and hence the out put appears as inverted that is with positive peak clamped to zero volt.

 

The direction of the diodes will determine which way the peak voltage goes.

When designing a demodulator like this, be sure the capacitors don't short the audio but do short the RF. It is a common mistake.

 

The D2 is used as positive peak clamper and hence the out put appears as inverted that is with positive peak clamped to zero volt.

No, the negative peak is clamped to (one diode drop below) zero volts.
The circuit is a voltage doubler (see Fig. 2).

 

They really should have used a small Schottky diode, like a BAT54. The BAT54's Vf is far lower than 1N4148/1N914 switching diodes.

 

No, the negative peak is clamped to (one diode drop below) zero volts.
The circuit is a voltage doubler (see Fig. 2).

Yeah Ron I first thought the same that -ve peak is clamped coz of diode's polarity but eng_alaakaa's scope's output is not conforming to that. Could you plz. explain?