signal processing - Envelope detector

I would like to have the envelope from a signal using a simple circuit composed of a diode and a low pass filter.

I know :

*Tam > t >> Tcarrier

*The diode must be a germanium diode because of the Vthreshold

I use:

*A germanium diode

*R = 1k

*C = 4.7nF

The wave is generate from a GBF :

*fcarrier = 1MHz *fam = 20 kHz

My circuit:

I took a capture from the oscloscope measuring the signal accross the resistance. My signal is shown bellow :

My questions are:

• How can I have a clean modulating signal?


• Some books talk about a High pass filter, why it could be necessary? (I tried and it's not better)


• The circuit is currently designed for a specific carrier but if my carrier has a defined range (in my case I will have a range between 1MHz to 7MHz), how could I adapt the circuit (or is there another circuit)?


• Is the type of the capacitor change something?

Thank you,

Farad