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I am currently working on a project in which I have a modulated RF signal at a carrier frequency of 400kHz. The signal itself is fairly low voltage, and I need to extract it from the RF carrier. I was thinking about doing it using a simple diode + RC circuit envelope detector such as this one from here:

Envenope Detector

The problem with my current approach is that my diode's turn-on voltage is about .63V, but my signal is about 300mV p-p. Is there a simple way for me to DC-bias my signal to about ~600mV? That being said, is it wise to use an envelope-detector like to one I'm currently using for a low-noise application? If so, are there any other good alternatives you know of that would do well at demodulating?

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I would suggest using a bias tee for adding DC to your RF signal. More than 600 mV of DC voltage would be required, or your diode will only open at the maximum of the signal.

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(picture from Wikipedia)

You need to be careful about selecting the L and C values, so that the inductance represents a high enough impedance at the desired RF frequency but the capacitor's impedance should be lower than the characteristic impedance of the transmission line (coaxial cable or whatever you use).

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  • \$\begingroup\$ First of all, thanks for the response. I really do appreciate it. Secondly, I was wondering, at 400KHz, do you know/think if the parasitic capacitance of the inductor will come to play in this circuit and create a shunt path? Thanks! \$\endgroup\$ Commented Jan 30, 2015 at 10:13
  • \$\begingroup\$ It depends on the both value of your inductor – the higher the inductance, the lower the self-resonant frequency – and its "type" (certain inductors are designed to have really low parasitic capacitance). Overall, I don't think it will be an issue at 400 kHz using several hundred uH inductors, but I'm not familiar with typical parasitic values. You should check datasheets or try and measure the self-resonant frequency. \$\endgroup\$
    – hryghr
    Commented Jan 30, 2015 at 11:28

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