Adding an Envelope Detector to convert an RF input to a DC output
I connected the ADC Pi module to the GPIO header and stacked the DDS module on top. PCB support spacers were used to hold the modules together and to take the strain off the GPIO pins.
All that was left to do was to use a simple envelope detector to convert the RF coming out on the circuit under test into a DC voltage that could be measured by the ADC Pi module – time for a little bit of theory…
A simple envelope detector is shown above. It consists of only three components – a diode, a capacitor and a resistor. An RF signal (Vi) is fed into the detector and a DC signal (Vo) comes out the other end. The RF input (blue line) and DC output (red line) signals are plotted below.
When I tried using a simple envelope detector I discovered that I was getting little or no signal out of the envelope detector. So much for the theory! I measured the peak RF voltage coming out of the circuit under test I found it to be around 0.2Volts (200mV) – this was the cause of the problem! Time for some more theory…
With reference to the graph above, a diode needs a certain level of forward voltage “Vd” to “turn it on”. For a Silicon diode this voltage is about 0.7 Volts and for a Germanium this voltage is around 0.25 Volts (250 mV), which is around the same level as (or greater than) the peak RF signal voltage I was trying to detect. Therefore the RF signal I was trying to detect and measure was insufficient to drive the simple envelope detector
For more detail: A Slice of Raspberry Pi