Diodes and a tank circuit

I’m curious what the D1 and especially D3 do in the attached circuit. L1 and C1 form a tank circuit tuned to around 125 KHZ. C1 AC couples the output of the antenna to the rest of the circuit. When I take D1 out of the picture it seems to not have an effect. I’m not clear on what D3 does. Any ideas?



What is the context? 125kHz sounds like RFID sensing, but I wonder why tunnel diodes are used?

Yes, its an RFID reader, 125, 128 and 132.4 KHZ both ASK and FSK modulation. Right now I have the 125 ASK working but not FSK. I couldn't find a regular diode in Viso so I used the tunnel picture. I'm using 1N4001 diodes. Vo goes to an envelope detector to pull off the AM modulation

I don't understand that circuit really - usually the tank is driven at high voltage with a peak detector
circuit and the peak voltage changes are capacitively coupled to the rx amp. The peak detector would
use a diode, but not in that topology.

I don't understand it either. Its a modification of this design

Here's the full schematic: I believe D2, C2 and R1 form the envelop detector.

D2/D3 with associated components form a rectifier / peak ac detector.


ahh. thanks. the peak detector cuircuts ive seen only have d2 and components. what does d3 add?

The circuit in your first posting is not the same!!

You are right, I have a cap in the wrong place! Nice catch. So it seems D1 does nothing at least for an AM modulated signal. With D3 removed the modulation looks like a square wave at the output of C7. Putting D3 back in causes the modulation to look more like a saw tooth the peak of which defines the start of the pulse.

ahh. thanks. the peak detector cuircuts ive seen only have d2 and components. what does d3 add?

D3 is a clamping diode, also known as a DC restorer. You can search this on the web. Basically, it shifts the incoming AC signal by clamping one side of the waveform to a fixed DC voltage - in this case ground. The result is a higher, almost double, rectified voltage across C2.


A voltage double. Thanks for the info James.

Got it. So C7 and D3 form the clamp and move the signal from +/- into all positive territory referenced by the grounded side of D3.