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| RF Detector Help |
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| amoy:
Hello, I am building an RF FSM but I need some help understanding the circuit. My circuit is similar to the first half of the circuit in this link (http://www.qsl.net/n9zia/wireless/pics/fs_meter.png), before the amplification stage. Here are my questions: 1. What is the purpose of the diode configuration to detect RF signals 2. How does the capacitor/resistor configuration work? is it a high-pass/low-pass? 3. I am using 1N5711 Schottky diodes, and I am able to pick up signals produced by walkie-talkies (460 MHz) but not cell phones. Could this be a diode issue or filtering issue? Any help would be appreciated. Thanks. |
| PA0PBZ:
Hi, The diodes are there to rectify the signal, the meter works only on DC, not AC (or RF). The capacitors are supposed to get charged by the rectified signal, the resistor is there to discharge them when the signal is gone. Probably not needed without the opamps, the meter will take care of the discharge. And yes, you could look at it as some sort of low-pass. Not sure why you would not be able to detect a cell phone, or it must be that you forgot they don't transmit when there is no call going on? Did you build this with smd components and very short (or better, no) leads? *yes, I'm trying to keep it simple... |
| amoy:
Yes, I tested this with a transmitting cell phone (outgoing call), and I didn't bild this with smd components. I just soldered the components together with very short leads. I was wondering if the low-pass was filtering out higher frequencies, but I'm assuming that's not it either. It may just be that the diodes don't work for frequencies that high (1.9 GHz). |
| PA0PBZ:
Diodes really don't care about frequencies, it's their capacitance that stops them from working. Since these have a capacitance of about 1.5 pF it should not be a problem using them in this circuit for 1.9Ghz, and as you probably noticed the original diagram says something about a 2.4Ghz antenna. What are you using to pick up the signal? |
| amoy:
I'm using a 12cm wire, which is the length of a 2.4 GHz wavelength. I am about to test the circuit with a wifi antenna instead of the wire to check whether it is in fact an antenna issue. |
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