RF Detector Help - Page 1
Forum section: electronics / beginners

author: amoy
author_rank: Newbie
timestamp: April 02, 2012, 06:26:41 pm
content: 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 signals2. 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.

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 02, 2012, 07:06:46 pm
content: 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...

author: amoy
author_rank: Newbie
timestamp: April 02, 2012, 07:48:21 pm
content: 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).

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 02, 2012, 07:56:57 pm
content: 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?

author: amoy
author_rank: Newbie
timestamp: April 02, 2012, 08:49:42 pm
content: 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.

author: w2aew
author_rank: Super Contributor
timestamp: April 02, 2012, 09:16:06 pm
content: Most cellphone transmissions are bursty in nature, not continuous like your walkie talkie. This makes them harder to measure with a FSM because it won't respond that fast. You might also try increasing the value of the 51 ohm resistor (or even temporarily removing it) for increased sensitivity.

author: gregariz
author_rank: Frequent Contributor
timestamp: April 02, 2012, 09:28:35 pm
content: Quote from: amoy on April 02, 2012, 06:26:41 pm1. The circuit is just an AM detector. the extra diode to ground may not be needed, but it is recommended because there will be RF on the ground.2. Same as an AM detector RC is a low pass filter - where usually you want to ride the envelope however you have the option to extend the RC here to ride a CW3. Walkie Talkies are probably FM, so this detector will detect the amplitude component. Cell phones are digital modulation depending on your network, the average amplitude will be so low you wont see it.

author: M84AB1
author_rank: Contributor
timestamp: December 07, 2018, 04:51:51 am
content: Reviving an old thread here but I was equally unsuccessful (similar circuit, same diode 1n5711) to pick up signals from cell phones (iphone 6s). Walkie-Talkies (466Mhz) were no problem, neither were wireless home phones (40Mhz). I tried different capacitor values but the needle on my DC Microamp (50uA range) is not moving at all. I tried amplifying the rectified signal with an CA3130 op-amp (having trimmed the offset voltage to 0) but that produced nothing either. I have ordered a few bat62 diodes and will see how that goes.Also, from the reading that I have done it appears that cell phones use a form of frequency shift keying and should thus have a continuous (very much like FM) rf signal output. I would therefore expect the rf detector to pick that up, provided the diodes are up to the task.