Indication of the increase in the power of electromagnetic radiation - Page 1
Forum section: electronics / beginners

author: bipolunipol
author_rank: Contributor
timestamp: January 28, 2021, 02:39:06 am
content: Hello,I decided to create a hobby device that would be able to measure (relatively, roughly) the increase in the power of electromagnetic radiation in the environment (radiation measurement of 2G / 3G / 4G / 5G phones). I assumed a bandwidth from 100MHz to several GHz.Construction - theory: the system consists of an antenna from which the radio signal is rectified thanks D1. Later, the rectified signal goes to the operational amplifier, where it is amplified. Then there is a comparator where the reaction threshold is set. Finally, a transistor T1 is used, which inverts the signal phase, and a low-pass filter R11 and C9, which smoothes the signal from the comparator (integrator) going to the ADC of the microcontroller.Schematic in attachment.I'd like to ask you a few questions:1. Is the design of the input stage of the circuit correct? I wonder about the correct connection of the detector to the amplifier... Is it correct? I would like the system to respond to the widest possible band, because it is not about a precise spectrum analysis, but a statement that there is a device that radiates nearby with big power (this is supposed to work like a wiretap detector that responds to a wide range of frequencies).2. I plan to connect the output of the circuit to the ADC of the microcontroller. I would like to get the following relationship - increase in radiation intensity = proportional increase in voltage supplied to the ADC of the microcontroller. I would also like to be able to adjust the sensitivity of the circuit with an external potentiometer. Is this idea with comparator, transistor to invert the signal phase and filter OK?Thanks,bu

author: Terry Bites
author_rank: Super Contributor
timestamp: January 28, 2021, 12:33:30 pm
content: The antenna is unspecified, do you anticipate changing it for each band?What is a WCZ?What signal levels are you expecting from the antenna?

author: CaptDon
author_rank: Super Contributor
timestamp: January 28, 2021, 01:51:26 pm
content: The problem I see here is the A.C. coupling to the amplifiers? If the signal is steady state for R.F. level such as an FM modulated signal your a.c. coupling will establish a zero current condition and the signal will appear to have been turned off? An amplitude modulated signal would continue to produce an output waveform demodulated by the diode and coupled through the capacitors. I think you need a better front end.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 28, 2021, 03:13:00 pm
content: http://wilysis.com/networkcellinfo/22-features

author: geggi1
author_rank: Frequent Contributor
timestamp: January 28, 2021, 03:36:42 pm
content: If you want to build this kind of instument I recomend that you look into how a Logamp is working.There are several manufacturers of this type components.Analoge devices got the AD8307 that will work up to about 1Ghz and is garanteed to 500Mhz.Try to google "RF field strenght meter"

author: bipolunipol
author_rank: Contributor
timestamp: February 13, 2021, 03:26:57 am
content: Thanks for the answers,Quote from: Terry Bites on January 28, 2021, 12:33:30 pmNo, I want the antenna to be always the sameW.CZ - I marked the high frequency diodeIt's hard to say, my main purpose is only to indicate the presence of electromagnetic fields. This is my first project in radio electronics. For educational purposes. I need levels that can be amplified later with a cheap amplifier.Quote from: CaptDon on January 28, 2021, 01:51:26 pmThanks, so what solution might be appropriate?Quote from: Terry Bites on January 28, 2021, 03:13:00 pmI've seen it, but prefer to do something from 0Quote from: geggi1 on January 28, 2021, 03:36:42 pmThe AD8318 chip is also interesting. I'll check, thanksBTW, I found such a device on the Internet:https://safelivingtechnologies.com/acousticom-2-rf-detector/. How is it possible that they made them for such a wide frequency band? I can't find a schematic anywhere, but the solution seems interesting

author: pwlps
author_rank: Frequent Contributor
timestamp: February 13, 2021, 06:12:21 pm
content: Your front-end won't work because for small signals coming from the antenna the diode will remain in the linear regime (try to simulate it in LTspice, you will see how ridiculously small the output will be, basically zero), you need an RF preamp with enough gain to make the diode work as a detector. The easiest solution is to use an integrated RF detector (preamp+diodes) like the ADL5506 or MAX2015.

author: bipolunipol
author_rank: Contributor
timestamp: February 13, 2021, 07:08:32 pm
content: Ok, I'll check ADL5506. And what do you think about such a schematic as in the attachment?

author: pwlps
author_rank: Frequent Contributor
timestamp: February 13, 2021, 07:50:57 pm
content: Quote from: bipolunipol on February 13, 2021, 07:08:32 pmI think it's the same, you need a strong enough source or antenna close to the source. My experience with passive rf detectors (basically a load with a Schottky diode) is that sensitivity was very sharply decreasing below -20dBm or so. Here the diodes are biased but I can't tell what the difference will be for small signals. But maybe people who want to test for rf exposure are not concerned with small signals, only the big onesAnyway a log-scale detector like the ADL5506 is a nice solution because it can sense both strong and weak signals.

author: bipolunipol
author_rank: Contributor
timestamp: February 18, 2021, 01:50:16 am
content: I did some research and I found another interesting amp chip with the symbol LMH2110. This is a 45-dB Logarithmic RMS power detector particularly suited for accurate power measurement of modulated RF signals that exhibit large peak-to-average ratios. The device has an RF frequency range from 50 MHz to 8 GHz. Details:https://www.ti.com/lit/ds/symlink/lmh2110.pdfI designed a simple schematic with this chip. The signal is induced in the antenna made on the PCB. I would make the antenna as shown in the attachment. The system amplifies the signal from the antenna, and then the amplified signal goes through the RC filter connected to the ADC input of the ATMEGA8 microcontroller.I've included a schematic in the attachmentIs the idea correct? I've got the biggest doubts about the input circuit, because in the datasheet I have a COUPLER in the input...

author: radiolistener
author_rank: Super Contributor
timestamp: February 18, 2021, 08:43:11 am
content: Quote from: bipolunipol on February 13, 2021, 03:26:57 amThat's the problem, because the antenna sensitivity highly dependents on a frequency.So, the antenna output amplitude will depends on the frequency.Another issue here is a diode. Because it has some open voltage which is not zero. And diode have bad linearity near the open voltage. So, the minimum measurement amplitude of your detector will be limited with a diode open voltage, which is about 1 V.For 50 Ohm impedance 1 V corresponds to 0.01 Watt or 10 dBm power. You will need to put the antenna into a very strong field to get so high power on the output.You can use logarithmic detector instead diode to measure low power RF signals.But also you will need different antenna for different frequency band, otherwise your device will works ok just for one frequency band.

author: radiolistener
author_rank: Super Contributor
timestamp: February 18, 2021, 08:57:00 am
content: Quote from: bipolunipol on February 18, 2021, 01:50:16 am45 dB is too small. You're needs for about 70 dB or more dynamic range to detect and measure different flux density from a low power and high power transmitters.For 0-400 MHz I recommend you to use Chinese logarithmic detector module onAD8310, it has 95 dB dynamic range:https://www.aliexpress.com/item/4000532223547.htmlFor 400 MHz and higher frequency, there is logarithmic detector module onAD8318, it has just 70 dB dynamic range, but can work up to 8 GHz:https://www.aliexpress.com/item/32798085351.htmlThese Chinese modules have not well impedance matched RF input, but it will be good enough for your purposes.The output voltage of logarithmic detector depends on the input amplitude by a logarithmic law. So, you can convert voltage to dBm power. The higher input amplitude leads to lower output voltage.You can attach it to ADC, calibrate it with some known power source and use to measure antenna output power.

author: pwlps
author_rank: Frequent Contributor
timestamp: February 18, 2021, 09:56:35 am
content: Quote from: bipolunipol on February 18, 2021, 01:50:16 amR21 is not needed since the RF input is 50 ohms. (The 50ohm load on the coupler was connected to a different coupler port).The LMH2110 is at least 10dB less sensitive than ADL5506 so it depends how strong signals you want to measure. (For really small signals I would either add a preamp or choose a higher gain device like the AD8318). It is more accurate (true RMS) but this will be meaningless unless you carefully calibrate the antenna in the band of interest. In fact the antenna is the most uncertain component here, you may have resonances giving enhanced sensitivity at some frequencies and almost zero sensitivity at other frequencies. I don't have experience with printed microwave antennas so I can't advice you on that, if you want a broadband antenna covering several bands you might find the relevant info here:https://www.academia.edu/33736544/Broadband_Microstrip_Antenna.

author: bipolunipol
author_rank: Contributor
timestamp: February 19, 2021, 03:31:54 am
content: Quote from: radiolistener on February 18, 2021, 08:43:11 amI know there may be a problem with antenna for different bands. I will make that small PCB antenna for testing - it should work fine for 2.4GHz, then I will check its sensitivity for other frequencies with an analyzerQuote from: radiolistener on February 18, 2021, 08:57:00 amThank you! So I ordered the AD8318 from this link. I'll probably wait a bit for the package with the module. In the meantime I decided to test LMH2110 because I've easy access to it at my local electronics store. I'll check if its sensitivity will actually be too low in my project. I will do a simple prototype layout, I'm designing a PCBQuote from: pwlps on February 18, 2021, 09:56:35 amThank you for the information! I removed R21 and, however, I decided to test LMH2110 while waiting for the AD8318 module. I corrected the schematic and now it looks like in the attachment. Is it OK? I'm wondering about the value of C16 - it probably doesn't matter that much, because at such high frequencies its impedance will approach 0?

author: pwlps
author_rank: Frequent Contributor
timestamp: February 19, 2021, 03:16:12 pm
content: Quote from: bipolunipol on February 19, 2021, 03:31:54 amAgree, but check that the cap is rated for the frequency range used (low inductance, reasonably low esr at high freqs). I think most of standard SMD ceramic caps will be fine. Maybe you don't need as much as 47nF (Z=0.03ohm at 100MHz, even 10 times more would still be small compared to 50 ohm).