gimmicky field strength meter - Page 1
Forum section: electronics / rf-microwave

author: djacobow
author_rank: Super Contributor
timestamp: April 30, 2019, 12:58:40 am
content: Years ago I saw some toys that lit up when a cell phone was in use nearby. I figured these were just an antenna tuned for cell phone frequencies and a simple diode detector, comparator, LEDs.Is there more to it? I'm thinking something that could pick up a cell phone transmitting maybe two meters away.What sorts of diodes would be appropriate for 1700/1900 MHz or whatever the kids are using these days?

author: coppercone2
author_rank: Super Contributor
timestamp: April 30, 2019, 01:58:42 am
content: is this like a starter GI-JOE kit for signals espionage golf-of-tonkien boat? get your kids started with spy crafts (8 years and above?)

author: djacobow
author_rank: Super Contributor
timestamp: April 30, 2019, 02:23:14 am
content: Yeah, basically, but for grownups. Kind of as a party trick, but the espionage angle could be fun, too.I distinctly remember one was a car air freshener that also lit up when you made a call. This would have been late 90s / early 00s.!

author: radiolistener
author_rank: Super Contributor
timestamp: April 30, 2019, 02:46:29 am
content: You can use this module:https://www.aliexpress.com/item/1-8000MHz-AD8318-RF-Logarithmic-Detector-70dB-RSSI-Measuring-Power-Meter-Module-W-store-Jan11/32966621495.htmlIt is much sensitive and better than diodes.Just connect some GSM antenna to SMA connector, 7-15V power supply to GND/Vcc and multimeter to OUT.Smaller voltage on the output = stronger EMF fields.You can even calculate power in dBm:power in dBm = -15.443 * x^3 + 49.478 * x^2 - 93.55 * x + 44.116where x = voltage on the OUT pins.This is simplified formula, it gives approximation value.It works well for +5...-40 dBm range, if you're needs more precise or wide range, you're needs to calibrate it.

author: djacobow
author_rank: Super Contributor
timestamp: April 30, 2019, 02:59:18 am
content: That's a cool module, but a purpose designed chip that is actually capable of giving calibrated output is not in the spirit of what i am after.Let's say i want to make s few dozen of these for friends.

author: David Hess
author_rank: Super Contributor
timestamp: April 30, 2019, 03:11:03 am
content: Quote from: djacobow on April 30, 2019, 12:58:40 amSmall signal schottky diodes are typically used today. These are the same parts used for microwave mixers.Small signal transistor base-emitter junctions can be used as microwave silicon diodes.I have used 1N270 germanium diodes up to 1.2 GHz but their performance has significantly dropped off by then. They worked better than 1N4148 switching diodes.

author: djacobow
author_rank: Super Contributor
timestamp: April 30, 2019, 03:09:01 pm
content: Infineon has an app note on microwave schottky diode detectors, and they used a BAT15-04W in their two diode example. Sot-23 part has both diodes in it and costs $0.60 qty 1, so that's appealing.Now to design a suitable PCB antenna...

author: radiolistener
author_rank: Super Contributor
timestamp: April 30, 2019, 07:39:50 pm
content: try this one:https://www.banggood.com/GSM-Mobile-Phone-Signal-Flash-LED-Kit-Radiation-Power-Supply-DIY-Kit-NO-Need-Power-ICSK032A-p-1167499.html

author: xaxaxa
author_rank: Regular Contributor
timestamp: May 01, 2019, 08:11:55 am
content: bat17, but you probably also want to amplify (and filter) the signal first; to improve sensitivity you can also bias the diode just before conduction.

author: biastee
author_rank: Regular Contributor
timestamp: May 03, 2019, 05:46:56 am
content: Quote from: David Hess on April 30, 2019, 03:11:03 amIf given a choice between medium barrier (n-type) Schottky intended for mixer and low barrier (p-type) intended for zero-bias detector, go for the latter because it will result in a more sensitive detector.Quote from: djacobow on April 30, 2019, 12:58:40 amThe HSMS-2852 zero bias Schottky in the voltage doubler configuration can produce ~ 7mV DC for an input signal of 1.8 GHz -40 dBm (see fig. 8 and 11 athttps://sites.google.com/site/randomwok/Home/electronic-projects/cellphone-radiation-detector). For best sensitivity, connect the detector's output to a low-offset (< 2 mV) comparator such as LM393A.

author: OwO
author_rank: Super Contributor
timestamp: May 03, 2019, 06:19:12 am
content: If the application is powered then why not just bias the diode instead of using an ex🅱ensive low forward drop diode?The most sensitive configuration is simply to forward bias the diode with a tiny current through a large resistor, ac couple the RF to the diode, then detect the dc voltage on the diode (by a r-c lowpass filter followed by comparator). Use another identical diode with slightly less bias as the reference voltage for the comparator.

author: biastee
author_rank: Regular Contributor
timestamp: May 04, 2019, 03:15:00 am
content: @OwO. Thank you for counter-suggesting the biased alternative. Honestly, I was remiss in jumping straight into the zero-bias detector (ZBD) without comparing it with the biased alternative. :-) Retrospectively, some reasons (read: excuses) that I can summon up in support of ZBD:1. In a one-off hobby project, paying half a dollar more for a ZBD may be a worthy trade-off for a simpler circuit.2. The cost difference may be actually smaller than portrayed by the diodes' prices. The biased diode has a static forward voltage that must be removed by comparing it with a matched reference diode. The following extra components are needed: biasing resistors, reference diode/s, and RF shielding for the reference diode.3. A biased detector appropriate for 1.8 GHz is a hybrid diode (e.g. HSMS-282x). The hybrid construction has about 4x more junction cap than the ZBD, degrading the former's sensitivity.

author: OwO
author_rank: Super Contributor
timestamp: May 04, 2019, 03:32:53 am
content: I tried looking up prices but the HSMS-2852 is listed as obsolete everywhere, and on broadcom's website it seems every schottky diode they used to sell are discontinued. Do you know of any similar parts?

author: dcarr
author_rank: Regular Contributor
timestamp: May 04, 2019, 07:14:30 am
content: Check out Skyworks---SMS7621 might work. Still very disappointing though to see all those parts go away...

author: djacobow
author_rank: Super Contributor
timestamp: May 04, 2019, 04:09:17 pm
content: Upthread, i mention the Infineon BAT15-04W, which seems similar to the HSMS-2852.

author: David Hess
author_rank: Super Contributor
timestamp: May 04, 2019, 10:50:29 pm
content: Quote from: OwO on May 04, 2019, 03:32:53 amAvago's discrete microwave parts were all discontinued after they became Broadcom which is a shame since they were the best documented.

author: biastee
author_rank: Regular Contributor
timestamp: May 09, 2019, 08:54:59 am
content: Quote from: David Hess on May 04, 2019, 10:50:29 pmNot only the discrete parts, but the MMICs (MGAs & MSAs, etc) and RFICs (ALMs & AMMPs, etc) are all gone. Only semi-custom cellphone parts such as filters, front-end modules and PAs survived the cut.

author: David Hess
author_rank: Super Contributor
timestamp: May 09, 2019, 05:05:34 pm
content: Quote from: biastee on May 09, 2019, 08:54:59 amIt is too bad that some other company did not buy the production facilities to make them under their own name.

author: biastee
author_rank: Regular Contributor
timestamp: May 14, 2019, 08:40:16 am
content: Quote from: djacobow on May 04, 2019, 04:09:17 pmOut of curiosity, I simulated a voltage doubling detector using the BAT15-04W. The diode SPICE model, which I hope is accurate, was downloaded from Infineon website. Coming back to the simulation, two parameters caught my attention:1. the BAT15-04W is way more sensitive (good) than the HSMS-2852.2. the BAT15-04W detector is incredibly narrow band compared to the HSMS-2852 (0.2% vs. 10% of centre frequency at the -10 dB return loss points). What this means is that the impedance matching components will likely require fine-tuning (difficult at 1.8 GHz). I anticipate this will be a deal-breaker.Hope this is of help to you. I will be grateful if someone can shed light on which diode parameter is responsible for the above differences. And whether the differences could have been anticipated by comparing the datasheets?

author: djacobow
author_rank: Super Contributor
timestamp: May 14, 2019, 06:06:26 pm
content: So I designed a board with two copies of a simple detector with the BAT15-04W and 0805 components. One with a PCB meandery 1/4 wave antenna and the other with my attempt at a PIFA using formulas gleaned from YT videos. I regarded this as a bit of shot in the dark, because I don't have an appropriate signal generator or network analyzer to see what's going on.It didn't work very well. I have at least two problems: the antennas and filter networks not being tuned properly and the comparator inputs to my uC having strangely low impedance, like ~400 kOhm @DC. I know it's not the uC itself, whose inputs are really like open circuits at DC. I think it must be something ohmic on the board, maybe flux residue -- though I cleaned it as best as I could.Anyway, I'm thinking about if/how to proceed given a limited budget.

author: coppercone2
author_rank: Super Contributor
timestamp: May 15, 2019, 12:47:25 am
content: I wonder how small you can make em, if you can make a mass produced one that runs on the smallest coincell as a passive detector to scatter around perhaps with magnets if you need to quickly secure a room. They would communicate a ping to a master unit if they detected something i.e bug

author: djacobow
author_rank: Super Contributor
timestamp: May 15, 2019, 04:55:03 pm
content: Quote from: coppercone2 on May 15, 2019, 12:47:25 amYou can make them quite small. The largest components will be the antenna and the battery, by far. A coin cell could definitely power such a thing. The antenna could be a coil of wire sticking out or it could be something on the PCB. In the latter case, that would take up some space. Half of a 1/4 wave dipole at 1700 MHz is ~4.4 cm.