How to check for a RF output? - Page 1
Forum section: electronics / repair

author: uslrs
author_rank: Contributor
timestamp: November 24, 2019, 11:12:47 am
content: Hi, I would really appreciate some help with this issue - I have a number of PCB's with damaged RJ sockets/battery carriers, etc which are a relatively straight forward to repair. These boards communicate with a base unit to identify location and alarm situations and as far as I can make out, this is accomplished using an Si 4431 chip, which according to the data sheet, can be programmed to operate in the 240-930 MHz range. The layout on the board looks similar to the example schematic in the data sheet (attached). I am looking for a (simple?) way to identify there's a RF output, so I'm not wasting time and money on pointless repairs but I'm not trying to interpret or capture the output in any way. I have a 60MHz scope. logic analyser and the usual range of multimeters, etc, but I'm struggling to think of a quick and simple go/no-go test. The board has an aerial with what appears to be a test point but I can't see anything on the scope - I've shown the connection point to the Si4431 in red - Any ideas?

author: Miti
author_rank: Super Contributor
timestamp: November 24, 2019, 11:41:18 am
content: You could buy an RF Explorer spec an and diy a passive rf probe. They are quite cheap and, in this case, it would do the job.Edit: On second thought, reading the datasheet, I see that the chip supports frequency hopping so it may be hard to catch with a spec an. Maybe use one of those cheap power meters from ebay.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 24, 2019, 12:03:42 pm
content: Does the board include the microcontroller? The chip will not do anything unless instructed to do so.

author: Muttley Snickers
author_rank: Unknown
timestamp: November 24, 2019, 12:13:27 pm
content: Quote from: uslrs on November 24, 2019, 11:12:47 amMany wireless security system devices were designed to transmit their alarm conditions not more than once every minute to conserve the batteries. Some systems required the battery cover to be removed or a jumper link to be enabled before the device could be enrolled or registered into the main control panel or receiver, this process would also enable longer transmission times which is what you may require to verify their operation.

author: uslrs
author_rank: Contributor
timestamp: November 24, 2019, 12:17:27 pm
content: Quote from: Miti on November 24, 2019, 11:41:18 amThanks Miti - I've found this one on Ebay: RF Power Meter Digital Radio Frequency Attenuation Value -75~+16 dBm DetectionIs that what you're suggesting? They appear to operate in the range 1-600 MHz, so not sure if that's adequate?

author: uslrs
author_rank: Contributor
timestamp: November 24, 2019, 12:22:44 pm
content: Quote from: PA0PBZ on November 24, 2019, 12:03:42 pmYes, the board has an ATXMEGA32A4U mpu connected to the Si4431 on pins 16 & 17 as in the datasheet example.

author: uslrs
author_rank: Contributor
timestamp: November 24, 2019, 12:29:31 pm
content: Quote from: Muttley Snickers on November 24, 2019, 12:13:27 pmThanks Muttley - there does appear to be a delay/repeat capacity, but there's no jumper/link/switch on the board - the alarm/call buttons are used in sequence to program the location and ID for each unit, but I don't think there is any associated inhibit function .

author: Muttley Snickers
author_rank: Unknown
timestamp: November 24, 2019, 12:41:55 pm
content: I probably could have explained that last statement a bit better but what I was trying to relay is that generally speaking most wireless security devices do not transmit continuously and short periodic bursts can be difficult to identify, handheld remote keyfobs on the other hand can transmit at will.

author: BFX
author_rank: Frequent Contributor
timestamp: November 24, 2019, 01:07:27 pm
content: Cheap SDR will do the job.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 24, 2019, 01:25:20 pm
content: Look up the FCC ID to figure out what frequency it operates at, then use a RTLSDR.

author: uslrs
author_rank: Contributor
timestamp: November 24, 2019, 02:16:26 pm
content: Quote from: BFX on November 24, 2019, 01:07:27 pmPlease excuse my ignorance - is this (SDR) a software-defined radio? If so, could you point me to an example. Many thanks.

author: uslrs
author_rank: Contributor
timestamp: November 24, 2019, 02:21:25 pm
content: Quote from: NiHaoMike on November 24, 2019, 01:25:20 pmI can't find anything relating to a FCC ID and would this be appropriate? RTL2832U DVB-T Tuner Dongles

author: David Hess
author_rank: Super Contributor
timestamp: November 24, 2019, 09:28:18 pm
content: Before spending a lot of money, I would hack together a simple RF probe for my multimeter using some small signal schottky or germanium diodes.https://www.electroschematics.com/rf-probe-circuit/

author: uslrs
author_rank: Contributor
timestamp: November 25, 2019, 09:48:14 am
content: Quote from: David Hess on November 24, 2019, 09:28:18 pmMany thanks David - this looks perfect for my requirements!!

author: uslrs
author_rank: Contributor
timestamp: November 25, 2019, 04:23:35 pm
content: Quote from: David Hess on November 24, 2019, 09:28:18 pmI've made the RF probe and am very happy to report it works like a dream! Had all the components and took no time at all to put together, so well pleased. Thanks again David.