RF Frequency monitor - where i need to start? - Page 1
Forum section: electronics / rf-microwave

author: virtapp
author_rank: Contributor
timestamp: March 31, 2021, 11:41:25 am
content: Hi guys,I need to monitor a power rf transmitter that would (in normal conditions) work at 10Mhz carrier.Sometimes, when the devices has some issue (due to hardware issues, overheat, etc...), this initial value will go lower or above the initial 10Mhz carrier...I need to monitor that the device is working at 10Mhz carrier and then use my own logic (using external monitoring master working with RS485) in order to manage the carrier variation and alerting.Does enyone know if it exists something that i can attach between the coax cable antenna and the device that can monitor that?Sorry for the silly question...i'm not an expert in RF

author: Grandchuck
author_rank: Super Contributor
timestamp: March 31, 2021, 12:20:47 pm
content: https://www.minicircuits.com/pdfs/ZX47-40+.pdfThis covers 10MHz but uses SMA connectors.

author: geggi1
author_rank: Frequent Contributor
timestamp: March 31, 2021, 08:55:23 pm
content: What do you want to monitor?Level or frequency?If its the level you can used the device described in the previous post.If its frequency you must use a frequency counter with a tap of some kind. You probably also need some kind of interface to get it converted to 485

author: bob91343
author_rank: Super Contributor
timestamp: March 31, 2021, 10:53:04 pm
content: You will have to put up with interference from WWV, which operates at precisely 10 MHz.

author: virtapp
author_rank: Contributor
timestamp: April 01, 2021, 01:00:26 pm
content: My only need is to monitor the frequency, basically i have to see when the frequency go away from 10Mhz and then do some stuff (alert the maintenance).My question is about the sensor/interface that i need to use...i think that the ZX47-40+ it is not okay: the range of my irradiating device is 1-15Mhz.

author: Grandchuck
author_rank: Super Contributor
timestamp: April 01, 2021, 03:52:47 pm
content: Quote from: virtapp on April 01, 2021, 01:00:26 pmCorrect ... that would serve for amplitude monitoring. I suppose you could use a programmable frequency counter with communication capability.

author: Bud
author_rank: Super Contributor
timestamp: April 01, 2021, 04:26:22 pm
content: Quote from: virtapp on March 31, 2021, 11:41:25 amFirst of all you need a coupling device, e.g. a directional coupler with appropriate power rating. You can insert such device between the trsnsmitter and antenna. RF couplers have small insertion loss not affecting much the thru power. Now, the output of the coupler is a fraction of the incident power and represents a reduced copy of the signal. Then you scale this copy to the level required by your monitoring device using either an amplifier or attenuator. Only then you actually can perform the monitoring task. A crude setup could involve a frequency counter connected to a computer, or a narrow band 10MHz filter followed by a power detector. It depends what capability you have and what the requirements are on portability of the monitoring circuit.

author: rfclown
author_rank: Frequent Contributor
timestamp: April 10, 2021, 12:37:15 am
content: As Bud said, you need two things, a "coupler" to tap a small portion of the signal to measure, and a frequency counter that has an output that you can monitor. Some frequency counters only have a display, but others have the ability to give their output via a digital interface (serial, GPIB, USB, etc).For the coupler, one kind is called a "directional coupler". It taps a portion of the signal going in one dirction. The specifications to look for are frequency (has to work at 10 MHz), power rating, and coupling factor (specified in dB usually). The coupling factor doesn't matter than much, but a 10 dB coupler will tap off 1/10th of your power, whereas a 20 dB coupler will only tap off 1/100th of your power. You don't usually need much to detect the signal. A few that work at 10 MHz are:https://www.minicircuits.com/WebStore/dashboard.html?model=ZABDC50-51HP%2Bhttps://elecraft.com/collections/cp1-model/products/cp1-directional-coupler-kitYou can probably find something cheaper on eBay. search for directional coupler. I posted the elecraft because I have one, and minicircuits because I regularly use their products.Then you need a frequency counter that has an interface. I bought a "FA-2" last year on eBay. It was $130 and it sends it's output on a USB virtual COM port interface. There are much cheaper things available on eBay, but this is what I have. (I also have an HP unit with GPIB interface, but that is more complicated).