[solved] How to attenuate TX but not RX of a transceiver?[solved]  - Page 1
Forum section: electronics / rf-microwave

author: rfspezi
author_rank: Regular Contributor
timestamp: February 27, 2019, 09:44:55 am
content: I need to attenuate the TX-power of a transceiver by 15 dB (can't go lower in the device settings).However, the RX sensitivity must remain the same.So i need some kind of "directional" attenuator.My thought was to use two circulators and an attenuator.However i guess there is a better solution to this that i don't know?Thanks

author: OwO
author_rank: Super Contributor
timestamp: February 27, 2019, 10:52:18 am
content: One circulator will do:Code:antenna -------- (0) --------- transceiver||resistor to gnd(assuming clockwise circulator)The resistor should be selected to have a return loss of -15dB.Circulators are not expensive, I can get new 800-950MHz 50W ones for $3.

author: rfspezi
author_rank: Regular Contributor
timestamp: February 27, 2019, 11:51:46 am
content: Thanks OwOThat solution is simply perfect

author: OwO
author_rank: Super Contributor
timestamp: February 27, 2019, 12:25:14 pm
content: Quote from: ogden on February 27, 2019, 12:05:11 pmhttps://item.taobao.com/item.htm?spm=a230r.1.14.32.68e447e1nMmCSd&id=40158399867&ns=1&abbucket=9#detailThis is the one I've bought before, it's 860-960 MHz, but it's an isolator (port 3 has a built in attenuator that can't be removed). I think you can find similar ones if you search "hyg 环形器" on taobao, and there are some datasheets here:http://www.htd-rf.com/product/"射频环形器" (rf circulator) also turns up some results. The difficulty is figuring out which prices are real and which ones you have to contact the seller and ask for the real price.

author: rfspezi
author_rank: Regular Contributor
timestamp: February 27, 2019, 01:39:40 pm
content: At mouser i found one for about 14,-€However it seems not to be easy to find circulators at the usual distributors.

author: Velund
author_rank: Regular Contributor
timestamp: February 27, 2019, 08:11:19 pm
content: Hey, guys! Topic starter is not indicated frequency band, but everyone started to discuss dirt-cheap cellular circulators that is scrapped by thousands from decomissioned cellular base stations.What if they need something in VHF range?It will be really expensive device, and not so easy to obtain.Some time ago I even thinked to look for some inexpensive used transceiver with blown PA to throw PA board away completely and put a pair of connectors for separate RX and TX paths (with millliwatt range output power) for transverters. It may be cheaper than decent VHF circulator, of course.

author: rfspezi
author_rank: Regular Contributor
timestamp: February 28, 2019, 07:48:02 am
content: The application is in the 5 GHz Wifi band and the "directional attenuation" is needed because the minimum TX-power of the transceiver is still too high.The "interface" to the transceiver is the pure coax cable - so no chance to extract a TX/RX signal for controling an external attenuator.I guess the circulator with a -15dB RL resistor (approx. 75 Ohms?) solution is the best way to go without having to design an active circuit.EDIT:By the way... an isolator with approx. 15 dB of isolation would do the same, right? (except that i have no control for trimming at all)

author: Bud
author_rank: Super Contributor
timestamp: February 28, 2019, 06:18:51 pm
content: Are you looking to kill 96% of TX power?

author: rfspezi
author_rank: Regular Contributor
timestamp: February 28, 2019, 06:43:57 pm
content: Quote from: Bud on February 28, 2019, 06:18:51 pmYes, -15dB of TX and as little as possible of incoming RX signal.

author: rfspezi
author_rank: Regular Contributor
timestamp: February 28, 2019, 07:10:20 pm
content: Quote from: ogden on February 28, 2019, 07:06:25 pmI just can say it is a very special application for measurement.However, i think i can close this thread since the perfect solution was already posted by Owo.

author: rfspezi
author_rank: Regular Contributor
timestamp: February 28, 2019, 07:36:16 pm
content: Funny... i feel like people enjoymore than presenting a solution.

author: ogden
author_rank: Super Contributor
timestamp: February 28, 2019, 09:24:37 pm
content: Quote from: rfspezi on February 28, 2019, 07:36:16 pmFunny... i feel like you have short memory. Whatever. Enjoy solution you have, hopefully it works for you as well. I make sure to not waste any further second of my time to ungrateful like you

author: mikerj
author_rank: Super Contributor
timestamp: March 01, 2019, 11:04:41 am
content: Quote from: ogden on February 28, 2019, 09:24:37 pmQuestioning someones requirements that have already been very clearly stated, and which are not typical XY problems, is wasting their time as much as your own.

author: ogden
author_rank: Super Contributor
timestamp: March 01, 2019, 11:22:48 am
content: Quote from: mikerj on March 01, 2019, 11:04:41 amNo RF measurements I know (and I know quite enough) requires asymmetric attenuators. Very often people ask solutions to imaginary problems or try to solve "problem" in wrong place or using wrong approach. - That's why often it is good idea to tell actual problem, not just requirements for solution.