RF switch IC selection  - Page 1
Forum section: electronics / projects

author: Asim
author_rank: Regular Contributor
timestamp: August 25, 2015, 03:39:17 pm
content: Hello everyone,I am working on a project where an NFC reader with multiple antennas is needed ( each antenna will be installed in a different place) . I am planning to use the NFC reader from Adafruit PN532http://www.adafruit.com/datasheets/pn532ds.pdf.For the RF switch, I am thinking of using HMC252QS24https://www.hittite.com/content/documents/data_sheet/hmc252qs24.pdf.I came across it while searching through digikey. it's package size makes it easy to solder by hand.My question is: Is it a good selection? any reason this wouldn't work ?Regards,Asim

author: KJDS
author_rank: Super Contributor
timestamp: August 25, 2015, 04:02:00 pm
content: How much isolation do you need and how much insertion loss can you tolerate?

author: rvalente
author_rank: Frequent Contributor
timestamp: August 25, 2015, 04:34:27 pm
content: RF is totally not my field, but I have used minicircuits in the past (for a impedance converter for AM/FM automatic testing for car radios)http://www.minicircuits.com/products/Switches.shtml

author: Neganur
author_rank: Unknown
timestamp: August 25, 2015, 07:08:20 pm
content: HMC252QS24 is so expensive for a single one (~15 EUR), I'd suggest looking for "SP6T" on ebay.You'll easily find something likeeBay auction: #281765034520. Unless of course, it has to be small and not second hand.Your NFC reader is at some 13 MHz so there might be some other solution available. Mind the cable loss. NFC is not a lot of power and not really meant for long passive transmission.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 25, 2015, 07:09:44 pm
content: I am not shure the output of the chips have a well defined impedance. So it might allready be difficult to only have a long cable between the chip and the antenna. Adding a switch might add significant damping to both sending and recieving.The frequency is not that high - so I don't think special HF switches are needed. Also isolation is likely less critical than on resistance.

author: nctnico
author_rank: Super Contributor
timestamp: August 25, 2015, 08:28:57 pm
content: At 13MHz you can use about any CMOS switch. A 74HC4016 for example has a bandwidth of 150MHz (NXP). There are also analog muxes out there.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2015, 10:58:48 pm
content: Yeah, just go with regular CMOS...74HC might be a little weak in the poop for a power signal (you'll lose 3dB easy from switch resistance), but any of the DG403 or similar analog switches will do the job no problem.Tim

author: Asim
author_rank: Regular Contributor
timestamp: August 26, 2015, 03:39:53 am
content: Thanks guys,Using a regular cmos device would be cheaper and easier to find.Tim, the GD403 is SPST, I am looking for sp4t or sp6t . Any suggestions for a high power analog switch. I will keep looking

author: LukeW
author_rank: Frequent Contributor
timestamp: August 26, 2015, 03:59:31 am
content: Note that the PN532 antenna port is balanced, so you'll need a balanced switch, it's not single-ended RF to the antenna.

author: nctnico
author_rank: Super Contributor
timestamp: August 27, 2015, 06:50:52 pm
content: Perhaps an easier way is to buffer the antenna TX signals with a differential output opamp (maybe with enable) and use an analog multiplexer to select between the various receiver signals. OTOH using multiple RFID chips and multiplex them at the MCU side could be a cheaper solution.