IS HIGH SPEED TRACKS ALLOWED UNDERNEATH THE RF ANTENNA TRACKS - Page 1
Forum section: electronics / rf-microwave

author: Anirudh_THABJUL_SB
author_rank: Contributor
timestamp: August 31, 2016, 12:45:16 pm
content: Hello Everyone,I am Anirudh THABJUL, electronic engineer working in a company called "Second-Bridge".We are working with 10 layers PCB where all the RF Components except the Matching network & Antennas are on the TOP layer and Matching network & antennas are on the Bottom layer connected from Top layer to Bottom layer by one Via (Refer the first attachment).In our application, We have to measure distance between two PCB, So when we measure distance, We are able to measure max of 90M with tranceiver section active alone but When we Switch-On our main processor measuring is reducing to 40M max distance (Refer the second attachment).So, We suppose there is a flaw in the layout design. We have went through main documents, It's written that high speed tracks should not come underneath the RF section and we have to maintain solid ground plan. Is it so? if yes we don't have any high speed tracks underneath TOP layer i.e. INNER LAYER 1 but we have DDRAM tracks underneath BOTTOM layer i.e. INNER LAYER 8.Does the Matching network and antenna are also considered as major RF components?Is this rule applicable rule bottom layer also like top layer?Please help to understand this better.Thank you.Regards,Anirudh

author: Ice-Tea
author_rank: Super Contributor
timestamp: August 31, 2016, 01:41:46 pm
content: Are you saying you have DRAM tracks directly above the matching components? Or is there a solid GND plane between it?

author: Anirudh_THABJUL_SB
author_rank: Contributor
timestamp: August 31, 2016, 02:48:27 pm
content: Quote from: Ice-Tea on August 31, 2016, 01:41:46 pmThank you for the reply.I mean we have DRAM tracks directly underneath the Matching circuit and Antenna Tracks layer i.e. INNNER LAYER 1 is DDRAM and TOP LAYER is Matching Network and Antenna.Regards,Anirudh

author: Ice-Tea
author_rank: Super Contributor
timestamp: August 31, 2016, 03:03:50 pm
content: Not sure I understand. Maybe you should post your stackup. But if you have DDR tracks below your RF Without a solid GND as a shield in between (how doesthathappen on a 10layer board???) then that's pretty bad...

author: Anirudh_THABJUL_SB
author_rank: Contributor
timestamp: August 31, 2016, 03:11:52 pm
content: Quote from: Ice-Tea on August 31, 2016, 03:03:50 pmWe have many other modules like NAND, SENSORS, WIFI etc... routes in other layers. So we if we have DDR tracks just below RF tracks it is an issue. I will try to avoid this mistake in our coming boards.Thank you for your help.

author: dmills
author_rank: Super Contributor
timestamp: September 01, 2016, 03:46:15 pm
content: Also if coupling RF between layers, you will want at least one stitching via between the reference planes somewhere nearby.I am surprised that it worked at all with a memory bus between the ground plane and the RF routing layers, sounds like noise city to me..Usual stackup for this kind of thing would have a ground plane on at least layer 2 and 9 (With plentiful stitching vias).How did you control the impedance of the RF nets without a reference plane?Regards, Dan.

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 01, 2016, 05:19:13 pm
content: Quote from: Anirudh_THABJUL_SB on August 31, 2016, 03:11:52 pmPlease take a little bit of advice, meant only in the best possible way... If you are implementing RF and the fact that you should not be running a digital bus right below any part of that RF section is news to you you should seek assistance. As an example: as dmills mentioned, aside from the noice injected in the antenna, how are you matching your antenna if you do not have a reference plane? This is all pretty basic stuff. If your company has failed to pick up on things like that, I can only imagine the state of the rest of your board.If this is a commercial project: get someone to review your schematics and layout.ASAP.