18 Replies Latest reply on Apr 9, 2021 2:37 PM by BigG

    How to build a simple low cost reliable antenna switching circuit

    BigG

      I'm looking to build a simple (MVP: minimal viable product) BLE antenna array circuit to test BLE direction finding (AoA/AoD) functionality.

       

      I'm not wanting precision just yet. So, for my MVP I'm looking at just 3 maybe 4 antennas. For that I will need an RF switch and if I look on any of the major distributor websites I can find plenty single pole 3 or 4 throw (SP3T/SP4T) options.

       

      My question is how to build the input control side, which usually consists of logic table like this one (for a SP4T RF switch):

       

      Input 1Input 2
      Output Mode
      00RF1 on
      01RF2 on
      10RF3 on
      11RF4 on

       

       

      The switching frequency is at the microsecond(s) level.

       

      My BLE chip is a single 3V3 MCU so I don't want to use this MCU to handle the switching as well as the other stuff. It's not a dual core PSoc, for example, where I could've used their hardware timer blocks etc.

       

      So what IC options do I have with a power rail of 3V3.

       

      Is it a case of using a precision Timer IC with some logic gates (e.g. OR, AND, NAND etc.) or would something like a tiny 8-bit MCU do the trick. Basically looking for as few components as possible which is low cost and reliable etc.

       

      Looking to experts for some practical advice.

       

      Thanks

        • Re: How to build a simple low cost reliable antenna switching circuit
          shabaz

          Hi Colin,

           

          By coincidence we're thinking on the same track : )

          I've been looking at a silabs reference design, and aim to work as closely as possible to it, although it will take longer and is a pain due to the component sizes (smaller than I've ever fitted, so I might get it done for me).

          However, generally for RF switches, traditional popular choices are Peregrine Semiconductor and Skyworks, although there are options from others now too, e.g. NXP may have some. They are easy-ish to use, provided a few rules are followed. They mostly work with 3.3V logic. The inputs/outputs would need to be microstrip, that would be easiest, and there are online calculators for it. Some of the Peregrine parts are hand-solderable, and others may be too.

          For things like SP4T as you mention, one technique is to just use SPDT, and arrange one to feed two of them, i.e. a total of three SPDT can make a SP4T. The benefit being that sometimes it is easier to more symmetrically make the paths if you can spread them across separate chips, so that all paths are the same length (if that's what is desired depending on usecase).

          The chips can be controlled by normal logic gates, they are compatible with that, so a microcontroller to directly control them is fine too.

          Other than that, some basic RF prototyping rules can be followed, like complete ground plane, and any top side ground (generally just around the perimeter) must have lots of vias (spaced a couple of millimetres apart at a guess) to the underside ground. You might also want to consider 4-layer, in case it helps with the microstrip, since the width of that is determined by the distance to the groundplane, as you'll see with the calculators or use thinner PCB. Also, boards will warp, so with 2-layer you might want to consider having slightly more ground plane on top again with lots of vias, it's a balancing act. 4-layer and using the closest-to-top layer as ground plane is an option. Personally I'd go with 4-layer for this.

          8 of 8 people found this helpful
          • Re: How to build a simple low cost reliable antenna switching circuit
            Fred27

            I recently needed to test out a NFC reader with multiple antennas. I grabbed a Peregrine Semi evaluation board (EK42441-03). It worked very well and it seems to fit your requirements, including the control logic. The development board wasn't cheap (£83 incl. VAT) but the PE42441 seems more reasonably priced if you're spinning up your own board.

            5 of 5 people found this helpful