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Re: How to build a simple low cost reliable antenna switching circuit
shabaz
Apr 7, 2021 9:10 AM
8 of 8 people found this helpfulHi 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.
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Re: How to build a simple low cost reliable antenna switching circuit
4 of 4 people found this helpfulHi Shabaz
There's a saying "great minds think alike" but I think in my case it's just coincidence
Yes, the Silabs reference design is very handy alright.
I've also come across a reference design from TI.
https://www.ti.com/lit/an/tida029/tida029.pdf
And this project on Hackaday: https://hackaday.io/project/174175-micro-music
My question is very basic and simply highlights my lack of electronics design know how and my uncanny knack of over complicating things.
I'm simply wanting to evaluate or look at different hardware design options to offload the clock timer signal to trigger the RF switch logic as I have this notion that a micro second interrupt timer on my MCU will get in the way of doing other things.
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Re: How to build a simple low cost reliable antenna switching circuit
shabaz
Apr 7, 2021 1:03 PM
4 of 4 people found this helpfulHi Colin,
As I understand (I could be wrong) the BLE stack on the chip is in control of the switching pins, so there's no need for any further consideration, apart from (perhaps) if you need to do any translation in logic to map to the signals required by the switches. If you're maybe aiming to do this yourself in your own code (wouldn't it need to be sync'd with the BLE chip tx or rx somehow? I don't know enough to be sure) then as far as microcontrollers go, as a ballpark, I think a 16MHz clocked processor would take about 1usec to get to a state from a timer interrupt where you have saved off enough registers and are ready to begin performing whatever you needed to do (of course, this is dependant on processor, RISC/CISC etc), so realistically you'd want at least a 48 MHz processor if not more. This is assuming you want to be using C and not assembler. I think I'd be wanting to process all three or 4 measurements within one timer interrupt when it occurs (and then you don't need as speedy a processor or as frequent timer interrupts). But this is still assuming that the BLE stack really cannot do it, so it's worth double-checking.
Finally if you're just looking at repeatedly switching between antennas asynchronously that could be done with logic gates but I don't think this will work, so I guess I've misunderstood the requirement.
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Re: How to build a simple low cost reliable antenna switching circuit
Thanks for the feedback Shabaz.
Once again it pays to ask questions to the community... as I hadn't considered that may be the case... looking at the silabs docs again, the statement "The firmware for direction finding allows users to select antenna parameters such as the antenna array type" now makes sense. I hadn't put two & two together and considered the firmware aspect relating to antenna hardware design.
I think I best read the documentation again.
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Re: How to build a simple low cost reliable antenna switching circuit
3 of 3 people found this helpfulshabaz wrote:
... the BLE stack on the chip is in control of the switching pins,...
Not the exact same use case, but in the ballpark:
On the 2 LoRa boards you gave me, there are two antenna connectors. They are for 433 MHz and the sub-1 GHz operation.
They use PE4259 3 MHz switches.
What's different:
- the frequency is lower than BLE, but it's getting there ..
- the destination are two different pins in the radio IC.
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Re: How to build a simple low cost reliable antenna switching circuit
shabaz
Apr 8, 2021 11:56 AM
Hi Jan,
That looks good, and it's an easy-to-solder package too. It's a currently active part, and low cost.
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Re: How to build a simple low cost reliable antenna switching circuit
I found Infineon RF switch products to be lowest cost but not so sure they're easy to solder (e.g. SPDT is a mere 0.7 x 1.1 mm). Their switch time however looks slow compared to other vendors. Circa. 2us versus say 150ns. Not sure how critical that is for this type of app.
SPDT: https://www.infineon.com/cms/en/product/rf-wireless-control/rf-switches-spxt-dpxt/bgs12p2l6/
SP3T: https://www.infineon.com/cms/en/product/rf-wireless-control/rf-switches-spxt-dpxt/bgs13s4n9/
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Re: How to build a simple low cost reliable antenna switching circuit
shabaz
Apr 9, 2021 7:24 AM
2 of 2 people found this helpfulFrom what I read, the timeslots are of that order (microsec), so 2us parts may be too slow. Some parts are faster in larger packages, e.g. PE4220 has 200nsec typical in MSOP package, so that might have been an option, except it's obsolete and hard to find : (
I've not found a convenient package part. The one that Silabs uses is 0.37mm pitch QFN.
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Re: How to build a simple low cost reliable antenna switching circuit
Thanks for confirming that 2us is probably too slow.
Yes agree, finding larger packages is proving a challenge. I suspect that's because these RF switch IC's are widely used in cellular/mobile phones as literally all are in the 1mm by 1mm size range.
Macom have a good few SPDT switches (such as MASWSS0136) which use the SC-70 package.
Otherwise, I quite like the parts from Qorvo such as the RFSW8008 (SPDT) and the slightly larger (2.0mm x 2.0mm DFN, 8-pin package) RFSW8006Q (SP3T). My thinking is that the SP3T is only fractionally more in price and could still be used as a SPDT.
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Re: How to build a simple low cost reliable antenna switching circuit
shabaz
Apr 9, 2021 12:44 PM
1 of 1 people found this helpfulHi Colin,
Nice find. They do look (especially the Macom one) a lot more convenient to use. Another option to add to the list is PE4251 in MSOP package (0.65 mm pitch). It's relatively expensive ($1.16 in single qty) compared to other options, and only seems to be available at Digikey. It has very good isolation, and only slightly more insertion loss (perhaps not a major issue).
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Re: How to build a simple low cost reliable antenna switching circuit
Thanks for the comment.
I was looking for other products with similar isolation values. It looks like this is what you pay for. So $1.16 is pretty good value, in my opinion. For example, I found another MSOP-8 packaged IC, which is the MASW-008543 from Macom. This is circa. $5 for single qty. Isolation @2.4GHz >60 dB.
EDIT: If you willing to pay more (say around $3) then Analog Devices have a few good ones, which are also at the easier solder size. E.g. HMC284AMS8GE
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Re: How to build a simple low cost reliable antenna switching circuit
3 of 3 people found this helpfulJust to add that when it costs less than $10, including postage, to get some prototype boards made I simply could not help myself...
So I went ahead with a basic design. My limited understanding is that if your RF traces are very short on the PCB then width of trace is not too critical. I went for u.Fl connectors like those used on Hackaday project rather than the more common SMA connector if you're not using pcb trace antennas.
This is a 2 layer board too. My intention was to use a 1mm board thickness as this provides me with a ground plane closer to the traces but it would've cost more so went with 1.6mm.
I'll find out soon enough if this was a waste of $10 (I like to learn by my mistakes and still only know electrical impedance and EMI/EMC theory).
I'm using a Skyworks SP3T on this board. They are still tiny!
https://www.skyworksinc.com/-/media/SkyWorks/Documents/Products/301-400/SKY13317_373LF_200914K.pdf
I'll check out the ones from Peregrine Semiconductor as you suggested as it looks like the Skyworks stock levels are very low.
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Re: How to build a simple low cost reliable antenna switching circuit
shabaz
Apr 7, 2021 1:14 PM
2 of 2 people found this helpfulHi Colin,
Even worst-case if it doesn't work to optimal performance, it's still a handy test board for testing out your code, confirming the switch behaviour, and checking footprints and so on.. definitely makes sense to test things cheaply quickly too.
By the way, for the next prototype iteration, one more benefit of 4-layer, since you're using very small parts, the low-cost manufacturers produce 4-layer boards to slightly tighter specs (I feel/believe from what I've seen so far, but I have no confirmed evidence!).
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Re: How to build a simple low cost reliable antenna switching circuit
Just found this AoA design for an automotive application by TI. Uses just 2 dipole antennas. https://www.ti.com/lit/ug/tidue70a/tidue70a.pdf
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Re: How to build a simple low cost reliable antenna switching circuit
shabaz
Apr 8, 2021 11:49 AM
Nice find, interesting to see that two antennas are sufficient for good results!
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Re: How to build a simple low cost reliable antenna switching circuit
Yes agree, for this application, this design is really good.
Hope my maths is right when using their chart.
A 10 degree error for an object 30 meters away at an angle of 80 degrees to the device equates to a ± 5 metre area of uncertainty (based on 10 degree of error). If object is closer and more centred to the device the accuracy is very good. For example if the object is 3 metres away and within 30 degree of receiver the error distance is equivalent to about 10 cm (based on 2 degree of error).
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Re: How to build a simple low cost reliable antenna switching circuit
shabaz
Apr 8, 2021 6:29 PM
1 of 1 people found this helpfulI only briefly scanned the doc, but if that chart is from their antenna chamber testbed, then that's quite unrealistic conditions too.
From what little I know, even if they can achieve 1 metre of accuracy they are doing extremely well. 2-3 metres of accuracy for 50% of measurements would still be considered good (i.e. many measurements can be taken and averaged, or throw out the outliers).
30 metres is pretty far, and room features/objects might also start impacting a lot (but I don't know for sure) so it might get significantly more untrustworthy at such a distance than what they have in the chart. I'm really speculating though! At such distances probably multiple devices are needed if the use-case required accuracy in those locations.
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Re: How to build a simple low cost reliable antenna switching circuit
5 of 5 people found this helpfulI 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.
