linnix wrote:> On Oct 27, 11:21 am, Tim Wescott <t...@seemywebsite.please> wrote: >> On Sat, 27 Oct 2012 10:59:34 -0700, linnix wrote: >>> On Oct 27, 10:41 am, "langw...@fonz.dk" <langw...@fonz.dk> wrote: >>>> On 27 Okt., 19:22, linnix <m...@linnix.info-for.us> wrote: >>>>> We are using the Microchip MRF24JA40 802.15.4 RF transceiver, >>>>> might look into Atmel's AT86RF231 and Freescale's MC13202 as well. >>>>> We have one working board talking to a Microchip pre-made module, >>>>> but two non-working boards. The only difference is the crystal. The >>>>> working one is 5 mm SMD and the non-working one is 11 mm 2 pins. >>>>> Could it be so sensitive to crystal selection? Do we need to >>>>> pre-screen them for productions? >>>> a different crystal will probably need different capacitors to be spot >>>> on frequency. Try measuring the frequency, 802.15.4 requires something >>>> like +/-40ppm >>> The non-working one says 20ppm, not sure if it's stability or tolerence. >>> However, the cheap stuff (10 for $1.95 including shipping from Hong >>> Kong) might be fake. The working one (80 cents + $3 shipping from >>> digikey) is actually 30ppm. Will try to order more test crystals later. >>>>> The Freescale chip allows crystal calibration, but won't solve drift >>>>> problem. My non-working board detects signals occasionally, but not >>>>> good enough to pull in any packet. It seems to be drifting in and >>>>> out of the channel. >>>> I believe if you get the caps right wor the crystal you should able to >>>> stay with in +/40ppm over temperature and aging >>>>> Also, the 1/4 length for 2.5GHz is 3cm. Most WiFI antenna seems to >>>>> be bigger. Are they 3/4 length? Would 1-1/4 be better? We are just >>>>> using a wire coiled on top of the PCB. >>>> all kinds of different antennas as long as it is matched I'd assume >>>> what ever their put on give an acceptable performance >>>> but if devices are close I don't think it matters much >>>> I worked on developing the freescale 802.15.4 stuff but that was many >>>> years ago >>>> -Lasse >>> I have a 9cm wire talking to the Microchip's PCB E antenna. My signal >>> (on the devices) is much weaker than the Microchip module (coordinator), >>> but seems to be OK. >> Pay attention to parasitic capacitances on the PCB, too. If you've got >> ground plane underneath your crystal lands, then you've got capacitors, >> and those capacitors' temperature behavior is no better than the PCB >> material's. > > Yes, will have better layout for next version. > >> See if your manufacturer of choice has any app notes on layout for good >> oscillator stability. > > They suggested 4 layers with 4 ground plane (Digital, Analog, Crystal > and RF). ...Four (!) different ground planes? That is usually a recipe for disaster.> ... But that would be too expensive. I am cutting corners with > 2 layers. Perhaps i am just lucky with the one working board.As Lasse wrote, check for correct burden caps. They may very well have to be different between crystal versions. If the burden caps are wrong oscillation start may be recalcitrant and your frequency can we off. Easy way to check if you don't have a good frequency counter: Program a timer to spit out 1MHz. Then borrow a shortwave receiver, pull in WWV or WWVB or 5, 10 or 15MHz, listen to the beat. If the beat is in the kHz range you won't be a happy camper with channel lock up at 2.45GHz. -- Regards, Joerg http://www.analogconsultants.com/
802.15.4 RF issues
Started by ●October 27, 2012
Reply by ●October 27, 20122012-10-27
Reply by ●October 27, 20122012-10-27
On Sat, 27 Oct 2012 10:22:23 -0700 (PDT), linnix <me@linnix.info-for.us> wrote:>Also, the 1/4 length for 2.5GHz is 3cm. Most WiFI antenna seems to be >bigger. Are they 3/4 length? Would 1-1/4 be better? We are just >using a wire coiled on top of the PCB.Others have addressed the crystal issue, so I'll do the antenna. Designer�s Guide to LPRF <http://www.ti.com/lit/sg/slya020a/slya020a.pdf> See Pg 39 for links to specifics on the various antenna styles. Antenna Selection, quick reference guide <http://www.ti.com/lit/an/swra351/swra351.pdf> Antenna Selection Guide (new version) <http://www.ti.com/lit/an/swra161b/swra161b.pdf> Antenna Selection Guide (old version) <http://www.prochild.co.kr/board/files/tb_3/AN058%20Antenna%20Selection%20Guide%20swra161.pdf> Selecting antennas for low-power wireless applications <http://www.ti.com/lit/an/slyt296/slyt296.pdf> When you disclose what you're trying to accomplish, and what limitations are involved (size, cost, placement, aesthetics, range, data rate, proximity to metals, etc), perhaps I can offer some clues as to which would be "better". Incidentally, your wire coiled on top of the PCB might work if you're lucky, but a well controlled and properly modeled antenna system would probably be more consistent and more reliable. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Reply by ●October 27, 20122012-10-27
"linnix" <me@linnix.info-for.us> wrote in message news:b7f16eac-a877-4609-b2a5-e011d240fde6@b9g2000pba.googlegroups.com...> On Oct 27, 10:41 am, "langw...@fonz.dk" <langw...@fonz.dk> wrote: >> On 27 Okt., 19:22, linnix <m...@linnix.info-for.us> wrote: >> >> > We are using the Microchip MRF24JA40 802.15.4 RF transceiver, >> > might look into Atmel's AT86RF231 and Freescale's MC13202 as well. >> > We >> > have one working board talking to a Microchip pre-made module, but >> > two >> > non-working boards. The only difference is the crystal. The >> > working >> > one is 5 mm SMD and the non-working one is 11 mm 2 pins. Could it >> > be >> > so sensitive to crystal selection? Do we need to pre-screen them >> > for >> > productions? >> >> a different crystal will probably need different capacitors to be >> spot >> on frequency. Try measuring the frequency, 802.15.4 requires >> something >> like +/-40ppm > > The non-working one says 20ppm, not sure if it's stability or > tolerence. However, the cheap stuff (10 for $1.95 including shipping > from Hong Kong) might be fake. The working one (80 cents + $3 > shipping from digikey) is actually 30ppm. Will try to order more test > crystals later. > >> >> > The Freescale chip allows crystal calibration, but won't solve >> > drift >> > problem. My non-working board detects signals occasionally, but >> > not >> > good enough to pull in any packet. It seems to be drifting in and >> > out >> > of the channel. >> >> I believe if you get the caps right wor the crystal you should able >> to >> stay with in +/40ppm over temperature and aging >> >> >> >> > Also, the 1/4 length for 2.5GHz is 3cm. Most WiFI antenna seems to >> > be >> > bigger. Are they 3/4 length? Would 1-1/4 be better? We are just >> > using a wire coiled on top of the PCB. >> >> all kinds of different antennas as long as it is matched I'd assume >> what >> ever their put on give an acceptable performance >> >> but if devices are close I don't think it matters much >> >> I worked on developing the freescale 802.15.4 stuff but that was many >> years ago >> >> -Lasse > > I have a 9cm wire talking to the Microchip's PCB E antenna. My signal > (on the devices) is much weaker than the Microchip module > (coordinator), but seems to be OK.Make sure the board is cleaned properly. The No clean flux can cause problems for crystals, especially under the crystal. Cheers
Reply by ●October 28, 20122012-10-28
On Saturday, October 27, 2012 1:14:50 PM UTC-7, Joerg wrote:> linnix wrote: > > > On Oct 27, 11:21 am, Tim Wescott <t...@seemywebsite.please> wrote: > > >> On Sat, 27 Oct 2012 10:59:34 -0700, linnix wrote: > > >>> On Oct 27, 10:41 am, "langw...@fonz.dk" <langw...@fonz.dk> wrote: > > >>>> On 27 Okt., 19:22, linnix <m...@linnix.info-for.us> wrote: > > >>>>> We are using the Microchip MRF24JA40 802.15.4 RF transceiver, > > >>>>> might look into Atmel's AT86RF231 and Freescale's MC13202 as well. > > >>>>> We have one working board talking to a Microchip pre-made module, > > >>>>> but two non-working boards. The only difference is the crystal. The > > >>>>> working one is 5 mm SMD and the non-working one is 11 mm 2 pins. > > >>>>> Could it be so sensitive to crystal selection? Do we need to > > >>>>> pre-screen them for productions? > > >>>> a different crystal will probably need different capacitors to be spot > > >>>> on frequency. Try measuring the frequency, 802.15.4 requires something > > >>>> like +/-40ppm > > >>> The non-working one says 20ppm, not sure if it's stability or tolerence. > > >>> However, the cheap stuff (10 for $1.95 including shipping from Hong > > >>> Kong) might be fake. The working one (80 cents + $3 shipping from > > >>> digikey) is actually 30ppm. Will try to order more test crystals later. > > >>>>> The Freescale chip allows crystal calibration, but won't solve drift > > >>>>> problem. My non-working board detects signals occasionally, but not > > >>>>> good enough to pull in any packet. It seems to be drifting in and > > >>>>> out of the channel. > > >>>> I believe if you get the caps right wor the crystal you should able to > > >>>> stay with in +/40ppm over temperature and aging > > >>>>> Also, the 1/4 length for 2.5GHz is 3cm. Most WiFI antenna seems to > > >>>>> be bigger. Are they 3/4 length? Would 1-1/4 be better? We are just > > >>>>> using a wire coiled on top of the PCB. > > >>>> all kinds of different antennas as long as it is matched I'd assume > > >>>> what ever their put on give an acceptable performance > > >>>> but if devices are close I don't think it matters much > > >>>> I worked on developing the freescale 802.15.4 stuff but that was many > > >>>> years ago > > >>>> -Lasse > > >>> I have a 9cm wire talking to the Microchip's PCB E antenna. My signal > > >>> (on the devices) is much weaker than the Microchip module (coordinator), > > >>> but seems to be OK. > > >> Pay attention to parasitic capacitances on the PCB, too. If you've got > > >> ground plane underneath your crystal lands, then you've got capacitors, > > >> and those capacitors' temperature behavior is no better than the PCB > > >> material's. > > > > > > Yes, will have better layout for next version. > > > > > >> See if your manufacturer of choice has any app notes on layout for good > > >> oscillator stability. > > > > > > They suggested 4 layers with 4 ground plane (Digital, Analog, Crystal > > > and RF). ... > > > > > > Four (!) different ground planes? That is usually a recipe for disaster. > > > > > > > ... But that would be too expensive. I am cutting corners with > > > 2 layers. Perhaps i am just lucky with the one working board. > > > > > > As Lasse wrote, check for correct burden caps. They may very well have > > to be different between crystal versions. If the burden caps are wrong > > oscillation start may be recalcitrant and your frequency can we off. > > Easy way to check if you don't have a good frequency counter: Program a > > timer to spit out 1MHz. Then borrow a shortwave receiver, pull in WWV or > > WWVB or 5, 10 or 15MHz, listen to the beat. If the beat is in the kHz > > range you won't be a happy camper with channel lock up at 2.45GHz. >Thanks for the tip. I would have to change the microcontroller to clock off the RF circuit, rather than internal RC. Or, i guess just an external controller for calibration.
Reply by ●October 28, 20122012-10-28
On Saturday, October 27, 2012 4:09:31 PM UTC-7, Jeff Liebermann wrote:> On Sat, 27 Oct 2012 10:22:23 -0700 (PDT), linnix > > <me@linnix.info-for.us> wrote: > > > > >Also, the 1/4 length for 2.5GHz is 3cm. Most WiFI antenna seems to be > > >bigger. Are they 3/4 length? Would 1-1/4 be better? We are just > > >using a wire coiled on top of the PCB. > > > > Others have addressed the crystal issue, so I'll do the antenna. > > > > Designer�s Guide to LPRF > > <http://www.ti.com/lit/sg/slya020a/slya020a.pdf> > > See Pg 39 for links to specifics on the various antenna styles. > > > > Antenna Selection, quick reference guide > > <http://www.ti.com/lit/an/swra351/swra351.pdf> > > > > Antenna Selection Guide (new version) > > <http://www.ti.com/lit/an/swra161b/swra161b.pdf> > > > > Antenna Selection Guide (old version) > > <http://www.prochild.co.kr/board/files/tb_3/AN058%20Antenna%20Selection%20Guide%20swra161.pdf> > > > > Selecting antennas for low-power wireless applications > > <http://www.ti.com/lit/an/slyt296/slyt296.pdf> > >Thanks. I'll do more reading.> > When you disclose what you're trying to accomplish, and what > > limitations are involved (size, cost, placement, aesthetics, range, > > data rate, proximity to metals, etc), perhaps I can offer some clues > > as to which would be "better". >There will be many PFD devices in addition to the coordinator. The coordinator will be main powered (standalone or via PC USB), using the Microchip module with inverted-E (or modified inverted-F) antenna. It seems to work fine and cost is not a big issue, since we only need one per system. However, the PFD devices are likely battery powered, and some might be buried underground with external antenna. Some handheld remote unit can have a weak wire antenna, as they are mobile and can be moved closer to the coordinator. I am using the nearby smart electrical meters as RSSI test. The coordinator can pull in 80% signals within 50m, so the range is fine. My working prototype can pull in around 20% of the packets from the smart meters, and 80% from the coordinator. So, the prototype board is good enough, if only we can reproduce them reliablely. The coordinator and underground sensor can cost more, but the remote units must be low cost. BOM should be less than $10.> > > Incidentally, your wire coiled on top of the PCB might work if you're > > lucky, but a well controlled and properly modeled antenna system would > > probably be more consistent and more reliable. > > > > > > -- > > Jeff Liebermann jeffl@cruzio.com > > 150 Felker St #D http://www.LearnByDestroying.com > > Santa Cruz CA 95060 http://802.11junk.com > > Skype: JeffLiebermann AE6KS 831-336-2558
Reply by ●October 29, 20122012-10-29
"langwadt@fonz.dk" wrote:>> > my point is that for each different type of xtal you need to measure >> > the resulting frequency and adjust the caps, it can be much more than >> > 40ppm >> > off >> >> Until i get the 2.5GHz frequency counter, i just have to keep trying >> different crystals and caps. �I wonder if it's because of the ceramic >> SMD vs. metal case HC-59 crystal, in terms of para. caps. > >you can just measure the xtal frequencywhere "just measure" means using a high impedance (low capacitance) probe or an output not connected to the crystal. Oliver -- Oliver Betz, Munich despammed.com is broken, use Reply-To:
Reply by ●October 29, 20122012-10-29
"Martin Riddle" wrote: [...]>Make sure the board is cleaned properly. The No clean flux can cause >problems for crystals, especially under the crystal.could you explain this further, what kind of influence are you talking about? Oliver -- Oliver Betz, Munich despammed.com is broken, use Reply-To:
Reply by ●October 29, 20122012-10-29
Oliver Betz wrote:> "Martin Riddle" wrote: > > [...] > > >>Make sure the board is cleaned properly. The No clean flux can cause >>problems for crystals, especially under the crystal. > > > could you explain this further, what kind of influence are you talking > about? > > OliverThink about that, the flux gets heated in the process and many fluxes are made with organics that can break down to carbon. Also, years ago, I had some flux that I stuck the probes of a cap meter into and I was very surprised at the dialectic value it had.. I am sure there are different formulas for specific applications.. Jamie
Reply by ●October 29, 20122012-10-29
On Monday, October 29, 2012 4:38:50 AM UTC-7, Oliver Betz wrote:> "langwadt@fonz.dk" wrote: > > > > >> > my point is that for each different type of xtal you need to measure > > >> > the resulting frequency and adjust the caps, it can be much more than > > >> > 40ppm > > >> > off > > >> > > >> Until i get the 2.5GHz frequency counter, i just have to keep trying > > >> different crystals and caps. �I wonder if it's because of the ceramic > > >> SMD vs. metal case HC-59 crystal, in terms of para. caps. > > > > > >you can just measure the xtal frequency > > > > where "just measure" means using a high impedance (low capacitance) > > probe or an output not connected to the crystal. >The MC13202 has a separate Clock out pin, driving the microcontroller. Together with the cap trimmings, we might be able to have the frequency calibration build-in. This might make it a winner over the MRF24J40 and AT86RF231.
Reply by ●October 29, 20122012-10-29
Oliver Betz wrote:> "Martin Riddle" wrote: > > [...] > >>Make sure the board is cleaned properly. The No clean flux can cause >>problems for crystals, especially under the crystal. > > could you explain this further, what kind of influence are you talking > about?This usually doesn't show up right away. It develops over weeks or months. Some fluxes, like "no-clean" can be pretty high impedance, especially right after soldering or reflow. Then, they absorb water, and the conductivity goes up. Many logic inverter-type oscillators need some amount of conductance between the two crystal terminals to put the inverter input near the transition. But, too much conductance and the oscillator may fail to start, or may hop in frequency. In some cases I've seen conductance between adjacent chip pins in the hundreds of Ohms, which would certainly kill the Q of an oscillator. Cleaning the boards carefully with a toothbrush and solvent will fix the problem. It can sometimes be hard to get solvent under SMT components, and that's where these deposits like to hide. Jon
