linnix wrote:> > 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 don't need a 2.5 GHz counter to check the crystal, it is running at a much lower frequency. Trying to count the modulated output of the transceiver will probably be a mess, too. Also, it is probably turning on and off for short bursts, which will also interfere with getting a valid count. Make a coil about the size of the crystal and maybe 5-10 turns. First hook to a scope probe and see if you can pick up the crystal oscillator. If it looks real fuzzy, that may indicate the frequency is unstable, ie. the crystal is not being resonated at high Q. You may also see the crystal is running on the 3rd harmonic, they REALLY like to do this. Increasing the resonating cap value usually pulls them back down into the right range. Generally, the crystal makers specify a resonating cap value, you double this value if you have a capacitor to ground at each end of the crystal. So, if they spec 15 pF, you put a 30 pF cap at each end of the crystal. If you can get a visible signal that is remotely within range of what it is supposed to be, then try the freq. counter to measure it more accurately. After changing the caps, power up many times to make sure it starts reliably every time, and maybe over a range of temperatures, too. Jon
802.15.4 RF issues
Started by ●October 27, 2012
Reply by ●October 29, 20122012-10-29
Reply by ●October 30, 20122012-10-30
Jamie 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? > > Think 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 think the air gap between the case and the board will contribute much more uncertainty. Oliver -- Oliver Betz, Munich (oliverbetz.de)
Reply by ●October 30, 20122012-10-30
On Mon, 29 Oct 2012 12:38:52 +0100, Oliver Betz <obetz@despammed.com> 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? > >OliverI have not been followed this discussion with great attention, but there are several issues related to PCB material humidity and temperature dependencies in oscillators. When building free running HF oscillators (e.g. VFOs) never use two (or multi) layer constructions near the LC resonant circuit. The stray capacitance between the resonant LC components and the PCB ground plane affects the frequency. Unfortunately, this stray capacitance varies with the air humidity and temperature, which affects the dielectric constant of the PCB material and hence affect the frequency. For simple oscillators in the UHF/microwave range, a free running oscillator made of 1/4 wavelength PCB traces are extremely sensitive to these issues. Anyway, one should remember that a frequency drift of 1 Hz at 25 MHz is 100 Hz at 2.45 GHz. I was once debugging a GHz signal source based on some VHF overtone crystals and wondered, why the frequency was shifting every few seconds. I finally discovered that I was breathing on the crystal, that cased the frequency drift :-). When using HF fundamental or VHF overtone crystals as a reference to frequency multipliers or PLLs, you really need to pay attention to the PCB material and layout around the crystal oscillator.
Reply by ●October 30, 20122012-10-30
Hello Jon, not announcing a Followup-To: is bad. So again for c.a.e: hundreds of ohms between nets will kill most circuits, anyhow. If they don't kill them immediately, they will do slowly by electrochemical migration. Cleaning boards is not easy. No clean often means "no chance to clean". Oliver -- Oliver Betz, Munich (oliverbetz.de)
Reply by ●October 30, 20122012-10-30
On Sat, 27 Oct 2012 22:15:19 -0400, "Martin Riddle" <martin_rid@verizon.net> wrote:>Make sure the board is cleaned properly. The No clean flux can cause >problems for crystals, especially under the crystal.True. In a past life, I designed marine radios. As one would expect, marine radios tend to get wet, usually from condensation. Ionic contaminants on the PCB are NOT much of a problem, until the board gets wet. Then, the stuff really conducts. One board that I ran through a worst case test in our then modern wave soldering machine showed about 20K/square sheet resistivity. To high impedance circuits, long parallel traces, and voltage threshold activated circuits, that's almost like a short circuit. The general solution is to design using low impedances wherever possible. However, that won't work for crystal oscillators, which are high impedance devices. So, you're stuck with keeping the board clean, or at least the area around the crystal clean. Once you get it clean, you might also need some conformal coating. (Not the entire board as that makes rework difficult. Just the areas that are deemed moisture sensitive). When we switched from rosin flux to water soluable flux in the 1970's, we had nothing but problems. Initially, it was rather stupid problems, such as using an unfiltered water rinse with far too much calcium both in the water and sitting in the bottom of the water heater. Later, they became more difficult, such as uneven rinsing in the modified dish washer that was used for washing. Every board had several test traces which were used to estimate resistivity. When we knew they were baked dry, and all the rinse water was gone, they were conformal coated, usually with acrylic. In short, board cleaning after soldering with water soluable flux was not a trivial exercise. I've never done anything with Zigbee, so I'm not really familiar with the frequency stability requirements. Googling... This app note goes into the requirements in detail: Reference Oscillator Crystal Requirements for the MC1320x, MC1321x, MC1322x, and MC1323x IEEE 802.15.4 Devices <http://cache.freescale.com/files/rf_if/doc/app_note/AN3251.pdf> (Note that the chips have internal switched capacitors for both coarse and fine tuning.) Basically, they want +/-40ppm over the operating temperature range. That should be possible without any elaborate external TCXO style temperature compensation. Just buy a decent AT cut series resonant crystal, with a low series resistance, and keep the board clean. -- 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 30, 20122012-10-30
> Reference Oscillator Crystal Requirements for the MC1320x, MC1321x, > > MC1322x, and MC1323x IEEE 802.15.4 Devices > > <http://cache.freescale.com/files/rf_if/doc/app_note/AN3251.pdf> > > (Note that the chips have internal switched capacitors for both coarse > > and fine tuning.) Basically, they want +/-40ppm over the operating > > temperature range. That should be possible without any elaborate > > external TCXO style temperature compensation. Just buy a decent AT > > cut series resonant crystal, with a low series resistance, and keep > > the board clean.I was luck to have the first prototype working with this crystal: 50ppm stability and 20ppm tolerance http://www.digikey.com/product-detail/en/445C25D20M00000/CTX1164CT-ND/3135104 However, board #2 and #3 does not work with cheaper crystal: 20ppm as they claim. I'll spend a little more crystals and probably get the 10ppm, when i am ready to order more parts. Yes, the Freescale MC13202's cap trimming would be helpful in temp. compensation, which could be a problem later on.
Reply by ●November 10, 20122012-11-10
On Oct 30, 11:30�am, m...@linnix.info-for.us wrote:> > Reference Oscillator Crystal Requirements for the MC1320x, MC1321x, > > > MC1322x, and MC1323x IEEE 802.15.4 Devices > > > <http://cache.freescale.com/files/rf_if/doc/app_note/AN3251.pdf> > > > (Note that the chips have internal switched capacitors for both coarse > > > and fine tuning.) �Basically, they want +/-40ppm over the operating > > > temperature range. �That should be possible without any elaborate > > > external TCXO style temperature compensation. �Just buy a decent AT > > > cut series resonant crystal, with a low series resistance, and keep > > > the board clean. > > I was luck to have the first prototype working with this crystal: > 50ppm stability and 20ppm tolerance > > http://www.digikey.com/product-detail/en/445C25D20M00000/CTX1164CT-ND... > > However, board #2 and #3 does not work with cheaper crystal: > 20ppm as they claim. > > I'll spend a little more crystals and probably get the 10ppm, when i am ready to order more parts. > > Yes, the Freescale MC13202's cap trimming would be helpful in temp. compensation, which could be a problem later on.Followup on this: All my MRF24J40s are working now, with the CTX SMD ceramic crystal. ICAP|ECAP|ESR| STAB|TOL CTX1164 |18pf|18pf| 40|50PPM|20PPM // OK CTX1172 |18pf|18pf| 40|30PPM|20PPM // OK 445C33L20M|12pf|18pf| 40|30PPM|20PPM // Out of stock, will try 445C22L20M|12pf|18pf| 40|20PPM|20PPM // 1K min, wlll try BTW, the MC13202 spec for ICAP|ECAP|ESR| STAB|TOL 9pf|18pf| 40|10PPM|10PPM // Custom order? However, nothing in the spec of the non-working HC-59S would indicate why it won't work.
