802.15.4 RF issues

a different crystal will probably need different capacitors to be spot on frequency. Try measuring the frequency, 802.15.4 requires something like +/-40ppm

I believe if you get the caps right wor the crystal you should able to stay with in +/40ppm over temperature and aging

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

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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.

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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.

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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

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-Lasse

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.

See if your manufacturer of choice has any app notes on layout for good oscillator stability.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com

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Yes, will have better layout for next version.

They suggested 4 layers with 4 ground plane (Digital, Analog, Crystal and RF). But that would be too expensive. I am cutting corners with

2 layers. Perhaps i am just lucky with the one working board.

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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.

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you can just measure the xtal frequency

-Lasse

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Would it be possible to temperature compensate the crystal with the MC13202? That might force us to switch part.

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seems to indicated that is wouldn't be necessary but with the switchable load caps and temperature measurement I guess you could

-Lasse

Four (!) different ground planes? That is usually a recipe for disaster.

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/

Others have addressed the crystal issue, so I'll do the antenna.

Designer?s Guide to LPRF See Pg 39 for links to specifics on the various antenna styles.

Antenna Selection, quick reference guide

Antenna Selection Guide (new version)

Antenna Selection Guide (old version)

Selecting antennas for low-power wireless applications

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

Make sure the board is cleaned properly. The No clean flux can cause problems for crystals, especially under the crystal.

Cheers

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.

Thanks. I'll do more reading.

There will be many PFD devices in addition to the coordinator. The coordin ator will be main powered (standalone or via PC USB), using the Microchip m odule with inverted-E (or modified inverted-F) antenna. It seems to work f ine 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 buri ed 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 coord inator.

I am using the nearby smart electrical meters as RSSI test. The coordinato r can pull in 80% signals within 50m, so the range is fine. My working pro totype 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.

where "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:

[...]

could you explain this further, what kind of influence are you talking about?

Oliver

--
Oliver Betz, Munich 
despammed.com is broken, use Reply-To:

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 am sure there are different formulas for specific applications..

Jamie

n

ceramic

The MC13202 has a separate Clock out pin, driving the microcontroller. Tog ether with the cap trimmings, we might be able to have the frequency calibr ation build-in. This might make it a winner over the MRF24J40 and AT86RF23

1.

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