802.15.4 RF issues

I 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.

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

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 (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.

If the metal case is NOT grounded, it might have difficulties passing FCC Part 15 (incidental radiation) as the case makes a dandy capacitively coupled antenna at the oscillator frequency. It's not made from solderable metal for decoration.

I can usually pickup enough RF from a clock oscillator with a small loop attached to one end of a 50 ohm cable to the freq counter. The problem is that this picks up not only the clock oscillator, but all the other garbage being generated by the circuit. For low frequencies, I sometimes use a scope probe, loop, and move things around until I see something on the scope that looks usable. I then connect the counter to the vertical output of the scope. If that doesn't work, I throw together an LC parallel resonant circuit at the expected frequency, and use that instead of the untuned loop. I've tried low capacitance amplified probes for looking at oscillators. Sometimes it works, but I'm never quite sure if it's does or doesn't have an effect on the frequency. So, I don't use amplified probes.

I'm more familiar with 802.11 than with 802.15.4. Some of the chipsets have handy commands that temporarily disable spectrum spreading and produces a CW carrier at 2.4GHz. This is also handy for setting the frequency, especially when using ATE (automagic test equip). In dd-wrt, it's one of the Broadcom mfg test mode commands accessed with the "wl" command: carriersuprs syntax is: crsuprs Arg is channel number 1-14, or 0 to stop the test. Also see: freqacuracy syntax is: fqacurcy Arg is channel number 1-14, or 0 to stop the test. There might be something similar buried inside the 802.15.4 chipset.

I would lift the lead for high frequency crystals, but the 32,768 Hz crystals didn't radiate enough to matter. Also, trhese radios had multiple layers of aluminum shielding, and no FCC testing since they were custom built telemetry systems for the Aerospace industry. They met all FCC requiremnts, by design.

Some of our crystals were fundamental 125 MHz in a TO-5 can. You don't have a Grid Dip Oscillator, or two?

If it isn't buried under layers of NDA?

I have 3 grid dip meters. Two transistorize Heathkits, and one tube type EICO. I use them for tuning antennas.

Yeah, probably. It depends on the vendor. However, if he's working on a real ZigBee product, that requires intimate knowledge of the workings of the chipset, methinks he's probably already signed an NDA and has the necessary information. It's usually buried among the demo test software in the frequency set routines.

I have an Eico 710 & a James Millen.

Somewhere in a poorly written 10,000 page document with no index?

Regarding 2.5GHz test. If i can test the 16M/20M clock, why brother with 2 .5G?

Not Zigbee. Just native 802.15.4. Zigbee requires at least 20K code, whic h would be difficult to fit in a 16K µC.

Most 802.15.4 chip set docs are available without NDA. I read the spec for MRF24J40, AT86RF231 and MC13202 several times. Unfortunately, only the MC

Are you sure you can test at 16/24/32Mhz without affecting the frequency? Touch either lead of the crystal and you're going to move the frequnecy. You could build pickup coil into a test fixture, and maybe it won't pull your oscillator off frequency. Maybe add an emitter follower isolated test point on the PCB connected to the crystal osc output. If there was a test point furthur up the chain, it wouldn't be a problem, but I don't see one. So, you may need to measure at 2.4GHz simply becauses there's no other practical way to measure it elsewhere.

MRF24J40, AT86RF231 and MC13202 several times. Unfortunately, only the MC13202 provides a Clock Out interface based on the crystal.

Use an external cystal can oscillator.

h 2.5G?

There is a separate Clock Out (16M, 8M, 4M or 23K) pin, in addition to the two crystal pins. I can just feed this as the µC clock input, and calibr ate it again an external low-freq source. I guess Motorola Corp (sorry, Fr eescale) has more experiences in making RF chips.