CC2500 radios and crystal tolerances

21.11.2015 13:05

While working on Spectrum Wars (during a live demo in fact - this sort of things always happens during live demos) I found a pair of Texas Instruments CC2500 radios that were not able to communicate with each other. Even more puzzling, radio A was able to talk to radio B, but packets from radio B were not heard by radio A.

After checking for software problems (I learned from experience to be vary of CC2500 packet drivers) I found out that very short packets sometimes got through, while longer packets were dropped by the packet handling hardware due to bad CRC. I also noticed that this problem only occurred at low bit rate settings and hence most narrow band transmissions. This made me suspect that perhaps the transmission of radio B fell out of the radio A's reception bandwidth.

When configuring CC2500 radio for reception, you must configure the width of the channel filter. Neither the Texas Instrument's SmartRF Studio software nor the datasheet is very helpful in choosing the correct setting though. What the datasheet does mention is that 80% of the signal bandwidth must fall within the filter's bandwidth and warns that crystal oscillator tolerances must be taken into account. Unfortunately, determining what the signal bandwidth for the given modulation and bit rate is left as an exercise for the reader.

Some typical occupied bandwidth values for a few modulations and bit rates are given in the specifications, but of course, Spectrum Wars uses none of those. As an engineer-with-a-deadline I initially went with a guesstimate of 105 kHz filter bandwidth for 50 kbps bit rate and MSK modulation. It appeared to work fine at that time. After I noticed this problem, I continued with the practical approach and, once I had a reproducible test case, simply increased the filter bandwidth until it started to work at 120 kHz.

Just to be sure what the problem was, I later connected both problematic radios to the spectrum analyzer and measured their transmitted signals.

Measured spectra of two nodes transmitting MSK modulated packets.

The signal spectra are shown with green (radio A) and black traces (radio B). The cursors are positioned on their center frequencies. The blue and red vertical lines mark the (theoretical) pass bands of 105 kHz and 120 kHz receive filters on radio A respectively.

For both radios, the set central frequency of transmission was 2401 MHz. Their actual central frequencies as measured by the spectrum analyzer were:

radio Aradio B
f [MHz]2401.044902401.02171
δf [ppm]+18.7+9.0

The crystals used on radios are specified at ±10 ppm tolerance and ±10 ppm temperature stability. The accuracy of the frequency measurement was ±6 ppm according to the spectrum analyzer's documentation (a somewhat generous error margin for this instrument in my experience). Adding this up, it seems that the LO frequencies are within the maximum ±26 ppm range, although radio A looks marginal. I was measuring at room temperature, so I was not expecting to see deviations much beyond ±16 ppm.

On the other hand, it is obvious that a non-negligible part of the signal from radio B was getting clipped by the 105 kHz receive filter on radio A. The situation with the new 120 kHz setting in fact does not look much better. It is still too narrow to contain the whole main lobe of the signal's spectrum. It does appear to work though and I have not tried to measure what percentage of the signal's power falls within the pass band (it's not trivial with short packet transmissions).

As for why the problem was asymmetrical, I don't know for sure. It's obvious that this radio link is right on margin of what is acceptable. It might be that some other tolerances came into play. Perhaps minor differences in filter bandwidth or radio sensitivity tipped the scale in favor of packets going in one direction over the other. I've seen weirder things happen with these chips.

Posted by Tomaž | Categories: Analog

Comments

Just to tank you so much!
Had great problems with two cc2500 , receiving a bunch of malformed packets with CRC errors.
Afters growing up the "RX filter BW" from (MSK 250kBaud) 500khz to 800Khz everithing whent perfect , zero errors.
You sir deserve a Beer!!

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