What is the output impedance of CC1100/1? 

CC11xx=CC1101, CC1100, CC1150, CC1152, CC1110 and CC1111.
Al these devices use high order switch mode output amplifiers. These amplifiers have time varying impedance throughout the signal period.  The term output impedance is used for linear amplifiers or amplifiers that can be approximated by a linear equivalent. Output impedance is normally used to design complex conjugate impedance matching between amplifier and load. For linear amplifiers this is sufficient to secure optimum power transfer.  This method is not a valid for the CC11xx series of transceivers and transmitters. Due to the amplifiers nonlinear transfer characteristic the term output impedance not applicable.  And we are using the term load impedance instead. The load impedance is the differential impedance “seen” when looking into the RF_P RF_N pins from the chip. The optimum load impedance is found by using a method named load pull measurements. Load pull measurements are generally performed by connecting variable impedance to the amplifier output and monitoring one or more amplifier characteristics. Output power and harmonics is two examples of such characteristics. When a acceptable performance is found the load is disconnected from the amplifier and measured.  The CC1100 datasheet specifies the resulting load impedance to
315MHz:          122+j31
433MHz:          116+j41
868/915MHz:   86.5+J43 Impedance calculated based on analyzing datasheet and reference design schematic often deviate from these values. PCB parasitic and component imperfections generally accounts for these differences.
When operating at high frequencies PCB traces has to be modeled to account for phase shift, skin effect increased resistance, inductive and capacitive effects. Manufacturers of passives normally provide linear LCR models and/or S-parameter models representing their components at higher frequencies.  Be aware to check the valid frequency range for the models, and only use them within this range. Simulators often extrapolate the model data without warnings and simulation results become invalid. Remember that valid frequency range is the range up to the highest significant frequency component within your circuit. For CC11xx we recommend models and simulations to cower at least the 5th harmonic.

CC11xx operations are heavily dependent upon filter balun antenna impedance up to at least the fifth harmonic.  Matching load impedance only at fundamental frequency could easily result in high current consumption, low output power and high spurious/harmonics. If the load impedance at the harmonics is large and inductive we have generally found the CC11xx to operation satisfactory. Duplicating the load impedance at the harmonics frequencies from one of our reference design should lower the risk considerably.

It should be noted that the matching circuitry for the CC11xx series products, in addition to being an impedance matching circuits also hold BALUN and filtering functions. If the phase and amplitude balance within the BALUN portion of the function is reduced, performance is degraded. Typically increased current, lowered output power and increased 2nd harmonic. If the filter characteristic is changed harmonic and/or spurious emission may result, violating ETSI, FCC…
The filter characteristics can be substantial changed by minor layout changes. One example is butting the antenna close to RF_N/RF_P. Reduced isolation can then cause harmonics to bypass the filter and enter the antenna at to high levels. In addition to provide a load for the output amplifier the impedance match must represent adequate source impedance for the input LNA, generating reasonable receiver sensitivity. Our match, BALUN and filer designs generate what we believe to be the best combined RX and TX performance.  

The total complexity of the requirements for the balun, match and filter circuitry is quite extensive. We generally recommend designers un-experienced with Radio frequency design to copy our reference designs as closely as possible. The best is to download the Gerber files from web and copy the RF portion onto your own design using the same PCB thickness and dielectrics. If an exact copy is no option, change as little as possible and try to tweak performance back up by tuning individual component values one value up or down. This strategy has often proven to compensate for the layout change, but there is no guarantee.  


Q: Re: FAQ: "Output Impedance" not a valid term for CC11xx by 613920

 

 

Hi Sasidhar

 

When operated in receive one would expect that the LNA should be linear. This is however not completely true. Within CC1100/1 there is automatic gain control (AGC) functionality. This imply that the gain in the receive chain is adjusted depending upon the signal strength of the received signal. This results in that the LNA operates with different configuration at each discrete gain setting.

 

To the best of my knowledge nobody has undertaken this solely receive optimization task. As previously stated TI have found the recommended impedances by load-/source-pull over the full operational range and believe this to be the overall best compromise for TX and RX performance over the full operational range.

 

I assume that optimizing the noise figure is most interesting for the maximum gain setting. This is the gain setting when there is no input signal. If you where to perform this noise figure optimization, I recommend exploiting source pull techniques. If you attempt to measure the input impedance with a network analyzer you will find abrupt transitions in you results. These transitions will occur whenever CC1100/1 is adjusting its gain. The CC1100/1AGC gain can be fixed at maximum to avoid these transitions but then the LNA would probably enter compression at the power level that most normal network analyzer use during measurement, rendering the results invalid.

 

I am sorry, but the provided impedances and reference circuits are the information that I am aware of that can be used as a base for CC1100/1 design. But all this information is gained while trying to identify the best overall compromise performance. The only additional information that I might provide is that the CC1100/1 LNA  was designed to operate together with the output amplifier of CC1100/1 so large improvements is not expected.  This is still the situation even after recognizing that the current recommendations hardly are optimum one at any one nominal RX operation.  


Q: Re: FAQ: "Output Impedance" not a valid term for CC11xx by 1637307

Hi RFHWDude,

This is certainly understandable in the TX mode. Can you comment if the same holds good n hte receive mode? One would think that LNA are linear. Do LNAs also have time varying impedances throughout sinal period? If we were to optimize the noise figure of the system than the transmit power, would the load impedance recommendation stay the same in all the three bands? Please comment.


Thanks

Sasidhar


Q: Re: FAQ: "Output Impedance" not a valid term for CC11xx by 1187355

This is Chris Pinter with Pinter Electronics Consulting. I am an RF Engineer and will offer you some suggestions. If you need to get a hold of me directly please visit http://www.pinterec.ca/contact-us

Are you asking the questionsa for therectical purposes or for a practical application?    If the question is theretical then I would say you need to measure the two impendance lines seperately and consider adding the two impedances.   From a practical point of view the impedance of the circuit is best measured and tuned from the end of the balum, not the output pins.   I do not think the PCN fabricator would now very much about RF circuits most of the fabricators I know have a good understanding of the material and the changes during fabrication.   If you need help it is best to get the opinion of an RF Engineer.

We offer a number of package services that provide this service for a fixed cost. Please take a look at our cost effective solutions to help you. These services will help you reduce your costs and the time required to get your design functional. http://www.pinterec.ca/services/wireless-development/ti/low-power-rf-zigbee

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Q: Re: FAQ: "Output Impedance" not a valid term for CC11xx by 632988

Hi RFHWDUDE,

 

What would be the appropriate PCB trace differential impedance the differential pair of traces running from the RF_p and RF_n should be fabricated ?

Lets say I am connecting RF_N/RF_P ports to a balun. The balun data sheets indicate matching differential impedance to CC11xx at its input. I know the output of teh Balun is 50 ohms single ended, and can design the traces to match with 50 ohms. But what differential impedance must the PCB traces from CC11xx to the balun be designed for?  86.5 + j43 does not seem to ring any bells to the PCB fabricator. All he can understand is 90 ohms differential, 100 ohms differential, 110 ohms differential etc. He will tune the width/stack up/trace width/separation to match such a figure.Can you please guide me here?

 

Thanks,
Srikanth