CC1110 BALUN tuning with Network Analyzer and Spectrum Analyzer

[tonyctsiu]

Dear All,


I am trying to design a circuit using TI's CC1110. I am going to follow the TI's reference design for the balun.
The output impedance of the CC1110 at 915 mHZ is a differential complex impedance 86.5+j43. I have to then convert it into an unbalanced 50ohm port with a series of low pass filter and notch filter. The 50ohm output is headed to a SMA connector.

I have network analyzer and spectrum analyzer in my school laboratory. Can anyone suggest a practical method to verify the followings after the fibrication of PCB?

1. The network is matching properly.
2. The harmonics is supressed into an acceptance level

For the requirement 1 above, the difficulty is that I don't know how to use VNA to verify a differential output. Do I need a 4 port or 3 port VNA to do the job? Or do I need some kind of differential probe (if it exists)? Also the RF port of the CC1110 is just 2 pin of a QFN package. How can I probe it with a network analyzer?

For the requirement 2 above, can I just connect the SMA connector of my PCB to a spectrum analyzer to do the checking?

Do I need any other equipment to do the job?

Thanks for giving me directions!

Thank you and regards,

Tony

 

[E Kafeman]

1a. Assume that TI applications examples for a synthetic balun (discrete phase-shift 2*C and 2*L) are correct and use same values assumed that your PCB layout also is very similar.
1b. Buy a balun, connect it to VNA and do your impedance matching. Alternatively buy or build your own balun and calibrate the VNA for 200 Ohm and measure to find out best component values for a synthetic balun to convert down to 50 Ohm unbalanced. Coaxial balun is cheap and quick but you must know its limitations. It will only show correct values for one frequency and measured impedance, as showed on VNA, must be recalulated, if VNA is set for 50 Ohm.
If calculating a synthetic balun, I assume 86.5+j43 is not including any PCB load or trace length.

2. You must measure radiated harmonics for total circuit, including antenna. As an example, if less good values are selected for the synthetic balun, can it cause to much harmonics (personal experience with CC1101). A spectrum analyzer and a reference antenna and a measurement chamber, all calibrated according to actual standard, are needed to do a qualified job, but a bench test with a spectrum analyzer and any antenna can show if there are obvious problems with radiated harmonics.

CC1101 is in my opinion a bit sensitive and can cause a lot of harmonics if not correct matched and well decoupled at Vcc. Have never worked with CC1110 but guess it is similar in RF design.

 

[FvM]

2. You must measure radiated harmonics for total circuit, including antenna.

In the ETSI domain, both radiated and conducted (by external antenna connector) spurious components, which include harmonics, have to be measured, see ETSI EN 300 220-1. Because both contributions have to keep the limit values, a harmonics measurement at the antenna output only is meaningful, if you don't have the instrumentation for radiated measurements.

For the balun matching problem, achieving the specified output power will be a sufficient criterion. If you don't, considering the losses involved with the balun components, particularly inductor Q, you should think about adjusting the network.

For the single ended outputs, load pulling is the best method to determine the optimal output impedance and in return, possible matching network corrections. Unfortunately, with a balun, the balun's differential phase is a third independent parameter besides load R and X, that can't be manipulated from the single ended output port, so adjustment of the balun itself is necessary in differential load pulling.