J. KIEFFER ET AL.

Copyright © 2013 SciRes. CN

143

Figure 8. Impact of the different methods on EVM.

where ksyn is still the TDC gain in the synthesis mode

defined in equation (10) and which is now calibrated

before inserting the modulation into the PLL. Then, the

modulation, normalized by the wanted output frequency,

is applied on the TDC gain at each sample. This is the

third inputs of the proposed three-points modulator.

The Figure 8 shows the amelioration of the EVM with

the implementation of this new expression instead of

keeping the TDC gain filtered or constant after calibra-

tion. The PLL is still considered perfect.

Several observations can be made thanks to this figure.

At first, it is useless to increase the TDC gain filtering

bandwidth so that most of the modulation can pass with-

out alterations. On the contrary, the EVM increases with

the bandwidth enlargement.

Then, the proposed solution allows an EVM reduction

of 2% compared to a method where the TDC gain is kept

constant after the same calibration (filtering with 10 kHz-

bandwidth in this case). Moreover, the solution presents

certain robustness regarding a calibration error. In others

words, having an offset on the gain estimation has less

impact on EVM than not adding the modulation.The final

EVM is under 1%, below the EVM with the rest of the

PLL imperfections.

4. Conclusions

A nonlinear Digital PLL model has been developed to

bring out the impact of TDC gain estimation in polar

architectures. This causes indeed a large EVM degrada-

tion for wideband modulations such as WCDMA. The

proposed solution which finally amounts to add the mod-

ulation on the TDC gain transforms the classical

two-points modulator in a three-points modulator and

allows a good reduction of the EVM degradation due to

this contributor. This shows that the phase path is not

straightforward for wideband modulations from the con-

clusions derived from GMSK/EDGE polar modulator.

5. Acknowledgements

The authors thank Samuel Dubouloz for his help on the

modulator and demodulator.

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