Careful examination of Fig. 4.1(b) will shows that the even harmonic mixer(IF=RF-2*LO) can be converted to a fundamental mixer (IF=RF-LO) by connecting the 0oand 180 LO inputs together ,and also the 90 and 270 LO inputs together. The result is a fundamental LO Gilbert cell mixer whose input stage is identical to the even harmonic mixer. Table 4.2 shows the comparison of the even harmonic mixer to the fundamental mixer. The simulated NF for fundamental mixer is about 3dB lower than the even harmonic mixer and conversion gain is about 5dB higher than it.
Parameter Even Harm. Fundamental Conversion
Table 4.2: Comparison of sub-harmonic mixer to fundamental mixer
Chapter 5
C ONCLUSIONS A ND F UTURE P ROSPECTS
5.1 Conclusions
A fully integrated 2.4GHz CMOS frequency synthesizer is demonstrated.
Table 5.1 lists the measurement data compared to the spec of Bluetooth requires. The data shows that we achieve the spec requirement of Bluetooth except to phase error. However, this design suffered from strong spurious tones, and consumed large power in VCO buffer.
Measurement Spec of Bluetooth
Power supply 2.5V N.A
Tuning range of VCO 2.392 ~ 2.514GHz 2.4~2.481GHz Phase noise -102dBc/Hz@1MHz -120dBc/Hz@3MHz
Spurious tones -40dB N.A
Table. 5.1 Measurement data compared to spec requirement
5.2 Future Prospects
There’re several directions for future work. First, a more accurate method must be set to measure the phase error. We should integrate the down conversion mixer to convert the LO signal to lower frequency for decreasing the bond wire effect to the measurement of phase error. Second, the spurious tones are strong and seriously influence the signal. We have some tactics to alleviate this problem, narrow the bandwidth of the loop filter or minimize the interference of crystal oscillator by designing a current matching circuit in Charge Pump. Through the above two ways, spurious tones can be suppressed more effectively. Third, the VCO buffer should be re-designed to lessen power consumption and counterwork process variation.
Besides, the architecture of conventional integer-N frequency synthesizer suffers from many draw back. The limited bandwidth of integer-N frequency synthesizer causes out off band noise from VCO. Large divide number intensifies the in-band noise from the reference and phase detector noises. The architecture of fractional-N frequency synthesizer solves the problems above.
Moreover, the settling time can be faster by using fractional-N frequency synthesizer [20]. Therefore, the future research should be focused on the architecture of fractional-N frequency synthesizer.
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